A series of experiments made on this plan have revealed the fact that
if a number of vertical insulated wires are hung up in the air and
rather near together, the electrical capacity of the whole of the
wires in parallel is not nearly equal to the sum of their individual
capacities. If a number of parallel insulated wires are separated by a
distance equal to about 3 per cent. of their length, the capacity of
the whole lot together varies roughly as the square root of their
number. Thus, if we call the capacity of one vertical wire in free
space unity, then the capacity of four wires placed rather near
together will only be about twice that of one wire, and that of
twenty-five wires will only be about five times one wire.
This approximate rule has been confirmed by experiments made with long
wires one hundred or two hundred feet in length in the open air. Hence
it points to the fact that the ordinary plan of endeavouring to obtain
a large capacity by putting several wires in parallel and not very far
apart is very uneconomical in material. The diagrams in Fig. 8 show
the various methods which have been employed by Mr. Marconi and others
in the construction of such multiple wire aerials. If, for instance,
we put four insulated stranded 7/22 wires each 100 feet long, about
six feet apart, all being held in a vertical position, the capacity of
the four together is not much more than twice that of a single wire.
In the same manner, if we arrange 150 similar wires, each 100 feet
long, in the form of a conical aerial, the wires being distributed at
the top round a circle 100 feet in diameter, the whole group will not
have much more than twelve times the capacity of one single wire,
although it weighs 150 times as much.
[Illustration: FIG. 8.--VARIOUS FORMS OF AERIAL RADIATOR. _a_, single
wire; _b_, multiple wire; _c_, fan shape; _d_, cylindrical; _g_,
Conical.]
The author has designed an aerial in which the wires, all of equal
length, are arranged sufficiently far apart not to reduce each other's
capacity.
As a rough guide in practice, it may be borne in mind that a wire
about one tenth of an inch in diameter and one hundred feet long, held
vertical and insulated, with its bottom end about six feet from the
ground, has a capacity of 0.0002 of a microfarad, if no other earthed
vertical conductors are very near it. The moral of all this is that
the amount of electric energy which can be stored up in a simple
Marconi aerial is very limited, and is not much more than one-tenth of
a joule or one-fourteenth of a foot-pound, per hundred feet of 7/22
wire. The astonishing thing is that with so little storage of energy
it should be possible to transmit intelligence to a distance of a
hundred miles without connecting wires.
One consequence, however, of the small amount of energy which can be
accumulated in a simple Marconi aerial is that this energy is almost
entirely radiated in one oscillation or wave. Hence, strictly
speaking, a simple aerial of this type does not create a train of
waves in the ether, but probably at most a single impulse or two.
[Illustration: FIG. 9.--MARCONI-BRAUN SYSTEM OF INDUCING ELECTROMOTIVE
FORCE IN AN AERIAL, A. B, battery; K, key; I, induction coil; S, spark
gap; C, Leyden jar; E, earth plate; _ps_, oscillation transformer.]
We shall later on consider some consequences which follow from this
fact. Meanwhile, it may be explained that there are methods by which
not only a much larger amount of energy can be accumulated in
connection with an aerial, but more sustained oscillations created
than by the original Marconi method. One of these methods originated
with Professor Braun, of Strasburg, and a modification was first
described by Mr. Marconi in a lecture before the Society of Arts of
London.[7] In this method the charge in the aerial is not created by
the direct application to it of the secondary electromotive force of
an induction coil, but by means of an induced electromotive force
created in the aerial by an oscillation transformer. The method due to
Professor Braun is as follows: A condenser or Leyden jar has one
terminal, say, its inside, connected to one spark ball of an induction
coil. The other spark ball is connected to the outside of the Leyden
jar or condenser through the primary coil of a transformer of a
particular kind, called an oscillation transformer (see Fig. 9). The
spark balls are brought within a few millimetres of each other. When
the coil is set in operation, the jar is charged and discharged
through the spark gap, and electrical oscillations are set up in the
circuit consisting of the dielectric of the jar, the primary coil of
the oscillation transformer and the spark gap. The secondary circuit
of this oscillation transformer is connected in between the earth and
the insulated aerial wire; hence, when the oscillations take place in
the primary circuit, they induce other oscillations in the aerial
circuit. But the arrangement is not very effective unless, as is
shown by Mr. Marconi, the two circuits of the oscillation transformer
are tuned together.
We shall return presently to the consideration of this form of
transmitter; meanwhile we may notice that by means of such an
arrangement it is possible to create in the aerial a far greater
charging electromotive force than would be the case if the aerial were
connected directly to one terminal of the secondary circuit of the
induction coil, the other terminal being to earth, and the two
terminals connected as usual by spark balls. By the inductive
arrangement it is possible to create in an aerial electromotive forces
which are equivalent to a spark of a foot in length, and when the
length of the aerial is also properly proportioned the potential along
it will increase all the way up, until at the top or insulated end of
the aerial it may reach an amount capable of giving sparks several
feet in length. From the remarks already made on the analogy between
the closed organ-pipe and the Marconi aerial wire, it will be seen
that the wave which is radiated from the aerial must have a wave
length four times that of the aerial if the aerial is vibrating in its
fundamental manner. It is also possible to create electrical
oscillations in a vertical wire which are the harmonics of the
fundamental.
All musicians are aware that in the case of an organ-pipe if the pipe
is blown gently it sounds a note which is called the fundamental of
the pipe. The celebrated mathematician, Daniel Bernouilli, discovered
that an organ-pipe can be made to yield a succession of musical notes
by properly varying the pressure of the current of air blown into it.
If the pipe is an open pipe, and if we call the frequency of the
primary note obtained when the pipe is gently blown, unity, then when
we blow more strongly the pipe yields notes which are the harmonics of
the fundamental one; that is to say, notes which have frequencies
represented by the numbers 2, 3, 4, 5, &c. If, however, the pipe is
closed at the top, then over-blowing the pipe makes it yield the odd
harmonics or the tones which are related to the primary tone in the
ratio of 3, 5, 7, &c., to unity. Accordingly, if a stopped pipe gives
as its fundamental the note C, its first overtone will be the fifth
above the octave or G'.
[Illustration: FIG. 10.--SEIBT'S APPARATUS FOR SHOWING STATIONARY
WAVES IN LONG SOLENOID A. I, induction coil; S, spark gap; L,
inductance coil; C_{1}C_{2}, Leyden jars; E, earth wire.]
As already remarked, the aerial wire or radiator as used in Marconi
telegraphy may be looked upon as a kind of ether organ-pipe or siren
tube, and its electrical phenomena are in every respect similar to the
acoustic phenomena of the ordinary closed organ-pipe. When the aerial
is sounding its fundamental ether note, the conditions which pertain
are that there is a current flowing into the aerial at the lower end,
but at that point the variation in potential is very small, whereas at
the upper end there is no current, but the variations of potential are
very large. Accordingly, we say that at the upper end of the aerial
there is an antinode of potential and a node of current, and at the
bottom an antinode of current and a node of potential. By altering the
frequency of the electrical impulses we can create in the aerial an
arrangement of nodes of current or potential corresponding to the
overtones of a closed organ-pipe. But whatever may be the arrangement
the conditions must always hold that there is a node of current at
the upper end and an antidote of current at the lower end. In other
words, there are large variations of current at the place where the
aerial terminates on the spark-gap and no current at the upper end.
The first harmonic is formed where there is a node of potential at
one-third of the length of the aerial from the top. In this case we
have a node of potential not only at the lower end of the wire, but at
two-thirds of the way up. In the same way we can create in the closed
organ-pipe, by properly overblowing the pipe, a region about
two-thirds of the way up the pipe, where the pressure changes in the
air are practically no greater than they are at the mouthpiece. We can
make evident visually in a beautiful manner the existence of similar
stationary electrical waves in an aerial by means of an ingenious
arrangement devised by Dr. Georg Seibt, of Berlin. It consists of a
very long silk-covered copper wire, A (see Fig. 10), wound in a close
spiral of single layer round a wooden rod six feet long and about two
inches in diameter. This rod is insulated, and at the lower end the
wire is connected to a Leyden jar circuit, consisting of a Leyden jar
or jars and an inductance coil, L, the inductance of which can be
varied. Oscillations are set up in this jar circuit by means of an
induction-coil discharge, and the lower end of the long spiral wire is
attached to one point on the jar circuit. In this manner we can
communicate to the bottom end of the long spiral wire a series of
electric impulses, the time period of which depends upon the capacity
of the jar and the inductance of the discharge circuit. We can,
moreover, vary this frequency over wide limits. Parallel to the long
spiral wire is suspended another copper wire, E (see Fig. 10), and
between this wire and the silk-covered copper wire discharges take
place due to the potential difference between each part of the wire
and this long aerial wire. If we arrange matters so that the impulses
communicated to the bottom end of the long spiral wire correspond to
its fundamental note or periodic time, then in a darkened room we
shall see a luminous glow or discharge between the vertical wire and
the spiral wire, which increases in intensity all the way up to the
top of the spiral wire. The luminosity of this brush discharge at any
point is evidence of the potential of the spiral wire at that point,
and its distribution clearly demonstrates that the difference of
potential between the spiral wire and the aerial increases all the way
up from the bottom to the top of the spiral wire. In the next place,
by making a little adjustment and by varying the inductance of the jar
circuit, we can increase the frequency of the impulses which are
falling upon the spiral wire; and then it will be noticed that the
distribution of the brush discharge or luminosity is altered, and that
there is a maximum now at about one-third of the height of the spiral
wire, and a dark place at about two-thirds of the height, and another
bright place at the top, thus showing that we have a node of potential
at about two-thirds the way up the wire (see Fig. 11), and we have
therefore set up in the spiral wire electrical oscillations
corresponding to the first overtone. It is possible to show in the
same way the existence of the second harmonic in the coil, but the
luminosity then becomes too faint to be seen at a distance.
[Illustration: FIG. 11.--HARMONIC OSCILLATIONS IN LONG SOLENOID SHOWN
WITH SEIBT'S APPARATUS.]
An interesting form of aerial devised by Professor Slaby, of Berlin,
depends for its action entirely on the fact that the electrical
oscillations set up in it which radiate are harmonics of the
fundamental tone.
[Illustration: FIG. 12.--NON-RADIATIVE CLOSED LOOP AERIAL.]
[Illustration: FIG. 13.--SLABY'S LOOP RADIATOR.]
A closed vertical loop, A_{1}A_{2} (see Fig. 12), is formed by
erecting two parallel insulated wires vertically a few feet apart and
joining them together at the top. At the bottom these wires are
connected, with the secondary terminals of an induction coil, a
condenser, C, or Leyden jar, being bridged across the terminals and a
pair of spark balls, S, inserted in one side of the loop. It will
readily be seen that on setting the coil in action, oscillations will
take place in these vertical wires, but that if the oscillations are
simply the fundamental note of the system, then at any moment
corresponding to a current going up one side of the loop of wire there
must be a current coming down the other. Accordingly, an arrangement
of this kind, forming what is called a closed circuit, will not
radiate or radiates but very feebly. Professor Slaby found, however,
that it might be converted into a powerful radiator if we give the two
sides of the loop unequal capacity or inductance and at the same time
earth one of the lower ends of the loop, as shown in Fig. 13. By this
means it is possible to set up in the loop electrical overtones or
harmonics of the fundamental oscillation, and if we cause the system
to vibrate so as to produce its first odd harmonic, there is a
potential node at the lower end of both vertical sides of the loop, a
potential node on both vertical sides at two-thirds of the way up, and
a potential antinode at the summit of the loop; then, under these
circumstances, the closed loop of wire is in the same electrical
condition as if two simple Marconi aerials, both emitting their first
odd harmonic oscillation, were placed side by side and joined together
at the top.
It is a little difficult without the employment of mathematical
analysis to explain precisely the manner in which earthing one side of
the loop or making the loop unsymmetrical as regards inductance has
the effect of creating overtones in it. The following rough
illustration may, however, be of some assistance. Imagine a long
spiral metallic spring supported horizontally by threads. Let this
represent a conductor, and let any movement to or fro of a part of the
spring represent a current in that conductor. Suppose we take hold of
the spring at one end, we can move it bodily to and fro as a whole. In
this case, every part of the spring is moving one way or the other in
the same manner at the same time. This corresponds with the case in
which the discharge of the condenser through the uniform loop
conductor is a flow of electricity, all in one direction one way or
the other. The current is in the same direction in all parts of the
loop at the same time, and, therefore, if the current is going up one
side of the loop it is at the same time coming down the other side.
Hence the two sides of the loop are always in exact opposition as
regards the effect of the current in them on the external space, and
the loop does not radiate. Returning again to the case of the spring.
Supposing that we add a weight to one end of the spring by attaching
to it a metal ball, and then move the other end to and fro with
certain periodic motion, it will be found quite easy to set up in the
spring a pulsatory motion resembling the movement of the air in an
open organ-pipe. Under these circumstances both ends of the spring
will be moving inwards or outwards at the same time, and the central
portions of the spring, although being pressed and expanded slightly,
are moving to and fro very little. This corresponds in the case of the
looped aerial with a current flowing up or down both sides at the same
time; in other words, when this mode of electrical oscillation is
established in the loop, its electrical condition is just that of two
simple Marconi aerials joined together at the top and vibrating in
their fundamental manner. Accordingly, if one side of the double loop
is earthed, we then have an arrangement which radiates waves.
Professor Slaby found that by giving one side of the loop less
inductance than the other, and at the same time earthing the side
having greater inductance at the bottom, he was able to make an
arrangement which radiated, not in virtue of the normal oscillations
of the condenser, but in virtue of the harmonic oscillations set up in
the conductor itself. The mathematical theory of this radiator has
been very fully developed by Dr. Georg Seibt.
It will be seen, therefore, that there are several ways in which we
may start into existence oscillations in an aerial. First, the aerial
may be insulated, and we may charge it to a high potential and allow
this charge suddenly to rush out. Although this process gives rise to
a disturbance in the ether, as already explained, it is analogous to a
pop or explosion in the air, rather than to a sustained musical note.
The exact acoustic analogue would be obtained if we imagine a long
pipe pumped full of air and then suddenly opened at one end. The air
would rush out, and, communicating a blow to the outer air, would
create an atmospheric disturbance appreciated as a noise or small
explosion. This is what happens when we cut the string and let the
cork fly out from a bottle of champagne. At the same time, the
inertia of the air rushing out of the tube would cause it to overshoot
the mark, and a short time after opening the valve the tube, so far
from containing compressed air, would contain air slightly rarefied
near its mouth, and this rarefication would travel back up the tube in
the form of wave motion, and, being reflected as condensation at the
closed end, travel down again; and so after being reflected once or
twice at the open or closed end, become damped out very rapidly in
virtue of both air friction and the radiation of the energy. In the
case, however, of the ordinary organ-pipe, we do not depend merely
upon a store of compressed air put into the pipe, but we have a store
of energy to draw upon in the form of the large amount of compressed
air contained in a wind chest, which is being continually supplied by
the bellows. This store of compressed air is fed into the organ-pipe,
with the result that we obtain a continuous radiation of sound waves.
The first case, in which the only store of energy is the compressed
air originally contained in the pipe, illustrates the operation of the
simple Marconi aerial. The second case, in which there is a larger
store of energy to draw upon, the organ-pipe being connected to a wind
chest, illustrates the Marconi-Braun method, in which an aerial is
employed to radiate a store of electric energy contained in a
condenser, gradually liberated by the aerial in the form of a series
of electrical oscillations and waves. In this arrangement the
condenser corresponds to the wind chest, and it is continually kept
full of electrical energy by means of the induction coil or
transformer, which answers to the bellows of the organ. From the
condenser, electrical energy is discharged each time the spark
discharge passes at a spark gap in the form of electrical oscillations
set up in the primary circuit of an oscillation transformer. The
secondary circuit of this transformer is connected in between the
earth and the aerial, and therefore may be considered as part of it,
and, accordingly, the energy which is radiated from the aerial is not
simply that which is stored up in it in virtue of its own small
capacity, but that which is stored up in the much larger capacity
represented by the primary condenser or, as it may be called, the
electrical wind chest. By the second arrangement we have therefore the
means of radiating more or less continuous trains of electric waves,
corresponding with each spark discharge. To create powerful
oscillations in the aerial, one condition of success is that there
shall be an identity in time-period between the circuit of the aerial
and that of the primary condenser. The aerial is an open circuit which
has capacity with respect to the earth, and it has also inductance,
partly due to the wire of the aerial and partly due to the secondary
circuit of the oscillation transformer in series with it. The primary
circuit or spark circuit has capacity--viz., the capacity of the
energy-storing condenser--and it has also inductance--viz., the
inductance of the primary circuit of the oscillation transformer. We
shall consider at a later stage more particularly the details of
syntonising arrangements, but meanwhile it may be said that one
condition for setting up powerful waves by means of the above
arrangement is that the electrical time-period of both the two
circuits mentioned shall be the same. This involves adjusting the
inductance and capacity so that the product of conductance and
capacity for each of these two circuits is numerically the same.
Instead of employing an oscillation transformer between the condenser
circuit and the aerial, the aerial may be connected directly to some
point on the condenser circuit at which the potential oscillations are
large, and we have then another arrangement devised by Professor Braun
(see Fig. 14). In this case, in order to accumulate large potential
oscillations at the top of the aerial, it is, as we have seen,
necessary that the length of the aerial shall be one quarter the
length of the wave. If, therefore, the electrical oscillations in the
condenser circuit are at the rate of N per second, in other words,
have a frequency N, the wave-length correponding to this frequency is
given by the expression,
3x10^{10}/N cms.
The number 3x10^{10} is the value in centimetres per second of the
velocity of the electromagnetic wave, and is identical with that of
light. The corresponding resonant length of the aerial is therefore
one-fourth of this wave-length, or 3x10^{10}/4N. Generally speaking,
however, it will be found that with any length of aerial which is
practicable, say, 200 feet or 6,000 cms., this proportion necessitates
rather a high frequency in the primary oscillation circuit. In the
case considered--viz., for an aerial 200 feet in height--the
oscillations in the primary circuit must have a frequency of one and a
quarter million. This high frequency can only be obtained either by
greatly reducing the inductance of the primary discharge circuit, or
reducing the capacity. If we reduce the capacity, we thereby greatly
reduce the storage of energy, and it is not practicable to reduce the
inductance below a certain amount.
[Illustration: FIG. 14.--BRAUN'S RADIATOR. B, battery; I, induction
coil; K, key; S, spark-gap; L, inductance coil; C, condenser; A,
aerial.]
Summing up, it may be said that there are three, and, as far as the
writer is aware, at present only three, modes of exciting the
electrical oscillations in an aerial wire. First, the aerial may
itself be used as an electrical reservoir and charged to a high
potential and suddenly discharged to the earth. This is the original
Marconi method. The second method, due to Braun, consist of attaching
the aerial to some point on an oscillation circuit consisting of a
condenser, an inductance coil and a spark gap, in series with one
another, and charging and discharging the condenser across the spark
gap so as to create alterations of potential at some point on the
oscillation circuit. The length of the aerial must then be so
proportioned as above described that it is resonant to this frequency.
Thirdly, we may employ the arrangement involving an oscillation
transformer, in which the oscillations in the primary condenser
circuit are made to induce others in the aerial circuit, the
time-period of the two circuits being the same. This method may be
called the Braun-Marconi method. Professor Slaby has combined together
in a certain way the original Marconi simple aerial with the resonant
quarter-wave-length wire of Braun. He constructs what he calls a
_multiplicator_, which is really a wire wound into a loose spiral
connected at one point to an oscillation circuit consisting of a
condenser inductance, the length of this wire being proportioned so
that there is a great resonance or multiplication of tension or
potential at its free end. This free end is then attached to the lower
end of an ordinary Marconi aerial, and serves to charge it with a
higher potential than could be obtained by the use of the induction
coil directly attached to it.
* * * * *
We have next to consider the appliances for creating the necessary
charging electromotive force, and for storing and releasing this
charge at pleasure, so as to generate the required electrical
oscillations in the aerial.
It is essential that this generator should be able to create not only
large potential difference, but also a certain minimum electric
current. Accordingly, we are limited at the present moment to one of
two appliances--viz., the induction coil or the alternating current
transformer.
It will not be necessary to enter into an explanation of the action of
the induction coil. The coil generally employed for wireless
telegraphy is technically known as a ten-inch coil--_i.e._, a coil
which is capable of giving a ten-inch spark between pointed conductors
in air at ordinary pressure. The construction of a large coil of this
description is a matter requiring great technical skill, and is not to
be attempted without considerable previous experience in the
manufacture of smaller coils. The secondary circuit of a ten-inch coil
is formed of double silk-covered copper wire; generally speaking, the
gauge called No. 36, or else No. 34 S.W.G. is used, and a length of
ten to seventeen miles of wire is employed on the secondary circuit,
according to the gauge of wire selected. For the precautions necessary
in constructing the secondary coil, practical manuals must be
consulted.[8]
Very great care is required in the insulation of the secondary circuit
of an induction coil to be used in Hertzian wave telegraphy, because
the secondary circuit is then subjected to impulsive electromotive
forces lasting for a short time, having a much higher electromotive
force than that which the coil itself normally produces.
The primary circuit of a ten-inch coil generally consists of a length
of 300 or 400 feet of thick insulated copper wire. In such a coil the
secondary circuit would require about ten miles of No. 34 H.C. copper
wire, making 50,000 turns round the core. It would have a resistance
at ordinary temperatures of 6,600 ohms, and an inductance of 460
henrys. The primary circuit, if formed of 360 turns of No. 12 H.C.
copper wire, would have a resistance of 0.36 of an ohm, and an
inductance of 0.02 of a henry.
An important matter in connection with an induction coil to be used
for wireless telegraphy is the resistance of the secondary circuit.
The purpose for which we employ the coil is to charge a condenser of
some kind. If a constant electromotive force (V) is applied to the
terminals of a condenser having a capacity C, then the difference of
potential (_v_) of the terminals of the condenser at any time that the
contact is made is given by the expression:
v = V(1 - e^{-t/RC}).
In the above equation, the letter e stands for the number 2.71828, the
base of the Napierian logarithms, and R is the resistance in series
with the condenser, of which the capacity is C, to which the
electromotive force is applied. This equation can easily be deduced
from first principles,[9] and it shows that the potential difference
_v_ of the terminals of the condenser does not instantly attain a
value equal to the impressed electromotive force V, but rises up
gradually. Thus, for instance, suppose that a condenser of one
microfarad is being charged through a resistance of one megohm by an
impressed voltage of 100 volts, the equation shows that at the end of
the first second after contact, the terminal potential difference of
the condenser will be only 63 volts, at the end of the second second,
86 volts, and so on.
Since _e_^{-10} is an exceedingly small number, it follows that in 10
seconds the condenser would be practically charged with a voltage
equal to 100 volts. The product CR in the above equation is called the
_time-constant_ of the condenser, and we may say that the condenser is
practically charged after an interval of time equal to ten times the
time-constant, counting from the moment of first contact between the
condenser and the source of constant voltage. The time-constant is to
be reckoned as the product of the capacity (C) in microfarads, by the
resistance of the charging circuit (R) in megohms. To take another
illustration. Supposing we are charging a condenser having a capacity
of one-hundreth of a microfarad, through a resistance of ten thousand
ohms. Since ten thousand ohms is equal to one-hundredth of a megohm,
the time-constant would be equal to one-ten-thousandth of a second,
and ten times this time-constant would be equal to a thousandth of a
second. Hence, in order to charge the above capacity through the above
resistance, it is necessary that the contact between the source of
voltage and the condenser should be maintained for at least
one-thousandth part of a second.
In discussing the methods of interrupting the circuit, we shall return
to this matter, but, meanwhile, it may be said that in order to secure
a small time-constant for the charging circuit, it is desirable that
the secondary circuit of the induction coil should have as low a
resistance as possible. This, of course, involves winding the
secondary circuit with a rather thick wire. If, however, we employ a
wire larger in size than No. 34, or at the most No. 32, the bulk and
the cost of the induction coil began to rise very rapidly. Hence, as
in all other departments of electrical construction, the details of
the design are more or less a matter of compromise. Generally
speaking, however, it may be said that the larger the capacity which
is to be charged, the lower should be the resistance of the secondary
circuit of the induction coil.
In the practical construction of induction coils for wireless
telegraphy, manufacturers have departed from the stock designs. We are
all familiar with the appearance of the instrument maker's induction
coil; its polished mahogany base, its lacquered brass fittings, and
its secondary bobbin constructed of and covered with ebonite. But such
a coil, although it may look very pretty on the lecture table, is yet
very unsuited to positions in which it may be used in connection with
Hertzian wave telegraphy.
Three important adjuncts of the induction coil are the primary
condenser, the interrupter and the primary key. The interrupter is the
arrangement for intermitting the primary current. We have in some way
or other to rapidly interrupt the primary current, and the torrent of
sparks that then appears between the secondary terminals of the coil
is due to the electromotive force set up in the secondary circuit at
each break or interruption of the primary circuit. We may divide
interrupters into five classes.
We have first the well-known hammer interrupter which Continental
writers generally attribute to Neef or Wagner.[10] In this
interrupter, the magnetisation of the iron core of the coil is caused
to attract a soft-iron block fixed at the top of a brass spring, and
by so doing to interrupt the primary circuit between two platinum
contacts. Mr. Apps, of London, added an arrangement for pressing back
the spring against the back contact, and the form of hammer that is
now generally employed is therefore called an Apps break.
As the ten-inch coil takes a primary current of ten amperes at sixteen
volts when in operation, it requires very substantial platinum
contacts to withstand the interruption of this current continuously
without damage. The small platinum contacts that are generally put on
these coils by instrument makers are very soon worn out in practical
wireless telegraph work. If a hammer break is used at all, it is
essential to make the contacts of very stout pieces of platinum, and
from time to time, as they get burnt away or roughened, they must be
smoothed up with a fine file. It does not require much skill to keep
the hammer contacts in good order and prevent them from sticking
together and becoming damaged by the break spark.
By regulating the pressure of the spring against the back contact, by
means of an adjusting screw, the rate at which the break vibrates can
be regulated, but as a rule it is not possible, with a hammer break,
to obtain more than about 800 interruptions per minute, or, say,
twelve a second. The hammer break is usually operated by the magnetism
of the iron core of the coil, but for some reasons it is better to
separate the break from the coil altogether, and to work it by an
independent electromagnet, which, however, may be excited by a current
from the same battery supplying the induction coil. For coils up to
the ten-inch size the hammer break can be used when very rapid
interruptions are not required. It is not in general practicable to
work coils larger than the ten-inch size with a platinum contact
hammer break, as such a butt contact becomes overheated and sticks if
more than ten amperes is passed through it. In the case of larger
coils, we have to employ some form of interrupter in which mercury or
a conducting liquid forms one of the contact surfaces.
The next class of interrupter is the vibrating or hand-worked mercury
break, in which a platinum or steel pin is made to vibrate in and out
of mercury. This movement may be effected by the attraction of an iron
armature by an electromagnet, or by the varying magnetism of the core
of the coil, or it may be effected more slowly by hand.
The mercury surface must be covered with water, alcohol, paraffin or
creosote oil to prevent oxidation and to extinguish the break spark.
The interruption of the primary current obtained by the mercury break
is more sudden than that obtained by the platinum contact in air, in
consequence of the more rapid extinction of the spark; hence the
sparks obtained from coils fitted with mercury interrupters are
generally from twenty to thirty per cent. longer than those obtained
from the same coil under the same conditions, with platinum contact
interrupters. The mercury breaks will not, however, work well unless
cleaned at regular intervals by emptying off the oil and rinsing well
with clean water, and hence they require rather more attention than
platinum interrupters. It is not generally possible to obtain so many
interruptions per minute with the simple vibrating mercury interrupter
as with the ordinary hammer interrupter. The mercury interrupter has,
however, the advantage that the contact time during which the circuit
is kept closed may be made longer than is the case with the hammer
break. Also, if fresh water is allowed to flow continuously over the
mercury surface, it can be kept clean, and the break will then operate
for considerable periods of time without attention. The mercury
interrupter may be worked by a separate electromagnet or by the
magnetism of the core of the induction coil.
The third class of interrupter may be called the motor interrupter, of
which a large number have been invented in recent years. In this
interrupter some form of a continuously-rotating electromotor is
employed to make and break a mercury or other liquid contact. In one
simple form the motor shaft carries an eccentric, which simply dips a
platinum point into mercury, or else a platinum horseshoe into two
mercury surfaces, making in this manner an interruption of the primary
circuit at one or two places. As a small motor can easily be run at
twelve hundred revolutions per minute, or twenty per second, it is
possible to secure easily in this manner a uniform rate of
interruption of the primary current at the rate of about twenty per
second. If, however, much higher speeds are employed, then the time of
contact becomes abbreviated, and the ability of the coil to charge a
capacity is diminished.
Professor J. Trowbridge has described an effective form of motor break
for large coils, in which the interruption is caused by withdrawing a
stout platinum wire from a dilute solution of sulphuric acid, and by
this means he increased the spark given by a coil provided with hammer
break and condenser from fifteen inches to thirty inches when using
the liquid break and no condenser.[11]
A good form of motor-interrupter, due to Dr. Mackenzie Davidson,
consists of a slate disc bearing pin contacts fixed on the prolonged
steel axle of a motor placed in an inclined position; the disc and the
lower part of the axle lie in a vessel filled one-third with mercury
and two-thirds with paraffin oil. The circuit is made and broken by
the revolution of the disc causing the pins to enter and leave the
mercury. The speed of the motor can be regulated by a small
resistance, and can be adapted to the electromotive force used in the
primary circuit. When the motor is running slowly the interrupter can
be used with a low electromotive force, that is to say, something
between twelve and twenty volts, but with a higher speed a large
electromotive force can be used without danger of overheating the
primary coil, and with an electromotive force of about fifty volts,
the interruptions may be so rapid that an unbroken arc of flame,
resembling an alternating-current arc, springs between the secondary
terminals of the coil.
Mr. Tesla has devised numerous forms of rotating mercury break. In
one, a star-shaped metal disc revolves in a box so that its points dip
into mercury covered with oil, and make and break contact. In another
form, a jet of mercury plays against a similar shaped rotating wheel.
For details, the reader must consult the fuller descriptions in _The
Electrical World_ of New York, Vol. XXXII., p. 111, 1898; also Vol.
XXXIII., p. 247; or _Science Abstracts_, Vol. II., pp. 46 and 47,
1898.
The fourth class of interrupter is called a turbine interrupter. In
this appliance, a jet of mercury is forced out of a small aperture by
means of a centrifugal pump, and is made to squirt against a metal
plate, and interrupted intermittently by a toothed wheel made of
insulating material rotated by the motor which drives the pump. The
current supplying the coil passes through or along this jet of
mercury, and is therefore rendered intermittent when the wheel
revolves. In the case of this interrupter, the duration of the
contacts, as well as the number of interruptions per second, is under
control, and for this reason better results are probably obtained with
it than with any other form of break.
A description of a turbine mercury break devised by M. Max Levy was
given in the _Elektrotechnische Zeitschrift_, Vol. XX., p. 717,
October 12, 1899 (see also _Science Abstracts_, Vol. III., p. 63,
abstract No. 165) as follows:--
A toothed wheel made of insulating material carries from 6 to 24
teeth, and can be made to rotate from 300 to 1,000 times per minute,
the interruptions being thus regulated between 5 and 400 per second.
By raising or lowering the position of the jet of mercury and that of
the plate against which it strikes, the duration of the contact can
be varied, so that it is possible to regulate this period without
disturbing the number of interruptions per second.
The sparks obtained from a coil worked with a turbine interrupter have
more quantity than the sparks obtained with any other interrupter
under similar conditions, and the coil can be worked with a far higher
voltage than is possible when using the hammer break. In this manner,
the appearance of the secondary sparks can be varied from the thin
snappy sparks given by the hammer break to the thick flame-like arc
sparks given by the electrolytic break. This break can be adapted for
any voltage from twelve to two hundred and fifty volts, and the
primary circuit cannot be closed before the interrupter is acting. The
mercury in the break is generally covered with alcohol or paraffin oil
to reduce oxidation, and the appliance is nearly noiseless when in
operation. The mercury has to be cleaned at intervals, if the
interrupter is much used. If alcohol is used to cover the mercury, the
cleaning is very simple; the break requires only to be rinsed under a
water tap. When paraffin oil is used, the cleaning is generally
effected with the help of a few ounces of sulphuric acid in a very few
minutes. It is best, however, to clean the mercury continuously by
allowing the water to flow over it.
The motor driving the centrifugal pump and the fan can be wound for
any voltage, and it is best to have it so arranged that this motor
works on the same battery which supplies the primary circuit of the
coil, the two circuits working parallel together. A rheostat can be
added to the motor circuit to regulate the speed.
The turbine break driven by an independent motor, which is kept always
running, has another advantage over the hammer break in practical
wireless telegraphy, viz., that a useful secondary spark can be
secured with a shorter time of closure of the primary circuit, since
there is no inertia to overcome as in the case of the hammer break.
This latter form has only continued in use because of its simplicity
and ease of management by ordinary operators.
The mercury turbine interrupter has been extensively adopted both in
the German and British navies in connection with induction coils used
for wireless telegraphy.
Lastly we have the electrolytic interrupters, the first of which was
introduced by Dr. Wehnelt, of Charlottenburg, in the year 1899, and
modified by subsequent inventors. In its original form, a glass vessel
filled with dilute sulphuric acid (one of acid to five or else ten
parts of water) contains two electrodes of very different sizes; one
is a large lead electrode formed of a piece of sheet lead laid round
the interior of the vessel, and the other is a short piece of platinum
wire projecting from the end of a glass or porcelain tube. The smaller
of these electrodes is made the positive, and the large one the
negative. If this electrolytic cell is connected in series with the
primary circuit of the induction coil (the condenser being cut out)
and supplied with an electromotive force from forty to eighty volts,
an electrolytic action takes place which interrupts the current
periodically.[12] An enormous number of interruptions can, by suitable
adjustment, be produced per second, and the appearance of a discharge
from the secondary terminals of the coil, while using the Wehnelt
break, more resembles an alternate-current arc than the usual
disruptive spark.
At the time when the Wehnelt break was first introduced, great
interest was excited in it, and the technical journals in 1899 were
full of discussions as to the theory of its operation.[13] The general
facts concerning the Wehnelt break are that the electrolyte must be
dilute sulphuric acid in the proportion of one of acid to five or ten
of water. The large lead plate must be the cathode or negative pole,
and the anode or positive pole must be a platinum wire, about a
millimetre in diameter, and projecting one or two millimetres from the
pointed end of a porcelain, glass or other acid-proof insulating tube.
The aperture through which the platinum wire works must be so tight
that acid cannot enter, yet it is desirable that the platinum wire
should be capable of being projected more or less from the aperture by
means of an adjusting screw. The glass vessel which contains these two
electrodes should be of considerable size, holding, say, a quart of
fluid, and it is better to include this vessel in a larger one in
which water can be placed to cool the electrolyte, as the latter gets
very warm when the break is used continuously. If such an electrolytic
cell has a continuous electromotive force applied to it tending to
force a current through the electrolyte from the platinum wire to the
lead plate, we can distinguish three stages in its operation, which
are determined by the electromotive force and the inductance in the
circuit. First, if the electromotive force is below sixteen or twenty
volts, then ordinary and silent electrolysis of the liquid proceeds,
bubbles of oxygen being liberated from the platinum wire and hydrogen
set free against the lead plate. If the electromotive force is raised
above twenty-five volts, then if there is no inductance in the
circuit, the continuous flow of current proceeds, but if the circuit
of the electrolyte possesses a certain minimum inductance, the
character of the current flow changes, and it becomes intermittent,
and the cell acts as an interrupter, the current being interrupted
from 100 to 2,000 times per second, according to the electromotive
force and the inductance of the circuit. Under these conditions, the
cell produces a rattling noise and a luminous glow appears round the
tip of the platinum wire. Thus, in a particular case, with an
inductance of 0.004 millihenry in the circuit of a Wehnelt break, no
interruption of the circuit took place, but with one millihenry of
inductance in the circuit, and with an electromotive force of 48
volts, the current became intermittent at the rate of 930 per second,
and by increasing the voltage to 120 volts, the intermittency rose to
1,850 a second.
The Wehnelt break acts best as an interrupter with an electromotive
force from 40 to 80 volts. At higher voltages a third stage sets in:
the luminous glow round the platinum wire disappears, and it becomes
surrounded with a layer of vapour, as observed by MM. Violle and
Chassagny; the interruptions of current cease, and the platinum wire
becomes red hot. If there is no inductance in the circuit, the
interrupter stage never sets in at all, but the first stage passes
directly into the third stage. In the first stage bubbles of oxygen
rise steadily from the platinum wire, and in the interrupted stage
they rise at longer intervals, but regularly. The cell will not,
however, act as a break at all unless some inductance exists in the
circuit.
In applying the Wehnelt break to an induction coil, the condenser is
discarded and also the ordinary hammer break, and the Wehnelt break is
placed in circuit with the primary coil. In some cases, the inductance
of the primary coil alone is sufficient to start the break in
operation, but with voltages above 50 or 60, it is generally necessary
to supplement the inductance of the primary coil by another inductive
coil. The best form of Wehnelt break for operating induction coils is
the one with multiple anodes (see Dr. Marchant, _The Electrician_,
Vol. XLII., p. 841, 1899), and when it has to be used for long
periods, the cathode may advantageously be formed of a spiral of lead
pipe, through which cold water is made to circulate.
Another form of electrolytic break was introduced by Mr. Caldwell. In
this, a vessel containing dilute sulphuric acid is divided into two
parts. In the partition is a small hole, and in the two compartments
are electrodes of sheet lead. The small hole causes an intermittency
in the current which converts the arrangement into a break. Mr.
Campbell Swinton modified the above arrangement by making the
partition to consist of a sort of porcelain test-tube with a hole in
the bottom. This hole can be more or less plugged up by a glass rod
drawn out to a point, and this is used to more or less close the hole.
This porcelain vessel contains dilute acid and stands in a larger
vessel of acid, and lead electrodes are placed in both compartments.
The current and intermittency can be regulated by more or less closing
the aperture between the two regions.
When the Wehnelt break is applied to an ordinary ten-inch induction
coil, and the inductance of the primary circuit and the electromotive
force varied until the break interrupts the current regularly and with
the frequency of some hundred a second, the character of the secondary
discharge is entirely different from its appearance with the ordinary
hammer break. The thin blue lightning-like sparks are then replaced by a
thicker mobile flaming discharge, which resembles an alternating-current
arc, and, when carefully examined or photographed, is found to consist
of a number of separate discharges superimposed upon one another in
slightly different positions.
Many theories have been adopted as to the action of the break, but
time will not permit us to examine these. Professor S. P. Thompson and
Dr. Marchant have suggested a theory of resonance.[14] One difficulty
in explaining the action of the break is created by the fact that it
will not work if the platinum wire is made a cathode.
Although the Wehnelt break has some advantages in connection with the
use of the induction coil for Roentgen ray work, its utility as far as
regards Hertzian wave telegraphy is not by any means so marked. It has
already been explained that, in order to charge a condenser of a given
capacity at a constant voltage, the electromotive force must be
applied for a certain minimum time, which is determined by the value
of the capacity and the resistance of the secondary circuit of the
induction coil. If the coil is a ten-inch coil and has a secondary
resistance of, say, 6,000 ohms, and if the capacity to be charged has
a value, say, of one-thirtieth of a microfarad, then the time-constant
of the circuit is 1/5,000 of a second. Therefore, the contact with the
condenser must be maintained for at least 1/500 of a second, during
the time that the secondary electromotive force of the coil is at its
maximum, so that the condenser may become charged to a voltage which
the coil is then capable of producing.
In the induction coil, the electromotive force generated in the
secondary coil at the "break" of the primary current is higher than
that at the "make," and this electromotive force, other things being
equal, depends upon the rate at which the magnetism of the iron core
dies away, and its duration is shorter in proportion as the whole time
occupied in the disappearance of the magnetism is less. The Wehnelt
break does not increase the actual secondary electromotive force, nor
apparently its duration, but it greatly increases the number of times
per second this electromotive force makes it appearance. Hence this
break increases the current, but not the electromotive force in the
secondary coil. It, therefore, does not assist us in the direction
required--viz., in prolonging the duration of the secondary
electromotive force to enable larger capacities to be charged.
The important point in connection with the working of a coil used for
charging a condenser is not the length of spark which the coil can
give alone, but the length of spark which can be obtained between
small balls attached to the secondary terminals, when these terminals
are also connected to the two surfaces of the condenser. Thus, a coil
may give a ten-inch spark if worked alone, but on a capacity of
one-thirtieth of a microfarad it may not be able to give more than a
five-millimetre spark. Hence, in describing the value of a coil for
wireless telegraph purposes, it is not the least use to state the
length of spark which the coil will give between the pointed
conductors in air, but we must know the spark length which it will
give between brass balls, say, 1 centimetre in diameter, connected to
the secondary terminals, when these terminals are also short-circuited
by a stated capacity, the spark not exceeding that length at which it
becomes non-oscillatory.
A good way of describing the value of an induction coil for wireless
telegraph purposes is to state the length of oscillatory spark which
can be produced between balls one centimetre in diameter connected to
the secondary terminals, when these balls are short-circuited by a
condenser having a capacity, say, of one-hundredth of a microfarad,
and also one-tenth of a microfarad.
If a hammer or motor interrupter is employed with the coil, then a
primary condenser must be connected across the points between which
the primary circuit is broken. This condenser generally consists of
sheets of tinfoil alternated with sheets of paraffin paper, and for a
ten-inch coil may have a capacity of about 0.4 or 0.5 of a
microfarad.[15]
Lord Rayleigh discovered that if the interruption of the primary
circuit is sufficiently sudden and complete, as when the primary
circuit is severed by a bullet from a gun, the primary condenser can
be removed and yet the sparks obtained from the secondary circuit are
actually longer than those obtained with the condenser and the
ordinary break.[16]
In the use, however, of the coil for Hertzian wave telegraphy, with
all interrupters except the Wehnelt break a condenser of suitable
capacity must be joined across the break points.
Turning in the next place to the primary key, or signalling
interrupter, it is necessary to be able to control the torrent of
sparks between the secondary terminals of the coil, and to cut them up
into long and short periods in accordance with the letters of the
Morse alphabet. This is done by means of the primary key. The primary
key generally consists of an ordinary massive single contact key with
heavy platinum contacts. As the current to be interrupted amounts to
about ten amperes and is flowing in a highly inductive circuit, the
spark at break is considerable. If the attempt is made to extinguish
this spark by making the contacts move rapidly away from one another
through a long distance, in other words, by using a key with a wide
movement, then the speed at which the signals can be set is greatly
diminished. The speed of sending greatly depends upon the time taken
to move the key up and down between sending two dots, and hence a
short range key sends quicker than a long range key. If it is desired
to use a short range key, then some method must be employed to
extinguish the spark at the contacts. This is done in one of three
ways: Either by using a high resistance coil to short-circuit these
contacts, or by a condenser, or by a magnetic blow-out, as in the case
of an electric tramcar circuit controller. Of these, the magnetic
blow-out is probably the best.
Mr. Marconi has designed a signalling key which performs the function
not only of interrupting the primary circuit, but at the same time
breaks connection between the receiving appliance and the aerial.
The author has designed for signalling purposes a multiple contact key
which interrupts the circuit simultaneously in ten or twelve different
places. The particular point about this break is the means which are
taken to make the twelve interruptions absolutely simultaneous. If
these interruptions are not simultaneous, the spark always takes place
at the contact which is broken first, but if the circuit is
interrupted in a dozen places quite simultaneously, then the spark is
cut up into a dozen different portions, and the spark at each contact
is very much diminished. By this break, voltages up to two thousand
volts may be quite easily dealt with.
Various forms of break have been devised in which the circuit is
broken under oil or insulating fluids, but, generally speaking, these
devices are not very portable, and a dry contact between platinum
surfaces with appropriate means for cutting up the spark and blowing
it out so that the mechanical movement of the switch may be small is
the best thing to use.
The signalling key is really a very important part of the transmitting
arrangement, because whatever may be the improvements in receiving
instruments, it is not possible to receive faster than we can send. A
great many statements have appeared in the daily papers as to the
possibility of receiving hundreds of words a minute by Hertzian wave
telegraphy, but the fact remains that whatever may be the sensibility
of the receiving appliance, the rate at which telegraphy of any kind
can be conducted is essentially dependent upon the rate at which the
signals can be sent, and this in turn is largely dependent upon the
mechanical movement which the key has to make to interrupt the primary
circuit, and so interrupt the secondary discharge.
In order to make the separation of the contact points of the switch as
small as possible, and yet prevent an arc being established, various
blow-out devices have been employed. The simplest arrangement for this
purpose is a powerful permanent magnet so placed that its inter-polar
field embraces the contact points and is at right angles to them.
As already explained, the applicability of the induction coil in
wireless telegraphy is limited by the fact of the high resistance of
the secondary circuit and the small current that can be supplied from
it. Data are yet wanting to show what is the precise efficiency of the
induction coil, as used in Hertzian wave telegraphy, but there are
reasons for believing that it does not exceed 50 or 60 per cent.
Where large condensers have to be charged--in other words, where we
have to deal with larger powers--we are obliged to discard the
induction coil and to employ the alternating-current transformer. But
this introduces us to a new class of difficulties. If an
alternating-current transformer wound for a secondary voltage, say, of
20,000 or 30,000 volts, has its primary circuit connected to an
alternator, then if the secondary terminals, to which are connected
two spark balls, are gradually brought within striking distance of one
another, the moment we do this an alternating-current arc starts
between these balls. If the transformer is a small one, there is no
difficulty in extinguishing this arc by withdrawing the secondary
terminals, but if the transformer is a large one, say, of ten or
twenty kilowatts, dangerous effects are apt to ensue when such an
experiment is tried. The short circuiting of the secondary circuit
almost entirely annuls the inductance of the primary circuit. There
is, therefore, a rush of current into the transformer, and if it is
connected to an alternator of low armature resistance the fuses are
generally blown and other damage done.
Let us supppse, then, that the secondary terminals of the transformer
are also connected to a condenser. On bringing together the spark
balls connected with the secondary terminals we may have one or more
oscillatory discharges, but the process will not be continuous,
because the moment that the alternating-current arc starts between the
spark balls it reduces their difference of potential to a
comparatively low value, and hence the charge taken by the condenser
is very small, and, moreover, the circuit is not interrupted
periodically so as to re-start a train of oscillations.
When, therefore, we desire to employ an alternating-current
transformer as a source of electromotive force, although it may have
the advantage that the resistance of the secondary circuit of the
transformer is generally small compared with that of the secondary
circuit of an induction coil, yet, nevertheless, we are confronted
with two practical difficulties: (1) How to control the primary
current flowing into the transformer, and (2) how to destroy the
alternating-current arc between the spark balls and reduce the
discharge entirely to the disruptive or oscillatory discharge of the
condenser.
The control over the current can be obtained, in accordance with a
plan suggested by the author, by inserting in the primary circuit of
the transformer two variable choking coils. The form in which it is
preferred to construct these is that of a cylindrical bobbin standing
upon a laminated cross-piece of iron. These bobbins can have let down
into them an =E=-shaped piece of laminated iron, so as to complete the
magnetic circuit, and thus raise the inductance of the bobbin. By
placing two of these variable choking coils in series with the primary
circuit, the current is under perfect control. We can fix a minimum
value below which the current shall not fall, by adjusting the
position of the cores of these two choking coils, and we can then
cause that current to be increased up to a certain limit which it
cannot exceed, by short-circuiting one of these choking coils by an
appropriate switch. Several ways have been suggested for extinguishing
the alternating current arc which forms between the spark balls
connected to the secondary terminals when these are brought within a
certain distance of one another. One of these is due to Mr. Tesla. He
places a strong electromagnet so that its lines of magnetic flux pass
transversely between the spark balls. When the discharge takes place
the electric arc is blown out, but if the balls are short-circuited by
a condenser the oscillatory discharge of the condenser still takes
place across the spark gap. Professor Elihu Thomson achieves the same
result by employing a blast of air thrown on the spark gap. This has
the effect of destroying the alternating-current arc, but still leaves
the oscillating discharge of the condenser. The action is somewhat
tedious to explain in words, but it can easily be understood that the
blast of air, by continually breaking down the alternating-current arc
which tends to form, allows the condenser connected to the spark balls
to become charged with the potential of the secondary circuit of the
transformer, and that this condenser then discharges across the spark
gap, producing an oscillatory discharge in the usual manner. The
author has found that, without the use of any air blast or
electromagnet, simple adjustment of the double choking coil in the
primary circuit of the transformer, as above described, is sufficient
to bring about the desired result, when the capacity of the condenser
is adjusted to be in resonance.
Another method, which has been adopted by M. d'Arsonval, is to cause
the spark to pass between two balls placed at the extremities of
metal rods, which are in rapid rotation like the spokes of a wheel. In
this case, the draught of air produced by the passage of the spark
balls blows out the arc and performs the same function as the blast of
air in Professor Elihu Thomson's method. When these adjustments are
properly made, it is possible, by means of a condenser and an
alternating-current transformer supplied with current from an
alternator, to create a rapidly intermittent oscillatory discharge,
the sparks of which succeed one another so quickly that it appears
almost continuous. When using a large transformer and condenser, the
noise and brilliancy of these sparks are almost unbearable, and the
eyes may be injured by looking at this spark for more than a moment.
In the construction of transformers intended to be used in this
manner, very special precautions have to be taken in the insulation of
the primary and secondary circuits, and the insulation of these from
the core.
It may be remarked in passing that experimenting with large
high-tension transformers coupled to condensers of large capacity is
exceedingly dangerous work, and the greatest precautions are necessary
to avoid accident. In the light, however, of sufficient experience
there is no difficulty in employing high-tension transformers in the
above-described manner, and in obtaining electromotive forces of
upwards of a hundred thousand volts supplied through transformers
capable of yielding any required amount of current.
On occasions where continuous current alone is available, a motor
generator has to be employed converting the continuous current into an
alternating current. This is best achieved by the employment of a
small alternator directly coupled to a continuous-current motor; or by
providing the shaft of a continuous-current motor with two rings
connected to two opposite portions of its armature, so that when
continuous current is supplied to the brushes pressing against the
commutator, an alternating current can be drawn off from two other
brushes touching the above-mentioned insulated rings.
The next element of importance in the transmitting arrangement is the
spark gap. In the case of those transmitters employing an ordinary
induction coil, the secondary spark, or the discharge of any condenser
connected to the secondary terminals can be taken between the brass
balls about half an inch or one inch in diameter, with which the
terminals of the secondary coil are usually furnished; and it is
generally the custom to allow this spark discharge to take place in
air at ordinary pressure. In the very early days of his work Mr.
Marconi adopted the discharger devised by Professor Rhigi, in which
the spark takes place between two brass balls placed in vaseline or
other highly insulating oil.[17] But whatever advantage may accrue
from using oil as the dielectric in which the spark discharge takes
place, when carrying out simple laboratory experiments on Hertzian
waves, there is no advantage in the case of wireless telegraphy. The
Rhigi discharger was, therefore, soon discarded. If discharges having
large quantity are passed through oil, it is rapidly decomposed or
charred, and ceases to retain the special insulating and
self-restoring character which is necessary in the medium in which an
oscillating spark is formed. The conditions when the discharges of
large condensers are passed between spark balls are entirely different
from those when the quantity of the spark, or to put it in more exact
language, the current passing, is very small. In the case of Hertzian
experiments it is necessary, as shown by Hertz, to maintain a high
state of polish on the spark balls when they are employed for the
production of short waves of small energy, but when we are dealing
with large quantities of energy at each discharge, those methods which
succeed for laboratory experiments are perfectly impracticable. The
conditions necessary to be fulfilled by a discharger for use in
Hertzian wave telegraphy are that the surfaces shall maintain a
constant condition and not be fused or eaten away by the spark, and,
next, that the medium in which the discharge takes place shall not be
decomposed by the passage of the spark, but shall maintain the
property of giving way suddenly when a certain critical pressure is
reached, and passing instantly from a condition in which it is a very
perfect insulator to one in which it is a very good conductor; and,
thirdly, that on the cessation of the discharge, the medium shall
immediately restore itself to its original condition.
When using the ordinary ten-inch induction coil, and when the capacity
charged by it does not exceed a small fraction of a microfarad, it is
quite sufficient to employ brass or steel balls separated by a certain
distance in air, at the ordinary pressure, as the arrangement of the
discharger. When, however, we come to deal with the discharges of very
large condensers, at high electromotive forces, then it is necessary
to have special arrangements to prevent the destruction of the
surfaces between which the spark passes, or their continual
alteration, and many devices have been invented for this purpose. The
author has devised an arrangement which fulfils the above conditions
very perfectly for use in large power stations, but the details of
this cannot be made public at the present time.
* * * * *
We have to consider in connection with this part of the subject the
dielectric strength of air under different pressures and for different
thicknesses. It was shown by Lord Kelvin, in 1860, that the dielectric
strength of very thin layers of air is greater than that of thick
layers.[18] The electric force, reckoned in volts per centimetre,
required to pierce a thickness of air from two to ten millimetres in
thickness, at atmospheric pressure, may be taken at 30,000 volts per
centimetre. The same force in electrostatic units is represented by
the number 100, since a gradient of 300 volts per centimetre
corresponds to a force of one electrostatic unit. It appears also that
for air and other gases there is a certain minimum voltage
(approximately 400 volts) below which no discharge takes place,
however near the conducting surfaces may be approximated. In this
particular practical application, however, we are only concerned with
spark lengths which are measured in millimetres or centimetres, lying,
say, between one or two millimetres and five or six centimetres. Over
this range of spark length we shall not generally be wrong in
reckoning the voltage required to produce a spark between metal balls
in air at the ordinary pressure to be given by the rule:
_Disruptive voltage_ = 3,000 x _spark-gap length in millimetres_.
If, however, the air pressure is increased above the normal by
including the spark balls in a vessel in which air can be compressed,
then the spark length, corresponding to a given potential difference,
very rapidly decreases. Mr. F. J. Jervis-Smith[19] found that by
increasing the air pressure from one atmosphere to two atmospheres
round a pair of spark balls he reduced the spark length given by a
certain voltage from 2.5 to 0.75 centimetre.
Professor R. A. Fessenden has also made some interesting observations
on the effect of using compressed air round spark gaps. He found that
if a certain voltage between metal surfaces would yield a spark four
inches in length, at the ordinary pressure of the air, if the spark
balls were enclosed in a cylinder, the air round them compressed at
50lb. per square inch, the spark length for the same potential
difference of the balls was only one quarter of an inch, or
one-sixteenth of its former value.
The writer has also made experiments with an apparatus designed to
study the effect of compressed air round the spark gap. The
experimental arrangements are as follows: A ten-inch induction coil
has one of its terminals connected to the internal coating of a
battery of Leyden jars. The external coating is connected through the
primary coil of an oscillation transformer with the other secondary
terminal of the coil, and these secondary terminals are also connected
to a spark gap consisting of two brass balls enclosed in a glass
vessel into which air can be forced by a pump, the air pressure being
measured by a gauge. The balls in the glass vessel are set at a
distance of about three millimetres apart. The secondary circuit of
the oscillation transformer is connected to another pair of spark
balls, the distance of which can be varied.
Suppose we begin with the air in the glass vessel containing the balls
connected to the secondary terminals of the induction coil, which may
be called the secondary balls, at atmospheric pressure, and create
oscillatory discharges in the primary coil of the oscillation
transformer, we have a spark between the balls, which may be called
the tertiary balls, connected to the secondary terminals of the
oscillation transformer. If the secondary balls are placed, say, three
millimetres apart, the air in the glass vessel enclosing them being at
the ordinary atmospheric pressure, then with one particular
arrangement of jars used, a spark twenty-five or twenty-six
millimetres long between the tertiary balls will take place. Suppose,
then, we increase the pressure of the air round the secondary balls,
pumping it by degrees to 10, 20, 30, 40 and 50lb. per square inch
above the atmospheric pressure. We find that the spark between the
tertiary balls will gradually leap a greater and greater distance, and
when the pressure of the air is 50lb. per square inch, we can obtain a
fifty-millimetre spark between the tertiary balls, whereas when the
air in the glass vessel is at atmospheric pressure, we can only obtain
a spark between the tertiary balls of half that length.
This experiment demonstrates that the effect of compressing the air
round the secondary terminals of the induction coil is to greatly
increase the difference of potential between these balls before the
spark passes. In fact, it requires about double the voltage to force a
spark of the same length through air compressed at 50lb. on the square
inch that it does to make a spark of identical length between the same
balls in air at normal pressure. This shows that there is a very great
advantage in taking the discharge spark in compressed air. A better
effect can be produced by substituting dry gaseous hydrochloric acid
for air at ordinary pressures.
One other incidental advantage is that the noise of the spark is very
much reduced. The continual crackle, of the discharge spark of the
induction coil in connection with wireless telegraphy is very annoying
to sensitive ears, but in this manner we can render it perfectly
silent.
Professor Fessenden also states that when the spark balls are
surrounded by compressed air, and if one of the balls is connected
with a radiator, the compression of the air, although it shortens the
spark-gap corresponding to a given voltage, does not in any way
increase the radiation. When, however, the air in the spark-ball
vessel is compressed to 60lb. in the square inch, there is a marked
increase in the effective radiation, and at 80lb. per square inch the
energy emitted in the form of waves is nearly three and a-half times
greater than at 50lb., the potential difference between the balls
remaining the same.
This effect is no doubt connected with the fact that the production of
a wave, whether in ether or in any other material, is not so much
dependent upon the absolute force applied as upon the suddenness of
its application. To translate it into the language of the electronic
theory, we may say that the electron radiates only whilst it is being
accelerated, and that its radiating power, therefore, depends not so
much upon its motion as upon the rate at which its motion is changing.
The advantage in using compressed air round the spark gap is that we
can increase the effective potential difference between the balls
without rendering the spark non-oscillatory. In air of the ordinary
pressure there is a certain well-defined limit of spark length for
each voltage, beyond which the discharge becomes non-oscillatory, but
by the employment of spark balls in compressed air, we can increase
the potential difference between the balls corresponding to a given
distance apart before a discharge takes place, or employ higher
potentials with the same length of spark gap. In addition to this, we
have, perhaps, the production of a more effective radiation, as
asserted by Fessenden, when the air pressure exceeds a certain
critical value.
The next element which we have to consider in the transmitting
arrangements is a condenser of some kind for storing the energy which
is radiated at intervals. Where a condenser other than the aerial is
employed for storing the electric energy which is to be radiated by
the aerial, some form of it must be constructed which will withstand
high potentials. As the dielectric for such a condenser, only two
materials seem to be of any practical use, viz., glass and micanite.
Glass condensers in the form of Leyden jars have been extensively
employed, but they have the disadvantage that they are very bulky in
proportion to their electrical capacity. The instrument maker's quart
Leyden jar has a capacity of about one-five hundredth of a microfarad,
but it occupies about 150 cubic inches or more. Professor Braun has
employed in his transmitting arrangements condensers consisting of
small glass tubes like test tubes, lined on the inside and outside
with tinfoil, which are more economical in space. The author has found
that condensers for this purpose are best made of sheet glass about
one-eighth or one-tenth of an inch in thickness, coated to within one
inch of their edge on both sides with tinfoil, and arranged in a
vessel containing resin or linseed oil, like the plates of a storage
battery. M. d'Arsonval has employed micanite, but although this
material has a considerably higher dielectric strength than glass, it
is much more expensive to obtain a given capacity by means of micanite
than by glass, although the bulk of the condenser for a given capacity
is less.
To store up a certain amount of electric energy in a condenser, we
require a certain definite volume of dielectric, no matter how we may
arrange it, and the volume required per unit of energy is determined
by the dielectric strength of the material. Thus, for instance,
ordinary sheet glass cannot be safely employed with a greater electric
force than is represented by 20,000 volts for one-tenth of an inch in
thickness, or, say, a potential gradient of 160,000 volts per
centimetre. This is equivalent to an electric force of about 500
electrostatic units. This may be called the safe-working force. The
electrostatic capacity of a condenser formed of two metal surfaces a
foot square separated by glass three millimetres in thickness is
between 1/360 and 1/400 of a microfarad. If this condenser is charged
to 20,000 volts, we have stored up in it half a joule of electric
energy, and the volume of the dielectric is 270 cubic centimetres.
Hence, to store up in a glass condenser electric energy represented by
one joule at a pressure of 20,000 volts, we require 500 cubic
centimetres of glass, and it will be found that if we double the
pressure and double the thickness of the glass, we still require the
same volume.[20] Hence, in the construction of high-tension condensers
to store up a given amount of energy, the economical problem is how to
obtain the greatest energy-storing capacity for the least money.
Glass fulfils this condition better than any other material. Although
some materials may have very high dielectric strength, such as paper
saturated with various oils, or resins, yet they cannot be used for
the purpose of making condensers to yield oscillatory discharges,
because the oscillations are damped out of existence too soon by the
dielectric.
In arranging condensers to attain a given capacity, regard has to be
taken of the fact that for a given potential difference there must be
a certain total thickness of dielectric, and that if condensers of
equal size are being arranged in parallel it adds to their capacity,
whilst joining them in series divides their capacity. If N equal
condensers or Leyden jars have each a capacity represented by C, and
if they are joined _n_ in series and _m_ in parallel, the joint
capacity of the whole number is _m_C/_n_, where the product _mn_ = N.
Passing on next to the consideration of oscillation transformers of
various kinds--these are appliances of the nature of induction coils
for transforming the current or electromotive force of electrical
oscillations in a required ratio. These coils are, however, destitute
of any iron core, and they generally consist of coils of wire wound on
a fibre, wooden or ebonite frame, and must be immersed in a vat of oil
to preserve the necessary insulation. No dry insulation of the nature
of indiarubber or gutta-percha will withstand the high pressures that
are brought to bear upon the circuits of an oscillation transformer.
In constructing these transformers we have to set aside all previous
notions gathered from the design of low-frequency iron-core
transformers. The chief difficulty we have to contend against in the
construction of an effective oscillation transformer is the inductance
of the primary circuit and the magnetic leakage that takes place. In
other words, the failure of the whole of the flux generated by the
primary circuit to pass through or be linked with the secondary
circuit. Mr. Marconi has employed an excellent form of oscillation
transformer, in the design of which he was guided by a large amount of
experience. In this transformer the two circuits are wound round a
square wooden frame. The primary circuit consists of a number of
strands of thick insulated cable laid on in parallel, so that it
consists of only one turn of a stranded conductor. The secondary
circuit consists of a number of turns, say, ten to twenty, of thinner
insulated wire laid over the primary circuit and close to it, so that
the transformer has the transformation ratio of one to ten or one to
twenty. In the arrangements devised and patented by Mr. Marconi, these
two circuits, with their respective capacities in series with them,
are tuned to one another, so that the time-period of each circuit is
exactly the same, and without this tuning the device becomes
ineffective as a transformer.[21] There is no advantage in putting a
number of turns on the primary circuit, because such multiplication
simply increases the inductance, and, therefore, diminishes the
primary current in the same ratio which it multiplies the turns, and
hence the magnetic field due to the primary circuit remains the same.
Where it is desired to put a number of turns upon a coil, and yet at
the same time keep the inductance down, the writer has adopted the
device of winding a silk or hemp rope well paraffined between the
turns of the circuit, so as to keep them further apart from one
another, and as the inductance depends on the turns per centimetre,
this has the effect of reducing the inductance.
The next and most important element in any transmitting station is the
aerial or radiator, and it was the introduction of this element by Mr.
Marconi which laid the foundation for Hertzian wave telegraphy as
opposed to mere experiments with the Hertzian waves. We may consider
the different varieties of aerial which have been evolved from the
fundamental idea. The simple single Marconi aerial consists of a bare
or insulated wire, generally about 100ft. or 150ft. in length,
suspended from a sprit attached to a tall mast. As these masts have
generally to be erected in exposed positions, considerable care has to
be taken in erecting them with a large margin of strength. To the end
of a sprit is attached an insulator of some kind, which may be a
simple ebonite rod, or sometimes a more elaborate arrangement of oil
insulators, and to the lower end of this insulator is attached the
aerial wire. As at the top of the aerial we have to deal with
potentials capable sometimes of giving sparks several feet in length,
the insulation of the upper end of the aerial is an important matter.
In the original Marconi system, the lower end of the aerial was simply
attached to one spark ball connected to one terminal of the induction
coil, and the other terminal and spark ball were connected to the
earth. In this arrangement, the aerial acted not only as radiator, but
as energy-storing capacity, and as already explained, its radiating
power was on that account limited. The earth connection is an
important matter. For long distance work, a good earth is essential.
This earth must be made by embedding a metal plate in the soil, and
many persons are under the impression that the efficiency of the earth
plate depends upon its area, but this is not the fact. It depends much
more upon its shape, and principally upon the amount of its "edge." It
has been shown by Professor A. Tanakadate, of Japan, that if a metal
plate of negligible resistance is embedded in an infinite medium
having a resistivity _r_, the electrical conductance of this plate is
equal to 4pi/_r_ times the electrostatic capacity of the same plate
placed in a dielectric of infinite extent. Hence in designing an
earth plate, we have to consider not how to give it the utmost amount
of surface, but how to give it the greatest electrostatic capacity,
and for this purpose it is far better to divide a given amount of
metal into long strips radiating out in different directions, rather
than to employ it in the form of one big square or circular plate. The
importance of the "good earth" will have been seen from our discussion
on the mode of formation of electric waves. There must be a perfectly
free access for the electrons to pass into and out of the aerial.
Hence, if the soil is dry, or badly conductive in the neighbourhood,
we have to go down to a level at which we get a good moist earth. In
fact, the precautions which have to be taken in making a good earth
for Hertzian wave telegraphy are exactly those which should be taken
in making a good earth for a lightning conductor.
Whilst on the subject of aerials, a word may be said on the
localisation of wireless telegraph stations on the Marconi system. For
reasons which were explained previously, the transmission of signals
is effected more easily over water than over dry land, and it is
hindered if the soil in the neighbourhood of the sending station is a
poor conductor. Hence, all active Hertzian wave telegraph stations,
like all active volcanoes, are generally found near the sea. In those
cases in which a multiple aerial has to be put up consisting of many
wires, one mast may be insufficient to support the structure, and
several masts arranged in the form of a square or a circle have to be
employed. The illustrated papers have reproduced numerous pictures of
the Marconi power stations at Poldhu in Cornwall, Glace Bay in Nova
Scotia, and Cape Cod in the United States. In these stations, after
preliminary failures to obtain the necessary structural strength with
ordinary masts, tall lattice girder wooden towers have been built,
about 215 feet in height, well stayed against wind pressure, and which
so far have proved themselves capable of withstanding any storm of
wind which has come against them.
An important question in connection with the sending power of an
aerial is that of the relation of its height to the distance covered.
Some time ago Mr. Marconi enunciated a law, as the result of his
experiments, connecting these two quantities, which may be called
Marconi's Law. He stated that the height of the aerial to cover a
given distance, other things remaining the same, varies as the square
root of the distance. Let D be the distance and let L be the length of
the aerial, then if both the transmitting and receiving aerial are the
same height, we may say that D varies as L^{2}. This relation may be
theoretically deduced as follows:--Any given receiving apparatus for
Hertzian wave telegraphy requires a certain minimum energy to be
imparted to it to make it yield a signal. If the resistance and the
capacity of the receiver is taken as constant, this minimum working
energy is proportional to the square of the electromotive force set up
in the receiving aerial by the impact on it of the electric waves.
This electromotive force varies as the length of the receiving aerial
and as the magnetic force due to the wave cutting across it, and the
magnetic force varies as the current in the transmitting aerial, and
therefore, for any given voltage varies as the capacity, and therefore
as the length of the transmitting aerial. If, therefore, the
transmitting and receiving aerial have the same length, the minimum
energy varies as the square of the electromotive force in the
receiving aerial, and therefore as the fourth power of the length of
either aerial, since the electromotive force varies as the product of
the lengths of the aerials. Hence, when the distance between the
aerials is constant, the minimum working energy varies as the fourth
power of the height of either aerial, but when the lengths of the
aerials are constant, the energy caught up by the receiving aerial
must vary inversely as the square of the distance D between the
aerials. Hence, if we call _e_ this minimum working energy, _e_ must
vary as 1/D^{2} when L is constant, or as L^4 when D is constant, and
since _e_ is a constant quantity for any given arrangements of
receiver and transmitter, it follows that when the height of aerial
and distance vary, the ratio L^4/D^2 is constant, or, in other words,
D^2 varies as L^4 or D varies as L^2--_i.e._, distance varies as the
square of the height of the aerial, which is Marconi's Law. The curve,
therefore, connecting height of aerial with sending distance for given
arrangements is a portion of a parabola.
Otherwise, the law may be stated in the form L = _a_[\sq]{D}, where _a_
is a numerical coefficient. If L and D are both measured in metres,
then, for recent Marconi apparatus as used on ships, _a_ = 0.15 roughly.
(See a report on experiments made for the Italian Navy, 1900-1901, by
Captain Quintino Bonomo--"Telegrafia senza fili," Rome, 1902.)
This law, however, must not be used without discretion. After Mr.
Marconi had transmitted signals across the British Channel, some
people, forgetting that a little knowledge is a dangerous thing,
predicted that aerials a thousand feet in height would be required to
signal across the Atlantic, but Mr. Marconi has made such improvements
of late years in the receiving arrangements that he has been able to
receive signals over three thousand miles in 1903 with aerials only
thirty-three per cent. longer than those which, in 1899, he employed
to cover twenty miles across the English Channel.
[Illustration: FIG. 15.--ALTERNATING-CURRENT DOUBLE-TRANSFORMATION
POWER PLANT FOR GENERATING ELECTRIC WAVES (Fleming). _a_, alternator;
H_{1}H_{2}, choking coil; K, signalling key; T, step-up transformer;
S_{1}S_{2} spark-gap; C_{1}C_{2} condensers; T_{1}T_{2}, oscillation
transformers; A, aerial; E, earthplate.]
We turn, in the next place, to the consideration of those devices for
putting more power into the aerial than can be achieved when the
aerial itself is simply employed as the reservoir of energy. Professor
Braun, of Strassburg, in 1899, described a method for doing this by
inducing oscillations in the aerial by means of an oscillation
transformer, these oscillations being set up by the discharges from a
Leyden jar or battery of Leyden jars, which formed the reservoir of
energy. The induction coil is employed to produce a rapidly
intermittent series of electrical oscillations in the primary coil of
an oscillation transformer by the discharge through it of a Leyden
jar. Mr. Marconi immensely improved this arrangement, as described by
him in a lecture given before the Society of Arts on May 17, 1901, by
syntonising the two circuits and making the circuit, consisting of the
capacity of the aerial and the inductance of the secondary circuit of
the oscillation transformer, have the same time-period as the circuit
consisting of the Leyden jars, or energy-storing condenser, and the
primary circuit of the oscillation transformer, and by so doing
immensely added to the power and range of the apparatus.
Starting from these inventions of Braun and Marconi, the author
devised a double transmission system in which the oscillations are
twice transformed before being generated in the aerial, each time with
a multiplication of electromotive force and a multiplication of the
number of groups of oscillations per second. This arrangement can best
be understood from the diagram (see Fig. 15).
In this case a transformer, T, or transformers receive alternating
low-frequency current from an alternator, _a_, being regulated by
passing through two variable choking coils, H_{1} and H_{2}, so as to
control it. This alternating current is transformed up from a
potential of two thousand to twenty, forty or a hundred thousand, and
is employed to charge a large condenser, C_{1}, which discharges
across a primary spark-gap, S_{1}, through the primary coil of an
oscillation transformer, T_{1}. The secondary circuit of the
oscillation transformer is connected to a second pair of spark balls,
S_{2}, which in turn are connected by a secondary condenser, C_{2},
and the primary circuit of a third transformer, T_{2} and the
secondary circuit of this last transformer are inserted between a
Marconi aerial, A, and the earth E. When all these circuits are tuned
to resonance by Mr. Marconi's methods, we have an enormously powerful
arrangement for creating electric waves, or rather trains of electric
waves, sent out from the aerial, and the oscillations are controlled
and the signals made by short-circuiting one of the choking coils.
Another transmitting arrangement, which involves a slightly different
principle, and employs no oscillation transformer, is one due also to
Professor Braun. In this case, a condenser and inductance are connected
in series to the spark balls of an induction coil, and oscillations are
set up in this circuit. Accordingly, there are rapid fluctuations of
potential at one terminal of the condenser. If to this we connect a long
aerial, the length of which has been adjusted to be one quarter of the
length of wave corresponding to the frequency, in other words, to make
it a quarter-wave resonator, then powerful oscillations will be
accumulated in this rod. The relation between the height (H) of the
aerial and the frequency is given by the equation 3 x 10^{10} = 4_n_H,
where _n_ is the frequency of the oscillations and H the height of the
aerial in centimetres. The frequency of the oscillations is determined
by the capacity (C) and inductance (L) of the condenser circuit, and can
be calculated from the formula
n = (5,000,000) / ([\sq]{C (in mfds.) x L (in cms.)}).
That is, the frequency is obtained by dividing into the number
5,000,000, the square root of the product of the capacity in
microfarads, and inductance in centimetres, of the condenser circuit.
It will be found, on applying these rules, that it is impossible to
unite together any aerial of a length obtainable in practice with a
condenser circuit of more than a very moderate capacity. It has been
shown that for an aerial two hundred feet in height the corresponding
resonating frequency is about one and a quarter million.[22] As we are
limited in the amount to which we can reduce the inductance of a
discharge circuit, probably to something like a thousand centimetres,
a simple calculation shows that the largest capacity we can employ is
about a sixtieth of a microfarad. This capacity, even if charged at
60,000 volts, would only contain thirty joules of energy, or about
22.5 foot-pounds, which is a small storage compared to that which can
be achieved when we are employing the above-described methods, which
involve the use of an oscillation transformer. In such a case,
however, it is an advantage to employ a spark-gap in compressed air,
because we can then raise the voltage to a much higher value than in
air of ordinary pressure without lengthening the spark so much as to
render it non-oscillatory.
When employing methods involving the use of an oscillation
transformer, it is possible to use multiple aerials having large
capacity, and hence to store up a very large amount of energy in the
aerial, which is liberated at each discharge. The most effective
arrangement is one in which the radiator draws off gradually a large
supply of energy from a non-radiating circuit, and so sends out a true
train of waves, and not mere impulses, into the ether, and as we shall
see later on, it is only when the radiation takes place in the form of
true wave trains that anything like syntony can be obtained.
There are a number of variants of the above methods of arranging the
radiator and associated energy-storing in circuit. Descriptions of
these arrangements will be found in patents by Mr. Marconi, Professor
Slaby and Count von Arco, Sir Oliver Lodge, Dr. Muirhead, Professor
Popoff, Professor Fessenden and others. In all cases, however, they
are variations of the three simple forms of radiator already
described.
Returning to the analogy with the air or steam siren suggested at the
commencement of this article, the reader will see in the light of the
explanations already given, that all parts of the air-wave producing
apparatus have their analogues in the electrical radiator as used in
Hertzian wave telegraphy. The object in the one case is to produce
rapid oscillations of air particles in a tube, which result in the
production of an air wave in external space; in the other case, the
arrangement serves to produce oscillations of electrons or electrical
particles in a wire, the movements of which create a disturbance in
the ether called an electrical wave. Comparing together, item by item,
it will be seen, therefore, that the induction coil or transformer
used in connection with electric-wave apparatus is analogous to the
air pump in the siren plant. In the electrical apparatus, this
electron pump is employed to put an electrical charge into a
condenser; in the air wave apparatus, the air pump is employed to
charge an air vessel with high pressure air. From the electrical
condenser the charge is released in the form of a series of electrical
oscillations, and in the air wave producing appliance, the compressed
air is released in the form of a series of intermittent puffs or
blasts. In the electrical wave producing apparatus, these electrical
oscillations in the condenser circuit are finally made to produce
other oscillations in an air wire or open circuit, just as the puffs
of air finally expend themselves in producing aerial oscillations in
the siren tube. Finally, in the one case we have a series of air waves
and in the other case, a series of electrical waves. These trains of
electric waves or air waves, as the case may be, are intermitted into
long and short groups, in accordance with the signals of the Morse
alphabet, and, therefore, the Hertzian wave transmitter, in whatever
form it may be employed, when operated by means of a Marconi aerial,
is in fact an electrical siren apparatus, the function of which is to
create periodic disturbances in the universal ether of the same
character as those which the siren produces in atmospheric air.
* * * * *
We have to consider in the next place the arrangements of the
receiving station and the various forms of receivers that have been
devised for effecting telegraphy by Hertzian waves. Just as the
transmitting station consists essentially of two parts, viz., a part
for creating electrical oscillations and a part for throwing out or
radiating electric waves, so the receiving-station appliances may be
divided into two portions; the function of one being to catch up a
portion of the energy of the passing wave, and that of the other to
make a record or intelligible signal in some manner in the form of an
audible or visible sign.
Accordingly, there must be at the receiving station an arrangement
called a receiving aerial, which in general takes the form of a long
vertical wire or wires, similar in form to the transmitting aerial,
There is, however, a distinct difference in the function of the
transmitting aerial and the receiving aerial. The function of the
first is effective radiation, and for this purpose the aerial must
have associated with it a store of energy to be released as wave
energy; but the function of the receiving aerial is to be the seat of
an electromotive force which is created by the electric force and the
magnetic force of the incident electric wave.
In tracing out the mode of operation of the transmitting aerial, it
was pointed out that the electric waves emitted consisted of
alternations of electric force in a direction which is perpendicular
to the surface of the earth, and magnetic force parallel to the
surface of the earth. These two quantities, the electric force and the
magnetic force, are called the _wave vectors_, and they both lie in a
plane perpendicular to the direction in which the wave is travelling
and at right angles to one another, the electric force being
perpendicular to the surface of the earth. In optical language, the
wave sent out by the aerial would be called a plane polarised wave,
the plane of polarisation being parallel to the magnetic force. Hence,
if at any point in the path of the wave we erect a vertical conductor,
as the wave passes over it, it is cut transversely by the magnetic
force of the wave and longitudinally by the electric force. Both of
these operations result in the creation of an alternating
electromotive force in the receiving aerial wire.
As in all other cases of oscillatory motion, the principle of
resonance may here be brought into play to increase immensely the
amplitude of the current oscillations thereby set up in the receiving
aerial. As already explained, any vertical insulated wire placed with
its lower end near the earth has capacity with respect to the earth,
and it has also inductance, the value of these factors depending on
its shape and height. Accordingly, it has a natural electrical
time-period of its own, and if the periodic electromotive impulses
which are set up in it by the passage of the waves over it agree in
period with its own natural time-period, then the amplitude of the
current vibrations in it may become enormously greater than when there
is a disagreement between these two periods. Before concluding these
articles we shall return to this subject of electric resonance and
syntony, and discuss it with reference to what is called the tuning of
Hertzian wave stations. Meanwhile, it may be said that for the sake of
obtaining, at any rate in an approximate degree, this coincidence of
time-period, it is generally usual to make the receiving aerial as far
as possible identical with the transmitting aerial. If the receiving
aerial is not insulated, but is connected to the earth at its lower
end through the primary coil of an oscillation transformer, we can
still set up in it electrical oscillations by the impact on it of an
electric wave of proper period; and if the oscillation transformer is
properly constructed we can draw from its secondary circuit electric
oscillations in a similar period.
One problem in connection with the design of a receiving aerial is
that of increasing its effective length and capacity so as to increase
correspondingly the electromotive force or current oscillations in it.
It is clear that if we put a number of receiving wires in parallel so
that each one of them is operated upon by the wave separately,
although we can increase in this way the magnitude of the alternating
current which can be drawn off from the aerial, we cannot increase the
electromotive force in it except by increasing the actual height of
the wires. Unfortunately, there is a limit to the height of the
receiving aerial. It has to be suspended, like the transmitting
aerial, from a mast or tower, and the engineering problem of
constructing such a permanent supporting structure higher than, say,
two hundred feet is a difficult one.
Since any one station has to send as well as receive, it is usual to
make one and the same aerial wire or wires do double duty. It is
switched over from the transmitting to the receiving apparatus, as
required. This, however, is a concession to convenience and cost. In
some respects it would be better to have two separate aerials at each
station, the one of the best form for sending, and the other of the
best form for receiving.
In Mr. Marconi's early arrangements, the so-called coherer or
sensitive wave-detecting appliance, to be described more in detail
presently, was inserted between the base of the insulated receiving
aerial and the earth, but it was subsequently found by him to be a
great improvement to act upon the receiving device, not directly by
the electromotive force set up in the aerial, but by the induced
electromotive force of a special form of step-up oscillation
transformer he calls a "jigger," the primary circuit of which was
inserted in between the receiving aerial and the earth plate, and the
secondary circuit was connected to the sensitive organ of the
telegraphic receiving arrangements.[23] A suggestion to employ
transformed oscillations in affecting a coherer, had also been
described in a patent specification by Sir Oliver Lodge, in 1897, but
the essence of success in the use of this device is not merely the
employment of a transformer, but of a transformer constructed
specially to transform electrical oscillations.
Turning, then, to the consideration of the relation existing between
the transmitting and receiving aerials, we note that in their simplest
form these consist of two similar tall rods of metal placed upright,
with their feet in good connection with the earth at two places. We
may think of them as two identical lightning conductors, well earthed
at the bottom, and supported by non-conducting masts or towers. These
rods must be in good connection with the earth, and therefore with it
form, as it were, one conductor. If, as usual, these aerials are
separated by the sea, the intermediate portion of this circuit is an
electrolyte. The operations which take place when a signal is sent are
as follows:--
At the transmitting station, we set up in the transmitting aerial
electric oscillations, of which the frequency may be of the order of a
million, _i.e._, the oscillations as long as they last are at the rate
of a million a second. Each spark discharge at the transmitter
results, however, only in the production of a train of a dozen or two
oscillations, and these trains succeed each other at a rate depending
upon the transmitting arrangements used. Each oscillation in the
transmitting aerial is accompanied by the detachment from it of
semi-loops of electric strain, as already explained. The alterations
of electric strain directed perpendicularly to the earth, and of the
associated magnetic force parallel to the earth, constitute an
electric wave in the ether, just as the alternations of pressure and
motion of air molecules constitute an air wave. Associated with these
physical actions above ground, there is a propagation through the
earth of electric action, which may consist in a motion or atomic
exchange of electrons. Each change or movement of a semi-loop of
electric strain above ground has its equivalent below ground in
inter-atomic exchanges or movements of the electrons, on which the
ends of these semi-loops of electric strain terminate. The earth must
play, therefore, a very important part in so-called "wireless
telegraphy," and we might also say the earth does as much as the ether
in its production.
The function of the receiving aerial is to bring about a union between
these two operations above and below ground. When the electric waves
fall upon it, they give rise to electromotive force in the receiving
aerial, and, therefore, produce oscillations in it which, in fact,
are electric currents flowing into and out of the receiving aerial. We
may say that the transmitting aerial, the receiving aerial and the
earth form one gigantic Hertz oscillator. In one part of this system,
electric oscillations of a certain period are set up by the discharge
of a condenser and are propagated to the other part. In the earth,
there is a propagation of electric oscillations; in the space above
and between the aerials, there is a propagation of electric waves. The
receiving aerial _feels_, therefore, what is happening at the distant
aerial and can be made to record it.[24]
We have next to consider the question of the wave-detecting devices
which enable us to appreciate and record the impact of a wave or wave
train against the aerial. At the very outset it will be necessary to
coin a new word to apply generally to these appliances. Most readers
are probably familiar with the term "coherer," which was applied by
Sir Oliver Lodge, in the first instance, to an electric wave-detecting
device of one particular kind--viz., that in which a metal point was
lightly pressed against another metal surface and caused to stick to
it when an electric wave fell upon it. As our knowledge increased, it
was found that there were many cases in which the effect of the
electric radiation was to cause a severance and not a coherence, and
hence such clumsy phrases as "anticoherer" and "self-decohering
coherer" have come into use. Moreover, we have now many kinds of
electric wave detectors based on quite different physical principles.
At the risk of incurring reprobation for adding to scientific
nomenclature, the author ventures to think that the time has arrived
when a simple and inclusive term will be found useful to describe all
the devices, whatever their nature, which are employed for detecting
the presence of an electric wave. For this purpose the term
_kumascope_, from the Greek [Greek: kuma] (a wave), is suggested. The
scientific study of waves has already been called _kumatology_, and in
view of our familiarity with such terms as _microscope_,
_electroscope_ and _hygroscope_, there does not seem to be any
objection to enlarging our vocabulary by calling a wave-detecting
appliance a _kumascope_. We are then able to look at the subject
broadly and to classify kumascopes of different kinds.
We may, in the first place, arrange them according to the principle on
which they act. Thus, we may have electric, magnetic, thermal,
chemical and physiological operations involved; and finally, we may
divide them into those which are self-restoring, in the sense that
after the passage or action of a wave upon them they return to their
original sensitive condition; and those which are non-restoring, in
that they must be subjected to some treatment to bring them back again
to a condition in which they are fit to respond again to the action of
a wave.
We have no space to refer to the whole of the steps of discovery which
led up to the invention of all the various forms of the modern
electric kumascope or wave detector. Suffice it to say that the
researches of Hertz in 1887 threw a flood of light upon many
previously obscure phenomena, and enabled us to see that an electric
spark, and especially an oscillatory spark, creates a disturbance in
the ether, which has a resemblance in Nature to the expanding ripples
produced by a stone hurled into water. Scientific investigation then
returned with fresh interest to previously incomprehensible effects,
and a new meaning was read into many old observations. Again and again
it had been noticed that loose metallic contacts, loose aggregations
of metallic filings or fragments, had a mysterious way of altering
their conductivity under the action of electric sparks, lightning
discharges and high electromotive forces.
As far back as 1852, Mr. Varley had noticed that masses of powdered
metals had a very small conductivity, which increased in a remarkable
way during thunderstorms;[25] and in 1866, C. and S. A. Varley
patented a device for protecting telegraphic instruments from
lightning, which consisted of a small box of powdered carbon in which
were buried two nearly touching metal points, and they stated that
"powdered conducting matter offers a great resistance to a current of
moderate tension, but offers but little resistance to currents of high
tension."[26]
We then pass over a long interval and find that the next published
account of similar observations was due to Professor T.
Calzecchi-Onesti, who described in an Italian journal, _Il Nuovo
Cimento_ (see Vol. XVI., p. 58, and Vol. XVII., p. 38), in 1884 and
1885 his observations on the decrease in resistance of metal powders
when the spark from an induction coil was sent through them.[27] These
observations did not attract much attention until Professor E. Branly,
of Paris, in 1890 and 1891, repeated them on an extended scale and
with great variations, making the important observation that an
electric spark _at a distance_ had a similar effect in increasing the
conductivity of metallic powders.[28] Branly, however, noticed that in
some cases of conductors in powder, such as the peroxide of lead or
antimony, the effect of the spark was to cause a decrease of
conductivity.
To Professor E. Branly unquestionably belongs the honour of giving to
science a new weapon in the shape of a tube containing metallic
filings or powder rather loosely packed between metal plugs, and of
showing that when the pressure on the powder was adjusted such a tube
may be a conductor of very high resistance, but that the electrical
conductivity is enormously increased if an electric spark is made in
its neighbourhood. He also proved that the same effect occurred in the
case of two slightly oxidised steel or copper wires laid across one
another with light pressure, and that this loose or imperfect contact
was extraordinarily sensitive to an electric spark, dropping in
resistance from thousands of ohms to a few ohms when a spark was made
many yards away.
It is curious to notice how long some important researches take to
become generally known. Branly's work did not attract much attention
in England until 1892, when Dr. Dawson Turner described his own
repetition of Branly's experiments with the metallic filings tube at a
meeting of the British Association in Edinburgh. In the discussion
which followed, Professor George Forbes made an important remark. He
asked whether it was possible that the decrease in resistance could be
brought about by Hertz waves.[29]
This question shows that even in 1892 the idea that the effect of the
spark on the Branly tube was really due to Hertzian waves was only
just beginning to arise. The following year, however, Mr. W. B. Croft
repeated Branly's experiment with copper filings before the Physical
Society of London, and entitled his short Paper "Electric Radiation on
Copper Filings."[30] He exhibited a tube containing copper filings
loosely held between two copper plugs and joined in series with a
galvanometer and cell. The effect of an electric spark at a distance,
in causing increase of conductivity, was shown, and the return of the
tube to its non-conducting state when tapped was also noticed.
In the discussion which followed the reading of this Paper, Professor
Minchin described the effects of electric radiation on his impulsion
cells. He followed up this by reading a Paper to the Physical Society
on November 24, 1893, on the action of Hertzian radiation on films
containing metallic powders, and expressed the opinion that the change
in resistance of the Branly tube was due to electric radiation.[31]
Thus, at the end of 1893, a few physicists clearly recognised that a
new means had been given to us for detecting those invisible ether
waves, the chief properties of which Hertz had unravelled with
surpassing skill six years before, by means of a detector consisting
of a ring of wire having a small spark-gap in it.
In June, 1894, Sir Oliver Lodge delivered a discourse at the Royal
Institution, entitled "The Work of Hertz," and at this lecture use was
made of the Branly tube as a Hertz wave detector. The chief object of
the lecture was to describe the properties of Hertzian waves and their
reflection, absorption and transmission, and many brilliant
quasi-optical experiments were exhibited. Although a Branly tube, or
imperfect metallic contact, then named by him a _coherer_, was
employed by Sir Oliver Lodge to detect an electric wave generated in
another room, there was no mention in this lecture of any use of the
instrument for telegraphic purposes.[32]
As we are here concerned only with the applications in telegraphy, we
shall not spend any more time discussing the purely scientific work
done with laboratory forms of this wave detector.
Without attempting to touch the very delicate question as to the
precise point at which laboratory research passed into technical
application, we shall briefly describe the forms of kumascope which
have been devised with special reference to Hertzian wave telegraphic
work. A very exact classification is at present impossible, but we may
say that telegraphic kumascopes may be roughly divided into six
classes. The first class includes all those that depend for their
action on the "coherer principle" or the reduction of the resistance
of a metallic microphone by the action of electromotive force. As they
depend upon an imperfect contact, they may be called _contact
kumascopes_. This class is furthermore subdivided into the
self-restoring and the non-self-restoring varieties. The second class
comprises the _magnetic kumascopes_ which depend upon the action of an
electrical oscillation as a magnetising or demagnetising agency. The
third class comprises the _electrolytic responders_, in which the
action of electric oscillations either promotes or destroys the
results of electrolysis. The fourth class consists of the
_electrothermal detectors_, in which the power of an electrical
oscillation as a high frequency electric current to heat a conductor
is utilised. The fifth class comprises the _electromagnetic_ or
_electro-dynamic_ instruments, which are virtually very sensitive
alternating-current ammeters, adapted for immensely high frequency.
The sixth class must be made to contain all those which cannot be well
fitted at present into any of the others, such as the sensitive
responder of Schaefer, the action of which is not very clearly made
out.
We may proceed briefly to describe the construction of the principal
forms of kumascope coming under the above headings. In the first
place, let us consider those which are commonly called the "coherers"
or, as the writer prefers to call them, the _contact kumascopes_. The
simplest of these is the crossed needle or single contact, which
originated with Professor E. Branly.[33] The pressure of the point of
a steel needle against an aluminium plate was subsequently found by
Sir Oliver Lodge to be a very sensitive arrangement when so adjusted
that a single cell sends little or no current through the contact.[34]
When an electric wave passes over it, good conducting contact ensues.
The point is, in fact, welded to the plate, and can only be detached
by giving the plate or needle a light shock or vibration. A variation
of the above form is a pair of crossed needles, one resting on the
other.