LINE AND WIRELESS TELEGRAPHS
A Ground Telegraph
Nearly every boy is interested in telegraphy, and it is a fascinating
field for study and experimental work, to say nothing of the amusement
to be gotten out of it. The instruments are not difficult to make, and
two boys can easily have a line between their houses.
The key is a modified form of the push-button, and is simply a contact
maker and breaker for opening and closing an electrical circuit. A
practical telegraph-key is shown in Fig. 1, and in Fig. 2 is given the
side elevation.
The base-board is four inches wide, six inches long, and half an inch in
thickness. At the front end a small metal connector-plate is screwed
fast, and through a hole in the middle of it a brass-headed
upholsterer’s tack is driven for the underside of the key to strike
against. Two [L] pieces of metal are bent and attached to the middle of
the board to support the key-bar, and at the rear of the board another
upholsterer’s tack is driven in the wood for the end of the bar to
strike on and make a click. The bar is of brass or iron, measuring
three-eighths by half an inch, and is provided with a hole bored at an
equal distance from each end for a small bolt to pass through, in order
to pivot it between the [L] plates. A hole made at the forward end will
admit a brass screw that in turn will hold a spool-end to act as a
finger-piece. The screw should be cut off and riveted at the underside.
A short, strong spring is to be attached to the back of the base-block
and to the end of the key-bar by means of a hook, which may be made
from a steel-wire nail flattened. It is bound to the top of the bar
with wire, as shown in Figs. 2 and 3.
[Illustration: FIG. 1]
[Illustration: FIG. 4]
The incoming and outgoing wires are made fast to one end of the
connector-plate and to one of the [L] pieces that support the key. When
the key is at rest the circuit is open, but when pressed down against
the brass tack it is closed, and whether pressed down or released it
clicks at both movements. A simple switch may be connected with the
[L]-plate and the connection-post at the opposite side of the key-base,
so that, if necessary, the circuit may be closed. Or an arm may be
caught under the screw at the [L]-plate, and brought forward so that it
can be thrown in against a screw-head on the connector-plate, as shown
in Fig. 3. The screw-head may be flattened with a file, and the
underside of the switch bevelled at the edges, so that it will mount
easily on the screw.
In Fig. 4 (page 191) a simple telegraph-sounder is shown. A base-board,
four inches wide, six inches long, and seven-eighths of an inch in
thickness, is made of hard-wood, and two holes are bored, with the
centres two inches from one end, so that the lower nuts of the horseshoe
magnet will fit in them, as shown in Fig. 5. This allows the yoke to
rest flat on the top of the base, and with a stout screw passed down
through a hole in the middle of the yoke and into the wood the magnets
are held in an upright position.
From the base-block to the top of the bolt the magnets are two inches
and a quarter high. The bar of brass or iron to which the armature (A in
Fig. 5) is attached is four inches and a half in length and
three-eighths by half an inch thick. At the middle of the bar and
through the side a hole is bored, through which a small bolt may be
passed to hold it between the upright blocks of wood. At the front end
two small holes are to be bored, so that its armature may be riveted to
it with brass escutcheon-pins or slim round-headed screws. The heads are
at the top and the riveting is underneath. A small block of wood is cut,
as shown in Fig. 6, against which the two upright pieces of wood are
made fast. This block is two inches and a half long, one inch and a
quarter high, and seven-eighths of an inch wide. The laps cut from each
side are an inch wide and a quarter of an inch deep, to receive the
uprights of the same dimensions.
At the top of this block a brass-headed nail is driven for the underside
of the bar to strike on. A hook and spring are to be attached to the
rear of the sounder-bar, as described for the key, and at the front of
the base two binding-posts are arranged, to which the loose ends of the
coil-wires are attached.
Just behind the yoke, and directly under the armature-bar, a long screw
is driven into the base-block, as shown at B in Fig. 5. It must not
touch the yoke, and the head should be less than one-eighth of an inch
below the bar when at rest. On this the armature-bar strikes and clicks
when drawn to the magnets. The armature must not touch the magnets;
otherwise the residual magnetism would hold it down. The screw must be
nicely adjusted, so that a loud, clear click will result.
[Illustration: FIG. 2
FIG. 3
FIG. 5
FIG. 6
FIG. 8
TELEGRAPH KEY AND SOUNDER]
When the sounder is at rest the rear end lies on the brass tack in the
block, and the armature is about a quarter of an inch above the top of
the magnets. The armature is of soft iron, two inches and a half long,
seven-eighths of an inch wide, and an eighth of an inch thick. These
small scraps of metal may be procured at a blacksmith’s shop, and, for a
few cents, he will bore the holes in the required places; or if you have
a breast or hand drill the metal may be held in a vise and properly
perforated.
By connecting one wire from the key directly with one of the
binding-posts of the sounder, and the other with the poles of a battery,
and so on to the sounder, the apparatus is ready for use. By pressing on
the key the circuit is closed, and the magnetism of the sounder-cores
draws the armature down with a click. On releasing the key the bar flies
back to rest, having been pulled down by the spring, and it clicks on
the brass tack-head. These two instruments may be placed any distance
apart, miles if necessary, so long as sufficient current is employed to
work the sounder. Two sets of instruments must be made if boys in
separate houses are to have a line. Each one must have a key, sounder,
and cell, or several cells connected in series to form a battery,
according to the current required.
In the plan of the telegraph-line connections (Fig. 7, page 196) a
clear idea is given for the wiring; and if the line and return wires are
to be very long, it would be best to have them of No. 14 galvanized
telegraph-wire, copper being too expensive, although much better. These
wires must not touch each other, and when attached to a house, barn, or
trees, porcelain or glass insulators should be used. If nothing better
can be had, the necks of some stout glass bottles may be held with
wooden pins or large nails, and the wire twisted to them, as shown in
Fig. 8. When the line is not in use the switches on both keys should be
closed; otherwise it would be impossible for the boy having the closed
switch to call up the boy with the open one. Take great care in wiring
your apparatus to study the plan, for a misconnected wire will throw the
whole system out of order.
[Illustration: FIG. 7]
To operate the line see that all switches are closed and that the
connections are in good condition. When the boy in house No. 2 wants to
call up his friend in house No. 1 he throws open the switchon key, as
shown in the plan, and by pressing down on the finger-key his sounder
and that in house No. 1 click simultaneously. As soon as he raises or
releases the key the armatures rise, making the up-click. If he presses
his key and releases it quickly the two clicks on the sounder in house
No. 1 are close together; this makes what is called a dot. If the key is
held down longer it makes a long time between clicks, and this is
called a dash. The dot and dash are the two elements of the telegraphic
code. You will understand that the boy in house No. 2 hears just what
the one in No. 1 is hearing, since the electric current passing through
both coils causes the magnets to act in unison. So soon as the operator
in house No. 2 has finished he closes his switch, and the other in house
No. 1 opens his switch on the key and begins his reply. This is the
simple principle of the telegraph, and all the improved apparatus is
based on it, no matter how complicated. The complete Morse alphabet is
appended:
[Illustration: =The Morse Telegraph Code=]
Any persevering boy can soon learn the dot-and-dash letters of the Morse
code, and very quickly become a fairly good operator. Telegraphic
messages are sent and received in this way, and are read by the sound of
the clicks. Various kinds of recording instruments are also employed, so
that when an operator is away from his table the automatic recorder
takes down the message on a paper tape. In the stock-ticker, employed in
brokerage offices, the recording is done by letters and numerals, and
the paper tape drops into a basket beside the machine, so that any one
picking up the strip of paper can see the quotations from the opening of
business up to the time of reading them. These quotations are sent out
directly from the floor of the exchanges, and by the action of one man’s
hand thousands of machines are set in operation all over the city.
Perhaps the most unique and wonderful telegraphic signal-apparatus is
that located on the floor of the New York Produce Exchange and the
Chicago Exchange. The dials, side by side, are operated by direct wire
from Chicago. When the New York operator flashes a quotation it appears
simultaneously on the New York dial and simultaneously on the Chicago
dial, and vice versa.
Electrical instruments are not the only means by which the Morse
alphabet may be transmitted, for in some instances instruments would be
in the way, while in others the wires might be down and communication
cut off.
This is interestingly illustrated by an event in Thomas A. Edison’s
life. When he was a boy and an apprentice telegraph operator on the
Grand Trunk Line, an ice-jam had broken the cable between Port Huron, in
Michigan, and Sarnia, in Canada, so that communication by electricity
was cut off. The river at that point is a mile and a half wide, the ice
made the passage impossible, and there was no way of repairing the
cable. Edison impulsively jumped on a locomotive standing near the
river-bank and seized the whistle-cord.
He had an idea that blasts of the whistle might be broken into long and
short sounds corresponding to the dots and dashes of the Morse code. In
a moment the whistle sounded over the river: “Toot, toot, toot,
toot,--toot, tooooot,--tooooot--tooooot--toot, toot--toot, toot.”
“Halloo, Sarnia! Do you get me? Do you hear what I say?”
No answer.
“Do you hear what I say, Sarnia?”
A third, fourth, and fifth time the message went across, to receive no
response. Then suddenly the operator at Sarnia heard familiar sounds,
and, opening the station door, he clearly caught the toot, toot of the
far-away whistle. He found a locomotive, and, mounting to the cab,
responded to Edison, and soon messages were tooted back and forth as
freely as though the parted cable were again in operation.
Some years ago the police of New York were mystified over a murder case.
The man they suspected had not fled, but was still in his usual place,
and attending to his business quite as though nothing had happened to
connect him with the tragedy.
Detectives in plain clothes had been following him and watching closely
his every move in and out of restaurants and shops and at social
affairs; but not the slightest proof could be secured against him.
One noon-time they followed him into a café, where he had gone with a
friend. The detectives took seats near him, but each of them sat at
different tables in the room full of people.
When in the café the suspect sat next the wall, a habit the detectives
had noticed. Consequently, only those persons who sat at one side of him
or directly in front could see his face. During the time they were in
the restaurant the detectives communicated with each other by tapping on
the table tops with a lead-pencil; and something the man said, which the
nearest detective heard, led to the climax. One detective rose, paid his
check, and loitered near the door; another got up a little later and
sauntered out, but returned with a cardboard sign. Going over to the
table where the suspected criminal and his friend sat, he deliberately
tacked it on the wall above them, then went out again, leaving the third
detective to watch the face of the man as he read:
$1000 REWARD
for information leading to the arrest of the murderer of ------------
on March --------, 1876
The man cast a glance about the restaurant, then said to his companion:
“Did I show any signs of agitation?” The third detective rose, stepped
over to the man, tapped him on the shoulder, and said, “I want you.”
There would have been a scene of violence had not the other two
detectives closed in on the man, and within six months he paid the
penalty of his crime.
If it had not been for the dot-and-dash alphabet, tapped out with
lead-pencils, the detectives could not have communicated; but like
Edison, they used the means at hand to open up and carry on a silent
conversation.
Wireless Telegraphy
Everybody nowadays understands that wireless telegraphy means the
transmission of electrical vibrations through the ether and earth
without the aid of wires or any visible means of conductivity. The feat
of sending an electrical communication over thousands of miles of wire,
or through submarine cables, is wonderful enough, for all that custom
has made it an every-day miracle. To accomplish this same end by sending
our messages through the apparently empty air is indeed awe-inspiring
and almost beyond belief. And yet we know that wireless telegraphy is
to-day a real scientific fact.
At first sight it would seem that the instruments must be complicated
and necessarily beyond the ability of the average boy to make, and far
too expensive as well. As a matter of fact, the young electrician may
construct his wireless apparatus at a very moderate cost, it being
understood that the sending and receiving poles may be mounted on a
housetop or barn.
But first let us consider the theory upon which we are to work. There is
no doubt but that electricity is the highest known form of vibration--so
high, indeed, that as yet man has been unable to invent any instrument
to record the number of pulsations per second. This vibration will occur
in, and can be sent through, the ordinary form of conductor, such as
metals, water, fluids and liquids, wet earth, air and ice. Also through
what we call the ether.
Now the ether of the atmosphere, estimated to be fifteen trillion times
lighter than air, is the medium through which the electrical vibrations
pass in travelling in their radial direction from a central point,
corresponding to the ripples or wavelets formed when a pond or surface
of still water is disturbed. Ether is so fine a substance that the
organs of sense are not delicate enough to detect it, and it is of such
a volatile and uneasy nature that it is continually in motion. It
vibrates under certain conditions, and when disturbed (as by a dynamo)
it undoubtedly forms the active principle of electricity and magnetism.
James Clark Maxwell believed that magnetism, electricity, and light are
all transmitted by vibrations in one common ether, and he finally
demonstrated his theory by proving that pulsations of light,
electricity, and magnetism differed only in their wave lengths. In 1887
Professor Hertz succeeded in establishing proof positive that Maxwell’s
theories were correct, and, after elaborate experiments, he proved that
all these forces used ether as a common medium. Therefore, if it were
not for the ether, wireless telegraphy, with all its wonders, would not
be possible. We understand, then, that the waves of ether are set in
motion from a central disturbing point, and this can be accomplished
only by means of electrical impulse.
Suppose that we strike a bell held high in the air. The sound is the
result of the vibrations of its mass sending its pulsating energy
through the air. The length of the sound-waves is measured in the
direction in which the waves are travelling, and if the air is quiet and
not disturbed by wind the sound will travel equally in all directions.
The sound of a bell will not travel so well against a wind as it will
with it, just as the ripples on a pond would be checked by an adverse
set of wavelets.
Now the ether can be made to vibrate in a similar manner to the air by a
charge of electricity oscillating or surging to and fro on a wire
several hundred thousand times in a second. These oscillations strike
out and affect the surrounding ether, so that, according to the
intensity of the disruptive charge at the starting-point, the ether
waves may be made to reach near or distant points.
This is, perhaps, more clearly shown by the action of a pendulum. In
Fig. 9 the rod and ball are at rest, but if drawn to one side and
released it swings over to the other side nearly as far away from its
central position of rest as from the starting-point. If allowed to swing
to and fro it will oscillate until at last it will come to rest in a
vertical position. This same oscillation (oscillation being a form of
vibration) takes place in the water when a stone has been flung into it,
and in the ether when affected by the electrical discharge. In Fig. 10
are shown the principal varieties of vibration--the oscillating,
pulsating, and alternating.
It is known that if these oscillations are damped, so that the
over-intense agitation of the central disturbance is lessened, a new
series of vibrations, such as the pulsating or alternating, is set up,
and these secondary vibrations possess the power to travel around the
world--yes, and perhaps to other worlds in the planetary cosmos.
[Illustration: FIG. 9
FIG. 10
FIG. 11
OSCILLATION AND VIBRATION]
The study of ether disturbances, wave currents, oscillating currents,
and the other phenomena dependant upon this invisible force is most
interesting and fascinating, and were it possible to devote more space
to this topic several chapters could be written on the scientific theory
of wireless telegraphy.[2]
[2] For further information on this subject the student is referred to
such well-known books as _Signalling Across Space Without Wires_, by
Prof. Oliver J. Lodge, and _Wireless Telegraphy_, by C. H. Sewall.
The principle difference between wire, or line, and wireless telegraphy
is that the overhead wire, or underground or submarine cable, is
omitted. In its stead the ether of the air is set in vibratory motion by
properly constructed instruments, and the communication is recorded at a
distance by instruments especially designed to receive the transmitted
waves.
It seems to be the popular impression that a wireless message sent from
one point to another travels in a straight line, as indicated by Fig.
11, B representing Boston, which receives the message from N. Y., or New
York. As a matter of fact, if several sets of wireless receiving
instruments were located on the circumference of a circle the same
distance from New York in all directions, or even at nearer or farther
points, they would all receive the same message. Instead of travelling
in one direction, the ether waves are set in motion by the electrical
disturbance, just as water is agitated by the stone thrown into it. The
ripples, or wavelets, are started from the central point of disturbance
and radiate out, so that instead of reaching Boston only the waves
travel over every inch of ground, or air space, in all directions, and
would be recorded in every town and village within the sphere of energy
set up by the original force that put the ether waves in motion. The
stronger this initial force the wider its field of action. This is shown
at Fig. 12, which is an area comprising Philadelphia, Pittsburg,
Buffalo, Washington, and other cities. Moreover, the waves of electrical
disturbance would carry far beyond in all directions, taking in the
cities of the north, south, and west, and at the east, going far out to
sea, beyond Boston harbor and below Cape Hatteras, where ships carrying
receiving instruments could pick up the messages. Like the ripples on
the water, the radiating waves, or rings, become larger as they reach
out farther and farther from the centre of disturbance, until at last
they are imperceptible, and lose their shape and force.
[Illustration: FIG. 12]
At great distances, therefore, the ether disturbance becomes so slight
that it is impossible to record the vibration or message sent out; and
until some improved forms of apparatus and coherer are invented, or the
original disturbing force is enormously increased, it will be impossible
to send messages at longer distances than four or five thousand miles
from a central point. Both Marconi and De Forrest assert that they are
perfecting coherers which will make it possible to girdle the earth with
a message, and that within the next few years an aerogram may be sent
out from a station, and, after instantly encircling the earth and being
recorded during its passage at all intermediate stations, it will return
and be received at the original sending-point. This, of course, is a
matter of future achievement; but now that messages across the Atlantic
are a commercial fact, it seems quite possible that the greater feat of
overriding space and reaching any point on the earth’s surface will soon
be a reality. And now to proceed from theory to the construction of a
practical wireless apparatus having a radial area of action over some
ten or fifteen miles.
The principal parts of a wireless apparatus include the antennas (or
receiving and sending poles with their terminal connections), the
induction-coil, strong primary batteries or dynamo, the coherer and
de-coherer, the telegraph key and sounder (or a telephone receiver), and
the necessary connection wires, binding-posts, and ground-plates.
A large induction-coil with many layers of fine insulated wire will be
necessary for the perfect operative outfit. The most practical coil for
the amateur is a Ruhmkorff induction-coil. (See the directions and
illustrations for constructing this coil, beginning on page 59 of
chapter iv.)
The sending apparatus is practically the same in all outfits, and
consists of a source of electrical energy, such as a battery, or dynamo,
the essential induction-coil and adjustable spark-gap between the brass
balls on terminal rods, and the make-and-break switch, or telegraph-key.
It is in the various forms of coherers and receiving apparatus that the
different inventors claim superiority and originality. The systems also
differ in their theory of harmonic tuning or vibratory sympathy. This is
accomplished by means of coils and condensers, so that the messages sent
out on one set of instruments will not be picked up or recorded by the
receiving apparatus of competitors.
Having made or purchased an induction-coil of proper and adequate size,
it will now be necessary to construct the parts so that an adjustable
spark-gap may be secured.
Make a hollow wooden base for the induction-coil to rest on. It should
be a trifle longer than the length of the coil and about seven inches
wide. This may be made from wood half an inch thick. The base should be
two inches high, so that it will be easy and convenient to make wire
connections under it. Mount the induction-coil on the base and make it
fast with screws, arranging it so that the binding-posts are on the side
rather than at the top of the coil, as shown in Fig. 13.
Cut a thin board and mount it across the top of the induction-coil on
two short blocks, and to this attach two double-pole binding-posts (P
P). The fine wires from the induction-coil are made fast to the foot of
each post, and from the posts the aerial wire (A W) and ground wire (G
W) lead out.
Fasten two binding-posts at the forward corners of the base, and to them
make connection-wires fast to the heavy or primary wires of the coil.
Wires B and C lead out from these posts to the battery and key, and to
complete this part of the sending, or transmitting apparatus it will be
necessary to have two terminal rods and balls attached to the top of the
binding-posts (P P). This part of the apparatus is generally called the
oscillator, and the rods are balanced on the posts, so that they can be
moved in order to increase or diminish the space (S G), or spark-gap,
between the brass balls.
When, after experiment, the proper space has been determined, the set
screw at the top of the posts will hold the terminal rods securely in
place.
Obtain a piece of brass, copper, or German-silver rod three-sixteenths
of an inch in diameter. Now cut two short rods, each six inches long,
and two inches from one end flatten the rods with a hammer, as shown at
A in Fig. 14. Flatten the rod in two places at the other end, as shown
at B B in Fig. 14; then bore holes through the flattened parts (A), so
that the binding-screws at the top of the posts (P P) will pass through
them.
Obtain two brass balls from one to one inch and a half in diameter. If
they are solid or cast brass they may be attached to the ends of the
terminal rods by threading, so that it will be easy to remove them. If
the balls are of spun sheet-metal it will be necessary to solder them
fast to the ends of the rods, and, when polishing the balls, the rods
will have to be removed from the binding-posts. It is imperative that
the balls should be kept polished and in bright condition at all times,
to facilitate the action of the impulsive sparks.
[Illustration: FIG. 13]
[Illustration: FIG. 14]
To counterbalance these balls there should be handles at the long ends
of the rods. These handles may be of wood, or made of composition molded
directly on the rods. A good composition that can be easily made and
molded is composed of eight parts plaster of Paris and two parts of
dextrin made into a thick paste with water. The dextrin may be purchased
at a paint-store, and is the color of light-brown sugar. Mix the dry
plaster and dextrin together, so that they are homogeneous; then add
water to make the pasty mass. Use an old table-knife to apply the wet
composition to the bars. The flattened parts will help to hold the mass
in place until it sets. It is best to make two mixtures of the paste and
put one on first, leaving it rough on the surface, so that the last coat
will stick to it. When the last coat is nearly dry it may be rubbed
smooth with the fingers and a little water, or allowed to dry hard, and
then smoothed down with an old file and sand-paper.
If solid brass balls are used for the terminals the composition handles
may be made heavier; but in any event the proper amount of composition
should be used, so that when the rod is balanced on a nail or piece of
wire passed through the hole it will not tip down at one end or the
other, but will remain in a horizontal position.
The overhead part of the apparatus employed to collect the electric
waves is called the antennæ, and in the various commercial forms of
wireless apparatus this feature differs. The general principle, however,
is the same, and in Figs. 15, 16, 17, and 18 some simple forms of
construction are shown.
Great care must be taken to properly insulate the rod, wire, or fingers
of these antennæ, so that the full force of the vibration is carried
directly down to the coherer and sounder or receiver. For this purpose,
porcelain, glass, or gutta-percha knobs must be employed.
In Fig. 15 the apparatus consists of an upright stick, a cross-stick,
and a brace, or bracket, to hold them in proper place.
Porcelain knobs are made fast to the sticks with linen string or stout
cotton line. Then an insulated copper wire is run through the holes in
the knobs, and from the outer knob a rod of brass, copper, or
German-silver, or even a piece of galvanized-iron lightning-rod, is
suspended. Care should be taken to see that the joint between rod and
wire is soldered so as to make perfect contact. Otherwise rust or
corrosion will cause imperfect contact of metals, and interrupted
vibrations would be the result. The upright stick should be ten or
fifteen feet high, and may be attached to a house-top, a chimney, or on
the corner of a barn roof.
Another form of single antenna is shown in Fig. 16. This is a rod held
fast in a porcelain insulator with cement. The insulator, in turn, is
slipped over the end of a staff, or pole, which is erected on a building
top or out in the open, the same as a flag-pole. Near the foot of the
rod, and just above the insulator, a conducting-wire is made fast and
soldered. This is run down through porcelain insulators to the
apparatus.
If the pole is erected on a house-top it may be braced with wires, to
stay it, but care must be taken not to have these wires come into
contact with the rod, or conducting-wire.
[Illustration: FIG. 15
FIG. 16
FIG. 17
FIG. 18
TYPES OF ANTENNÆ]
Another form of antennas is shown in Fig. 17, where rods are suspended
from a wire which, in turn, is drawn taut between two insulators. The
insulators are held in a framework composed of two uprights and a
cross-piece of wood.
This frame may be nailed fast to a chimney and to the gable of a roof,
as shown in the drawing; and to steady the rods, so that they will not
swing in a high wind, the lower ends should be tied together with cotton
string, the ends of which should be fastened to the uprights. The
leading-in wire is made fast to the top wire, from which the rods are
suspended, and all the exposed joints should be soldered to insure
perfect contact and conductivity. A modified form of the Marconi antennæ
is shown in Fig. 18. This is made of a metal hoop three of four feet in
diameter held in shape by cross-sticks of wood, which can be lashed fast
to the ring. Leading down from it are numerous copper wires which
terminate in a single wire, the whole apparatus resembling a funnel. The
upper unions where the wires join the ring need not be soldered, but at
the bottom, where they all come together and join the leading-in wire,
it is quite necessary that a good soldered joint be made. This funnel
may be hung between two upright poles on a house-top, or suspended from
the towers or chimneys.
Almost any metal plate will do for the ground, or the ground-wire (G W
in Fig. 13) may be bound to a gas or water pipe which goes down deep in
the ground, where it is moist. Rust or white lead in the joints of
gas-mains sometimes prevent perfect contact, but in water-pipes the
current will flow readily through either the metal or the water. To
insure the most perfect results, it is best to have an independent
ground composed of metal, and connected directly with the oscillator, or
coherer, by an insulated copper wire. A simple and easily constructed
ground is a sheet of metal, preferably copper, brass, or zinc, to the
upper edge of which two wires are soldered, as shown in Fig. 19. This is
embedded in the ground three or four feet below the surface. Another
ground-plate is a sheet of metal bent in [V] shape and then inverted.
Two wires are soldered to the angle, and the ends brought together and
soldered. This ground is buried three or four feet deep, and stands in a
vertical position, as shown at Fig. 20. At Fig. 21 a flat ground is
shown. This is a sheet of metal cut with pointed ends. The ground-wire
is soldered to the middle of it, and it is then buried deep enough to be
embedded in moist earth.
One of the best grounds is an old broiler with a copper wire soldered to
the ends of the handles, as shown at Fig. 22. This is buried deep in the
ground in a vertical position, and the insulated copper wire is carried
up to the instruments.
The most important part of the wireless telegraphic apparatus is now to
be constructed, and this requires some care and patience. The coherer is
the delicate, sensitive part of the apparatus on which hinges success or
failure. There are various kinds of coherers designed and used by
different inventors, but while the materials differ and the construction
takes various forms, the same basic principle applies to all.
[Illustration: FIG. 19
FIG. 20
FIG. 21
FIG. 22
TYPES OF GROUNDS]
The coherer can best be explained as a short glass tube in which iron or
other metallic filings are enclosed. Corks are placed in both ends of
the tube, and through these corks the ends of wire are passed, so that
they occupy the position shown in Fig. 23, the ends being separated a
quarter of an inch. Metal filings will not conduct an electric current
the same as a solid rod or bar of the same metal, but resist the passage
of current.
After long periods of experimenting with various devices to detect the
presence of feeble currents, or oscillations, in the ether, the coherer
of metal filings was adopted. When the oscillations surge through the
resonator, the pressure, or potential, finally breaks down the air film
separating the little particles of metal, and then gently welds their
sharp edges and corners together so as to form a conductor for the
current. Before this process of cohesion takes place these fine
particles offer a very high resistance to the electrical energy
generated by a dry cell or battery--so much so that no current is
permitted to pass. But once the oscillations in the ether cause them to
cohere--presto! the resistance drops from thousands of ohms to hundreds,
and the current from the dry cell now flows easily through the coherer
and deflects the needle of a galvanometer. This is the common principle
of all coherers of the granulated metal type, although there are many
modifications of the idea.
The action of the electric and oscillatory currents on particles of
metal can best be understood by placing some fine iron filings on a
board, as shown at Fig. 24, and then inserting the aerial and ground
wires in the filings, but separated by an eighth or a quarter of an
inch. A temporary connection may be made as shown in Fig. 25.
[Illustration: FIG. 23]
[Illustration: FIG. 26]
[Illustration: FIG. 27]
[Illustration: FIG. 24]
[Illustration: FIG. 25]
A A are aerials on both instruments; C is the open coherer, or board
with iron filings, in which the ends of the aerial and ground wires are
embedded; D C is a dry cell; and R is a telegraphic relay, or sounder.
If the wire across C was not parted and covered with filings, the dry
cell would operate R, but the high resistance of the particles of metal
holds back the current.
On the opposite side, I C is the induction-coil; K is the telegraphic
key, or switch, which makes and breaks the current; S B is the
storage-batteries, or source of electric energy; and S G the spark-gap
between the brass balls on the terminal rods. By closing the circuit at
K the current flows through the primary of the induction-coil, affects
the secondary coil, and causes a spark to leap across the gap between
the brass balls. This instantly sets the ether in motion from A on the
right, and the impulse is picked up by A on the left. This oscillation
breaks down the resistance of the filings at C, and the current from
battery, or dry cell (D C), flows through the filings and operates the
sounder, or relay (R). This operation takes place instantly, and the
particles of metal are seen to cohere, or shift, so that better contact
is established. But as soon as the spark has jumped across the gap the
action of cohesion ceases until the key (K) is again operated to close
the circuit and cause another spark to leap across the gap. The shifting
of the metal particles on the board (C) is what takes place in the glass
tube of the coherer, Fig. 23, but in this confined space the particles
will not drop apart again as on the flat surface, but will continue to
cohere. A de-coherer is necessary, therefore, to knock the particles
apart, so that the next oscillatory impulse will have a strong and
individual effect. There are several forms of de-coherers in use, but
for the amateur telegrapher an electric-bell movement without the bell,
or, in other words, a buzzer with a knocker on the armature, will answer
every purpose. (See description of buzzer on page 64.) It must be
properly mounted, so that on its back stroke, or rebound, the knocker
will strike the glass tube and shake the particles of metal apart. For
this purpose the vibrations of the armature should be so regulated as to
obtain the greatest possible speed, in order that the dots and dashes
(or short and long periods) will be accurately recorded through the
coherer and made audible by the sounder or telephone receiver.
Another form of coherer is shown in Fig. 26. This is made of a small
piece of glass tube, two rods that will accurately fit in the tube, some
nickel filings, two binding-posts, and a base-block three inches and a
half long. The two binding-posts are mounted on the block, and through
the holes in the body of the posts the rods are slipped. They pass into
the tube, and the blunt ends press the small mass of filings together,
as shown in the drawing. By means of the binding-posts these
coherer-rods may be held in place and the proper pressure against the
filings adjusted; then maintained by the set-screws. The nickel filings
may be procured by filing the edge of a five-cent piece. Obtain a few
filings from the edge of a dime and add them to the nickel, so that the
mixture will be in the proportion of one part silver to nine parts
nickel. This mixture will be found to work better than the iron filings
alone. The aerial and ground wires are made fast to the foot-screws of
the binding-posts, and the base on which the coherer is mounted may be
attached to a table or ledge on which the other parts of the receiving
and recording apparatus are also installed.
Another form of coherer is shown at Fig. 27. This is constructed in a
somewhat similar manner to the one just described. A glass tube is
provided with two corks having holes in them to receive the
coherer-rods. Two plugs of silver are arranged to accurately fit within
the tube, and into these the ends of the coherer-rods are screwed or
soldered. Between these silver plugs, or terminals, the filings of
nickel and silver are placed, and the rods are pushed together and
caught in the binding-posts. The aerial and ground wires are made fast
to the foot-screws of the posts.
For long-distance communication it is necessary to have a condenser
placed in series with the sparking or sending-out apparatus. (See the
type of condenser described and illustrated in chapter iv., page 72.)
An astatic galvanometer is also a valuable part of the receiving
apparatus, and the one described on page 111 will show clearly the
presence of oscillatory currents by the rapid and sensitive deflections
of the needle.
For local service, where a moderately powerful battery is employed, a
telegraph-key, such as described on page 190, will answer very well, but
for high-tension work, where a powerful storage-battery or small dynamo
is employed, it will be necessary to have a non-sparking key, so that
the direct current will not form an arc between the terminals of a key.
Most of the keys used for wireless telegraphy have high insulated
pressure-knobs, or the make and break is done in oil, so that the spark
or arc cannot jump or be formed between the points.
The plan of a simple non-sparking dry switch is shown at Fig. 28. This
is built up on a block three inches wide and five inches long. It
consists of a bar (A), two spring interrupters (B and C), a spring (D),
and the binding-posts (E E). They are arranged as shown in Fig. 28, and
a front elevation is given in Fig. 29. The strip (B) lies flat on the
block, and is connected with one binding-post by a wire attached under
one screw-head and run along the under side of the base in a groove to
the foot of the post. Strip C is of spring-brass, and is made fast to
the base with screws. This is “dead,” as no current passes through it,
and its only use is to interrupt. The bar (A) is arranged as explained
for the line telegraph-key, and the remaining binding-post is connected
to it by a wire run under the base and brought up to one of the
angle-pieces forming the hinge. A high wood or porcelain knob is made
fast at the forward end of the bar, so that when high-tension current is
employed the spark will not jump from the bar to the operator’s hand.
The complete key ready for operation is shown at Fig. 30, and to make it
permanent it should be screwed fast to the table, or cabinet, on which
the coil and condenser rest. The plan of a “wet” key is shown in Fig.
31, and the complete key in Fig. 32.
[Illustration: FIG. 28
FIG. 29
FIG. 30
FIG. 31
FIG. 32
DRY AND WET NON-SPARKING SWITCHES]
A base of wood three by five inches is made and given several coats of
shellac. Obtain a small rubber or composition pill or salve box, and
make it fast to the front end of the base with an oval-headed brass
screw driven down through the centre of the box. A wire leading to one
binding-post is arranged to come into contact with the screw, and the
other post is connected by wire to one hinge-plate supporting the bar.
The long machine screw, or rivet, passed down through the knob and into
the bar, extends down below the bar for half an inch or more, so that
when the knob is pressed down the end of the screw, or rivet, will
strike the top of the screw at the bottom of the box without the bar
coming in contact with the edge of the box. When in operation the
composition box is filled with olive oil or thin machinery oil, so that
when contact is made by pressing the knob down the circuit will be
instantly broken, the spring at the rear end of the bar drawing it back
to rest. The oil prevents any sparks jumping across; and also breaks an
arc, should one form between the contact-points. With the addition of a
good storage-battery (the strength of which must be governed by the size
of the induction-coil and the distance the messages are sent) and a
dry-cell or two for the receiving apparatus, the parts of the wireless
apparatus are now ready for assembling. Full directions for making
storage-cells is given in chapter ii., page 21, and for dry-cells in
chapter ii., page 29. For short-distance work the plan shown in Figs. 33
and 34 will be found a very satisfactory form of apparatus. One of each
kind of instrument should be at every point where communication is to be
established.
In the sending apparatus (Fig. 33) S C are the storage-cells, K the key,
and I C the induction-coil. T T are the terminals and balls, S G the
spark-gap, and P P the posts that hold the terminal rods. A W is the
aerial wire running up from one post, and G W the ground-wire connecting
the other terminal post with the ground-plates.
In the receiving apparatus (Fig. 34) C is the coherer, D C the
de-coherer, T S the telegraphic sounder, or relay, and A G the astatic
galvanometer. B is the dry-cell, or battery, and D C S the de-coherer
switch, so that when the apparatus is not in use the dry-cell will not
operate the buzzer or de-coherer. A W is the aerial wire and G W the
ground-wire. Two or more storage-cells may be connected in series (that
is, the negative of one with the positive pole of the other) until a
sufficiently powerful source of current is secured for the transmission
of messages.
[Illustration: FIG. 33]
[Illustration: FIG. 34]
To operate the apparatus, the circuit is closed with K, and the current
from S C flows around the primary coil in I C and affects the secondary
coil, causing the spark to leap across the gap (S G). This causes a
disturbance through the wires A W and G W, and the ether waves are set
in oscillatory motion from the antennæ on the house-top. This affects
the antennæ at the receiving-point, and the impression is recorded
through the coherer (C) on the telegraphic sounder or relay (T S), which
is operated by the current from dry-cell or battery (B), since the
oscillations have broken the resistance of the filings in the coherer
(C). The instant that the current passes through the coherer and
operates T S, the astatic galvanometer indicates the presence of current
by the deflected needle.
[Illustration: FIG. 35]
When the apparatus is in operation D C S is closed, so that the current
from B operates the coherer (D C). Directly communication is broken
off, the switch (D C S) should be opened; otherwise the buzzer would
keep up a continuous tapping. For long-distance work a more efficient
sending apparatus is shown in Fig. 35. This is composed of an
induction-coil, with the terminal rods and brass balls forming the
spark-gap, an oil key (K), and three or more large storage-cells, or a
dynamo (if power can be had to run it). A condenser is placed in
connection with the aerial and ground wires, so that added intensity or
higher voltage is given the spark as it leaps across the gap. In
operation this apparatus is similar to the one already described. Where
contact is made with K the primary coil is charged, and by induction the
current affects the secondary coil, the current or high voltage from
which is stored in the condenser. When a sufficient quantity is
accumulated the spark leaps across S G and affects wires A W and G W.
This action is almost instantaneous, and directly the impulse sets the
ether in motion the same impulse is recorded on the distant coherers and
sounders.
There are a great many modifications of this apparatus, but the
principles are practically the same, and while the construction of this
apparatus is within the ability of the average boy, many of the more
complicated forms of coherers and other parts would be beyond his
knowledge and skill. Marconi has realized his ambition to send messages
across the ocean without wires, and is now doing so on a commercial
basis, and at the rate of twenty-five words a minute. It is but the next
step to establish communication half-way around the world, and finally
to girdle the earth.