application of the electro-motor to the propulsion of carriages on
tramways and railways. For the latter, indeed, an attempt was made
half-a-century ago on the Edinburgh and Glasgow railway, to employ the
force of an electro-magnetic machine actuated by a battery. This was in
1842, and although this electric locomotive was fitted up completely, it
did not attain a speed of more than four miles an hour. The weight with
the batteries, carriage, etc., exceeded five tons. But in the recent
inventions which have been in practical operation in many places, it is
found quite easy to dispense with any current producer on the electric
locomotive itself, for the electricity is supplied by a fixed dynamo and
the current is transmitted along the line by a conductor from which a
sliding contact conveys it to the electro-motor, which is attached to
the framework of the carriage and acts on the driving axles of the
wheels directly or by toothed gear. In such cases the return current is
carried either by another conductor or by the rails themselves. In
another arrangement one rail conveys the current to the locomotive and
the other returns it. When the rails are so used they have, of course,
to be insulated from the ground and laid with special electrical contact
pieces joining their consecutive lengths, and all the carriage wheels
have to be insulated, so that the currents shall flow only through the
coils of the electro-motor. A railway on this system has been worked at
Berlin for some time, and a short tramway on the same plan has lately
been opened at Brighton. The Bessborough and Newry Electric Railway
(Ireland) uses a single separate conductor three miles long, and the
power is supplied at a very small cost from a dynamo station near the
middle of the line, where water power is taken advantage of to drive a
large turbine. Quite recently electric propulsion has been adopted on
some of the short tunnel lines in London, and it is quite probable that
ultimately the system will be adopted throughout the whole course of the
underground railways, with the view of obtaining a purer and more
agreeable atmosphere.
[Illustration:
FIG. 280_i_.—_Poles with Single Arms for Suburban, Roads.—The Ontario
Beach Railway, Rochester, N. Y._
]
A very light electric railway has been designed, in which the cars run
along rails attached to posts at such a height above the ground as may
be required to make the line level, or with only slight gradients. The
rails also serve as conductors. This is known as the telepherage system,
and it is found to be well adapted for light loads in an undulating
country.
[Illustration:
FIG. 280_j_.—_The Glynde Telepherage Line, on the system of the late
Fleeming Jenkin._
]
The other plan which makes use of accumulators commends itself for
application to ordinary tramway carriages, because no conductors are
required along the line, and each car can move independently. The chief
objection is the great weight of the accumulators and the space they
occupy, although they are usually placed under the seats without much
inconvenience. There are at present (January, 1890) six electric
tramcars running in London, and the accumulator system would no doubt
have been applied largely as the motive power for the ordinary street
omnibus, but for the difficulty of controlling them under the momentum
of the great mass of the accumulators, etc. The same objection lies
against the use of the accumulators and motors for propelling tricycles,
although such machines have really been used. But accidents such as
occasionally happen to such vehicles would be attended with additional
risks of injury from the acids of the secondary battery, etc. But there
is one mode of using electric propulsion, that is free from every
objection and, indeed, offers great advantages. Only two years ago the
first electric boat on the Thames was tried experimentally between
Richmond and Henley, and the result was entirely in favour of the
electric over the steam launch. The Faure battery, or so-called “storage
cells,” are arranged beneath the floor of the boat for most of its
length in the smaller boats, and the electro-motor is directly coupled
with the screw shaft. The electric launch has these advantages: perfect
safety, freedom from dirt and smoke, no thumping or vibrating, no noise
of steam discharge, or smell of hot oil, no engineer or stoker is
required, and much larger space available for passengers. One of these
electric launches, not going full speed, is able to travel sixty miles
without having the accumulators recharged. A considerable number of
these launches are already in use, and many more are in course of
construction. They are made of all sizes, from the smallest to those
that will carry quite a large company, and may be used for excursion
parties on the river. The description of one of these last states that
she is 65 feet in length, and 10 feet across the beam. She can carry
sixty passengers, and twenty can dine in the saloon at one time. There
are lavatories, pantries, dressing rooms, etc., and a brass railed upper
deck, with an awning. At night this boat is lighted up with electric
glow-lamps, the current for these also being supplied by the
accumulators. The Electric Launch Company has stations with Gramme
machines at work to charge cells ready to replace exhausted ones at
several places, namely Hampton, Staines, Maidenhead, Boulter’s Lock,
Henley, Reading and Oxford. There is every prospect of a general
extension of the electric propulsion of boats, and visitors to the
Electrical Exhibition at Edinburgh, in 1890, will find electric launches
taking holiday makers as far as Linlithgow. The boats will be like those
on the Thames, fitted with the Immisch motor. Some electricians are now
sanguine enough to believe that even for large vessels electricity will
yet be able to compete with steam in special cases.
The modes of using electric propulsion that we have just noticed furnish
a very interesting chain of conversions of one form of force into
another, with a reversal of the order of transformation at a certain
point. Let us begin with the carbonic acid gas that existed in the
atmosphere of the carboniferous geological period. The solar emanations
were absorbed, and used by the leaves of the plants to separate the two
elements of the gas,—the plant retaining the one in its substance and
returning the other to the air. The plant becomes coal; and ages
afterwards the particles of the two separated elements are ready to
re-unite and give out in the form of heat all the energy that was
absorbed by their separation. This heat is in the steam-engine converted
into the energy of mechanical power. This mechanical power is in the
dynamo expended in moving copper wires through a magnetic field. Every
schoolboy who has played with a common steel magnet—and what boy has
not?—knows that the space immediately round the magnet is the seat of
strange attractive and repulsive force, for he has felt their pulls and
pushes on pieces of iron or steel. This mysterious space is the magnetic
field, and although a person would not be able to perceive that
mechanical force is expended when he moves a single copper ring across
such a field, he will readily become conscious of the fact when he moves
a number at once that form a closed circuit; and he should not omit the
opportunity of feeling this for himself if he is allowed to turn the
handle of such a machine as that represented in Figs. 275 or 277. The
mechanical power is absorbed in the dynamo because the movement induces
an electric current that would of itself produce motion in the machine
in the opposite direction. However, the electricity induced by magnetism
and motion is made to pass through the Faure cell or accumulator, when
it does chemical work by separating oxide of lead from sulphuric acid,
leaving these substances in a position to unite together again, when
this action produces a reverse current of electricity through an
external metal circuit. The coils of the electro-motor form this
circuit; the electricity induces magnetism, and the magnetism gives rise
to visible motion and mechanical power.
From what has been already said, it will be obvious that a pair of
covered copper wires connecting a dynamo with an electro-motor becomes a
very convenient means of _carrying power_ from one place to another.
There are situations in which shafts, belts, or any other mechanical
expedients are troublesome or impossible to use for this purpose. For
instance, a dynamo working at the mouth of a tunnel or coal-pit may be
made to drive any machinery within with nothing between but the
motionless wires. Or a single dynamo will supply moderate power to a
number of small workshops, provided each has an electro-motor, with no
other connection than a pair of copper wires. This arrangement is found
very advantageous for light work and where power is required
occasionally, as in watch-making, the manufacture of philosophical
instruments, etc. Such moderate power is occasionally in demand also in
private houses, to drive sewing machines, lathes, etc.; and it is
obtainable from the same source as the current for lighting. Private
installations for lighting purposes usually have a dynamo driven by a
gas engine, and working into a set of accumulators. It seems not a
little remarkable that if the gas were burnt in the ordinary way instead
of being used in the gas engine, it would give only a fraction of the
amount of the light it causes to be given out by the electric light
lamps. But at the present time, houses and business premises are
supplied with electricity by companies who carry electric mains through
the streets. In England these electric mains, which are thick insulated
copper wires, are inclosed in iron pipes and laid beneath the pavement,
like the gas mains. In the United States, where electric illumination is
much used, the conductors have been usually carried overhead like
telegraph wires, but not a few fatal accidents have occurred from these
conductors falling into the streets. There is no reason to doubt but
that in a short time it will be as common for households to draw upon
such electric mains for their supply of light and power as it now is to
draw gas and water from common mains. The electric supply companies have
central stations in suitable positions, where very large and powerful
dynamos are regularly driven by steam power. These stations are provided
with appliances for measuring the currents and for duly controlling the
energy sent out. What will appear very extraordinary when we remember
that electricity is in itself unknown, is that the quantity supplied to
each house or establishment can be actually measured, and is paid for by
meter as in the case of gas. As already said (page 498) electricity can
only be measured by its effects, and it is the chemical effect which it
is found convenient to use for the purpose we are speaking of. The plan
is simply this: two plates of zinc dip into a solution of sulphate of
zinc, and from the one to the other there is sent through the solution
one-thousandth part of the current to be measured. While the current
passes, zinc is deposited on the plate towards which the current goes in
the solution, and if this plate is periodically weighed this furnishes
the measure of the total current. But how is just one-thousandth of the
whole current taken off from the rest and made to circulate through the
measuring apparatus? This is very easily done by taking advantage of the
law of derived circuits, which for our present purpose may be stated
thus: when a current of electricity finds two different circuits along
which it can pass, it will divide and circulate through both of them,
but the greater part will pass through the circuit of less resistance
(if there be any inequality), and by adjusting the resistances of the
circuits we can divide the current between the two partial or derived
circuits in any required proportions. Electric resistances, it may be
mentioned, depend upon the length, section, and nature of the conductor,
and are very easily measured and adjusted.
While the method just explained serves very well to measure the quantity
of electricity that has passed through a conductor in a given period,
provided that the current has always been in the same direction, it will
be sufficiently obvious that it would fail altogether in the case of
alternating currents. And, in fact, even in the case supposed this mode
of measurement does not take account of the real energy set in motion. A
reference to page 498, where the differences of electric currents are
mentioned that are commonly spoken of—_tension_ and _quantity_—will show
that electric effects depend upon more than the _quantity_ of
electricity passing. Forms of apparatus have been devised for recording
the total energy supplied; but their construction and principles are too
complex to be here explained. In some cases high tension currents are
required, in others it is quantity and not tension that is sought for;
and there are ways of transforming the qualities of currents so that the
same source shall supply electricity of either class. An example of this
may have been noticed in the action of the Ruhmkorff coil, where the
mere interruption of the primary or battery circuit, which possesses so
little tension that of itself it could not give rise to a spark,
nevertheless produces a wave of electricity in the secondary circuit of
a tension so high that sparks several feet long may be produced by it.
A somewhat recent application of the electric current of the dynamo may
be just mentioned here. It is what is known as electrical welding, and
depends upon the heat developed by currents being proportioned to the
electrical resistance for each part of the circuit. The heat thus
generated, where the current passes between two surfaces of metal, even
of considerable dimensions, is sufficient to bring them to a semi-fluid
condition, so that when simply pressed together they coalesce into one
mass. In this way pieces of iron work can be welded together in
situations where it would be either inconvenient or impossible to heat
them by furnaces.
The reader who has followed the last article will probably be prepared
to admit that “the magnetic field” is one of the most wonderful things
in the whole realm of inorganic nature, as all the powerful effects we
have been describing are the results of merely moving wires through it.
A wire conveying an electrical current so modifies the space surrounding
it, or so acts upon the unknown pervading medium, that conductors moved
in it, have other currents generated in them. An intermittent current,
like that in the primary circuit of the induction coil, is equivalent to
a movement of the magnetic field in regard to the secondary coil, so
that the general principle in the coil and the dynamo is fundamentally
the same. Quite recently, Professor Elihu Thomson has shown some very
novel mechanical effects of repulsions and rotations of conductors
placed near the poles of a coil through which rapidly alternating
currents are passing. [1890.]
We already hear of natural forces which have hitherto in a manner run to
waste being now utilised in man’s service by the advantage taken of the
capability of a slender wire to convey power. A notable instance is in
the case of the famous Falls of Niagara. Here the head of water is used
to drive turbines; our readers must not run away with any notion of huge
water-wheels being placed below the falls. But from the high level of
the water above the falls a tunnel has been cut which brings the water
into pipes 7½ feet in diameter, and these deliver it into three
turbines, in passing through which it develops a force of 5,000 horse
power, and this force is communicated to a steel shaft 2½ feet in
diameter, connected with the revolving parts of the dynamo. Mr. G.
Forbes, the engineer, states that the company who have undertaken this
enterprise are supplying, with a handsome profit to themselves,
electrical current or power at ⅛th of a penny per unit, for which
English companies charge sixpence. That is, Niagara supplies power at
1/48th of the price it can be obtained from coal.
The fact that mechanical power can be brought from a distance to
everyone’s door by a slender wire, and at small cost, suggests the
possibility of great social and industrial changes being effected in the
future by that one condition. Think of the abolition of factory chimneys
and smoke, nay, even of the abolition of the factory system itself, for
cheap power transmission seems to promise much in that direction, and
there is a shadowing forth of still more in
_THE NEW ELECTRICITY._
The Leyden jar and a few of its most obvious and common effects have
been touched upon already, (page 490); but the phenomena which are
revealed by a careful study of its charge and discharge show that these
are by no means of the simple kind that has generally been supposed.
Thus, for instance, if the magnetising effects of what is called current
electricity be borne in mind, especially the _definiteness_ of this
action as regards the _direction_ of the current (_cf._ Fig. 257), it
would follow that if instead of the iron bar in Fig. 265 we place within
the coil some unmagnetised steel needles we should find after passing a
current or discharge that these have become converted into permanent
magnets, and that their north poles are always towards the left of the
supposed current. Years ago experiments were made to ascertain whether
the discharges of a Leyden jar repeatedly passed through a coil would
magnetise needles in the same way, because it had been assumed that the
discharge is simply a current of extremely short duration and of quite
definite direction. As far back as 1824 it had, however, been observed
that the needles were magnetised sometimes in the wrong direction, yet
no attempt was made to explain this—it was sometimes merely mentioned in
the books as “anomalous magnetisation.” Dr. Henry of Washington, U.S.A.,
experimented on the subject, and in 1842 referred this action to a
condition of the discharge which had never before been suggested. He
says “we must admit _the existence of a principal discharge in one
direction, and then several reflex actions backward and forward, each
more feeble than the preceding, until the equilibrium is obtained_.”
Some five years afterwards Helmholtz had independently arrived at the
same conclusion, and from the fact that when a _succession_ of Leyden
jar discharges are sent through the voltameter (Fig. 263) the water is
indeed decomposed, but _both_ oxygen and hydrogen are evolved at _each_
electrode. Sir William Thomson (now Lord Kelvin) examined the question
from a theoretical point of view, and in a masterly mathematical paper
published by him in 1853 not only showed that the discharge must be of
an oscillating character, but gave the form of equation by which the
rate of oscillation is determined.
Faraday proved, as has already been stated, that the matter of the
dielectric takes part in such condensing actions as that of the Leyden
jar. The electrical charge enters into the glass, the particles of which
are thrown into a certain state of strain or tension (which Faraday
called polarisation), and the discharge of the jar is their release from
that tension. So that it appears that whatever electricity may be, it
can in some way become bound up with the particles of ordinary matter
like glass and other dielectrics, and exert force upon them, which force
acts always in two opposite directions. It is the opposition of the form
or direction in which the electrical effect is manifested that gave rise
to the conception of the two “fluids”—the “positive” and the “negative.”
If these “fluids” really existed it would surely have been possible to
give to an insulated body an absolute charge of either of them. But this
can never be done; if, for instance, you have in the middle of a room a
metallic sphere charged with positive electricity, the necessary
condition is that on the walls of the apartment or on surrounding
objects there is an exactly equivalent quantity or negative electricity.
The number of oscillations or alternate momentary currents in a single
discharge of a Leyden jar is enormous. Theory shows that under ordinary
circumstances they must be enumerated by hundreds of thousands, if not
by millions; that is, the apparently instantaneous spark is really made
up of say a million surgings to and fro of the electric influence. But
theory also shows that the frequency of these oscillations can be
controlled or adjusted through an indefinite range. A general notion of
the requisite conditions may be obtained by the analogy of sound, and
for this we may take the familiar case of the strings of a musical
instrument, say the violin, or the harp. Everybody knows that when a
stretched string or wire is pulled a little aside it is in a state of
lateral strain, striving by its elastic force to return to its position
of rest, and if it is suddenly let go it not only rapidly regains that
position, but by the inertia of its motion is carried beyond it against
its elastic force, which, however, again brings it back, and the
movement is continued nearly up to the point at which it was originally
released, this swinging movement persisting for an indefinite period,
during which the vibrations, which have an ascertainable and perfectly
regular frequency, are communicated to the sounding-board of the
instrument and from that to the air, by which they are conveyed to the
ear and affect the auditor as a musical note, which note is higher as
the number of vibrations per second is greater. Everybody will have
observed that in the violin the note yielded by each open string is
higher as the tension becomes greater by turning the peg to tighten it;
that the same string will, without any change in its tension, yield
higher notes as shorter lengths of it are employed. Another circumstance
upon which the pitch of the note depends may also be illustrated in the
violin, in which it will be noted that the G string, which gives the
lowest notes, is loaded with wire wound spirally round it. Here, then,
are three circumstances that collectively determine the pitch or number
of vibrations of a string—tension, length, weight; and if you give the
measures of these to a mathematician he can tell you the note the string
will emit, for the number of vibrations is given (when the measures are
expressed in the proper units) by the formula
_√t_
_n_ = ——————
_2l√w_
This shows that we have only to adjust suitably the tension, length, and
weight of a string in order to make it vibrate at any rate we please.
Now in the oscillation of currents in the Leyden jar discharge there are
conditions which correspond, by analogy at least, with those that
determine the vibrations of a stretched string. These conditions are of
course electrical, and they are definable in terms of electric units,
which need not be discussed here. As we are leading the reader to the
modern view of electricity, which sets aside the fluid theories and
regards electricity as having no separate existence, but as being merely
the manifestation of some condition of a universally pervading medium,
the same, in fact, as the luminiferous ether, it is curious to remark
that these electrical oscillations would seem to attribute to the
incompressible and imponderable ether something very much like the
characteristic property of matter we call inertia, by virtue of which
the released cord flies past its position of equilibrium to the other
side. Or may this quality be dependent on the matter of the dielectric
in which the ether is, as it were, entangled?
The oscillatory character of the Leyden jar discharge was elegantly
demonstrated before a large audience in a lecture given by Professor O.
Lodge at the Royal Institution a few years ago. Clearly it is impossible
to render perceptible to the senses the millions of periodic discharges
that take place in the marvellously short space of time taken up by a
spark, but by doing what is analogous to slackening the tension of the
stretched string or increasing its length, that is by increasing the
_static capacity_, which means using a large number of jars combined
into a battery, and at the same time causing the discharge to pass
through coils (the effect of these is to increase the _self-induction_
of the circuit—called also _impedance_), an arrangement corresponding
with loading the string, Dr. Lodge was able to bring down the rate of
oscillation to 5,000 per second, when, instead of the crack of the
ordinary discharge, a very shrill continuous sound was heard. The
addition of another coil gave another load, and when the rate was thus
reduced to about 500, the note emitted was that of the C above the
middle A of the piano. With the rate of oscillation thus reduced, it
became easy to render the discontinuity of the discharge visible by
means of revolving mirrors, as in the well-known acoustical
demonstrations.
[Illustration:
FIG. 280_k_.
]