Part 64

The electro-motor may, therefore, be considered simply as a dynamo worked backward, and almost any form of dynamo may in this way be used as an electro-motor, that is, a current being supplied either from a battery or from a dynamo, the motor converts the electrical energy into mechanical energy. Any dynamo that supplies a direct and continuous current can thus be used; but there are certain conditions which make it desirable to somewhat modify the proportions and arrangement of the several parts when the machine is for motor purposes.

In general, any source of current may be used, but in the applications of the electro-motor there are chiefly two methods in practice of supplying the current. The one takes the current from a dynamo in motion, the other from an accumulator which has previously been “charged” by a dynamo.

Both of these methods are used in the familiar and interesting 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.

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.

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_