THE TECHNICALITIES OF ELECTRIC LIGHTING,
ELECTRIC LIGHTING is so interesting, and still so little
understood, that we publish the following account of the various systems, though it is more fitted for the columns of a technical newspaper than our own :- Electricity, as now applied to • the illumination of our streets and houses, is well exhibited at the Crystal Palace, where everything which is new, and so far as it goes, per- fect, is to be seen in practical use. Lighting by electricity is divided into two distinct operations,-1st, the induction of the electric current ; 2nd, its distribution to the lamps, in which its presence is made manifest in the form of light, and to a certain, though limited, extent as heat. More recently, this second branch has been divided into lamps of two kinds,-1st, those in which carbon is consumed in the Voltaic arc; 2nd, those in which there is no consumption of carbon, and which, consequently, must be spoken of as being rendered in- candescent, not as burning. The dynamo-electrical machines, whereby the currents of electricity are induced, are to be seen of varied design, size, and efficiency. Of these, the most conspicuous are those of Gramme, Siemens, Brush, and Edison. But however much machines of this type may differ, not in general appearance only, but in the details of structure whereby the desired object is effected, they are all applications of the same principle,—namely, that when a coil of copper wire, or other conducting material in which the con- volutions are insulated from each other, and the ends of which are joined, is caused to move towards the pole of a magnet, an electric current is induced in a certain direction round the con- volutions of the coil, and in the reverse direction on the with- drawal of the coil from the magnetic pole. In some machines, the currents so induced are passed at once through the circuit of the lamps,—i.e., first in one direction, and then in the other ; these are termed " alternating-current machines," the number of the alternations being something like 500 times a second.
The more usual practice, however, is to collect the currents by means of a commutator, and to send them forth in one direction only; these are styled " continuous-current machines." Machines may further be divided into two classes,--those of high and low tension. High-tension mschines are usually con- structed to induce a comparatively small quantity of electricity —enough for one arc lamp—under what may be termed a high tension or pressure, that is, they are capable of forcing the electricity through a great resistance, as, for example, the Brush Company's " forty lighter," which induces no greater quantity of electricity than a " two-lighter," though it is capable, as its American name implies, of driving its cur- rents through forty arc lamps placed in a row, one after the other; and it requires a proportionately greater engine- power to turn it. This effect of high tension is produced by causing coils of wire containing a large number of con- volutions to revolve before the poles of electro-magnets.
Low-tension machines, though there is no strict line drawn between the two, induce a larger quantity of electricity, but are only capable of forcing it through a small resistance, as, for instance, a single lamp, though their quantity enables them to feed many lamps simultaneously, each lamp affording a distinct route for its own modicum of current, instead of taking it from and passing it to its neighbours, as in the previous example. These machines are constructed of wire, having a large eectional
area, with a lesser length than that employed in high-tension machines. Mr. Edison's machine is a type of this class. High- tension currents are those from which unpleasant shocks may be experienced, if the conducting-wires are incautiously handled; whilst those of low tension are absolutely safe in this respect. The question will naturally be asked,—Why, then, are high- tension currents used ? The answer is, that arc lamps are more economical, candle-power for candle-power, than incandescent lamps ; and that although the subdivision of the electric cur- rent has been thoroughly solved in the use of incandescent lamps, no means has yet been brought forward whereby arc lamps can be manipulated in a similar manner. To take our previous example, twenty small low-tension machines would be required to burn the forty arc lamps now worked off one large high-tension machine ; in addition to which, the length of conducting-cable would require to be increased almost in the proportion of twenty to one. The solution of this difficulty is still engaging the attention of electricians. All arc lamps have one point in common, namely, that the light is produced by the passage of electricity from one carbon rod to another, the distance between which, or length of arc, is usually maintained at from one-sixteenth to one-eighth of an inch. In one species, the candle-lamp, best known in the form introduced by M. Jablochkoff, the distance between the carbon rods is maintained constant by placing them parallel to each other, and allowing the arc to play between them ; but as the positive carbon (or that from which the current leaps) burns away more rapidly than the negative, these lamps require an alternating current, in order that the carbons may be consumed at an uniform rate. In most other forms of lamp, one carbon, usually the negative, is fixed in position, and the other one fed towards it automatically, in such a manner as to main- tain an approximately constant length of arc. This feeding or regulating action is accomplished with more or less success in a variety of ways, in some cases the operation being effected by gravity, in some by small, electric motors ; whilst in others the attraction of magnets or a train of clockwork is used. In no-ease has the electric current sufficient tension to leap across from carbon to carbon in the first instance ; thus, if a lamp be examined when not in use the carbons will be seen to touch each other, and it is only after the current has been established that the carbon points are separated to the required distance, draw- ing, as it were, the arc out between them. The points to be considered in the design of an arc lamp are :-1. That the car- bons shall fall together when no current is passing. 2. That they be separated to the required distance automatically the instant the current is established. 3. That this distance be Maintained constant so long as the lamp is in action. 4. That an automatic arrangement be provided. where- by, in the event of accident happening to the lamp, the "current may pass on to the next, for should the cir- cuit be interrupted in any one place, all the lamps on that cir- cuit are extinguished. The incandescent lamps exhibited are those of Edison, Swan, Maxim, and Lane Fox. The principle and structure of these lamps are in each case, roughly speaking, the same, viz., a thin filament of carbon hermetically sealed into -a glass globe, from which the air is afterwards exhausted. Whilst being used, these filaments offer resistance to the pas- sage of the electric current, owing to which heat is generated and the carbon renderel incandescent. These filaments, unlike the carbon rods of the arc lamp, are not consumed, as they have no oxygen wherewith to combine ; they are prepared in various ways, those of Swan and Maxim being carbonised from a veget- able parchment, the Former made of cotton, the latter of cardboard; whilst those of Edison and Lane Fox are carbonised directly from a natural fibre, Edison using bamboo and Lane Fox a rooty fibre, known as French wisk, and largely used in the manufacture of brushes. The character of the light emitted from these lamps is that of a white hot coal, and is entirely free from the unpleas- ant, blue glare of the electric arc, as well as from the painful fluctuations due to defective feeding apparatus and want of homogeneity in the carbons burnt. The relative efficiency of arc and incandescent lamps per unit of current passed through them is about ten to one in favour of the electric arc. On the other hand, the advantages of a pleasant and steady light, comparatively under control, and free from danger, must be awarded to the smaller lights. If they are not at pre- sent as durable as they might be, this is, in great measure, compensated for by the continual cleaning and renewal of car- bons required by their more powerful rivals. Great as the
success attained has been in the transformation of mechanical power luta electrical energy, and thence into heat and light, still more wonderful is the storage of such power by electrical means to which this success has in great measure led. To M. Plante is due the credit of having been the first to accomplish the task in a form of practical utility, to M. Camille Faure of having brought it to the notice of the engineer and within the bounds of a commercial undertaking, and others are fast follow. ing in the same direction. Storage of electricity it certainly is not; if it can be called storage of anything other than potential energy, it is that of chemical action, which, through its rever- sibility, is capable of refunding a large per-tentage of the power whereby it was effected. It is impossible to foresee the limits to which this principle may not ultimately be applied. We already know that by its aid the power of wind, of falling water, of the tides, and of steam can be stored up for future use, and this irrespective of kind, and practically of distance ; thus the energy of a morning tide and an afternoon breeze may both be stored in the same reservoir, and either or both supple- mented by a steam or gas engine, if insufficient to effect the purpose desired, the whole power of the reservoir being drawn upon at a later hour for purposes of illumination ; or the order of things may be reversed, and the power stored during the night utilised to drive machinery by day. And yet this won- derful reservoir of power consists of nothing more complex or mysterious than plates of lead immersed in acidulated water, through which combination an electric current has been caused to circulate for a sufficient period.
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