To return now to transformers, the advantages of which I have to explain. If we transmit 1000 amperes at a pressure of 100 volts for 2 miles, the size of cable must be very large and costly, but if we transmit this electrical energy of 100,000 watts at a pressure of 10,000 volts, we shall only require a cable to carry 10 amperes, instead of 1000, and the cost will be enormously reduced. If it were not for such facilities, the transmission of energy to any considerable distance would be impracticable.

The alternating-current transformer itself is nothing more than an "induction" or "shocking" coil, an apparatus which is frequently Been at fairs and such places, where the proprietor will allow anyone to receive a shock for the sum of one penny. The principle is very simple: If some turns of insulated copper wire are wound upon a piece of iron, as shown in Fig. 619, and through this wire (called the primary coil) a current is passed, the iron will become magnetic, as previously described. Now if, on top of this copper wire, some more insulated copper wire is wound, called the secondary coil, a current can be induced in it from the coil below. In other words, there are two distinct coils wound on a piece of iron, and if we put electrical energy in one, we can get the same amount (less the small losses due to inefficiency) out of the other. In each coil, of course, the gauge of wire is arranged to suit the current, and if we require to transform down from a high tension, as is generally necessary in electric lighting, the primary coil would consist of many turns of fine wire, as the current would small and the pressure high, while the secondary coil would be of thick wire, as the current would be great and the pressure low. For example, if 2 amperes at 20 volts are passed through the first coil. 20 amperes at 2 volts can be obtained from the second, so we shall have obtained a similar amount of energy to that expended, hut at a different pressure.

The current on commencing to flow in the primary coil induces a current in the Opposite direction in the secondary coil, and, on ceasing to flow, reverses the current in the secondary coil, so that the secondary coil gives an alternating current. If, however, the current in the primary coil were to be continuous and uninterrupted,, the second coil would not continue to produce an induced current: in the shocking coils referred to, the primary coil is fed from a battery whieh gives a continuous current, but in circuit with it is a little apparatus termed an interrupter, and at each interruption, as described above, a current is induced in the secondary coil in each direction, i.e. alternating. If the primary coil were fed with an alternating current, the interrupter could be dispensed with, and at each alternation of the primary current, a current would be induced in the secondary coil, also of course alternating in direction.

Fig. 619. - View of Transformer.

Such alternating-current transformers give out a buzzing sound, in conse-quence of the molecular disturbance caused by the current reversals.

Continuous-current transformers, which have the disadvantage of a moving part, and consequently require some little attention, are based upon the principle of the electro-motor.

If a dynamo be connected to a second dynamo by a couple of wires, and the first be driven by an engine, the current it produces will enter the second dynamo and cause it to rotate, when it is called a motor. The reason for this rotating is simple, as the current from the first dynamo or generator enters the armature of the motor, and magnetizes it. but since the magnets of the motor are also magnetized by the same current, the armature is repelled round from the poles of the magnets, the like poles repelling one another. We will suppose that this motor spindle is connected to a third dynamo by a belt. Then this third can be made to generate at whatever pressure is required. So the first dynamo, as a generator, supplies the second, which absorbs its energy and rotates, driving the third dynamo as a generator. If the first were wound for 1000 volts and 10 amperes, the second could be wound to receive such a current, and the third could be wound to generate (say) 100 volts and 100 ampei

The continuous-current transformer is a combination of the second and third machines, such transformer having one armature, but being wound with two circuits, each with its commutator. One of these circuits absorbs the energy from the first generator, and the other generates at the final pressure required.

We have so far considered dynamos as the chief means of obtaining current for lighting purposes but have not gone into the question of driving them, whieh is a matter of great importance. There are many motive powers, Bach as the steam-engine, gas-engine, oil-engine, and water-turbine. Of these a selection should be made to suit each individual case. Nothing is cheaper than waterpower, provided the first cost of utilizing it is not excessive; after this steam-power is undoubtedly the cheapest, if the steam is required in sufficient quantity and for a sufficient length of time. Gas and oil engines come last upon the list. when the favourable circumstances alluded to above cannot be ensured, but otherwise they are, in many cases, the best means of obtaining power; for instance, where the power is required in only small quantities, and then only occasionally.

Accumulators are very important adjuncts of electric-lightining apparatus. They are not necessarily desirable where electricity can be continuously generated, but it frequently happens that, even if it can be continuously generated, it is at such inconvenience, or causes so much wear and tear, as to justify the use of accumulators.

The storage or accumulation of electricity is effected by means of chemical action; and the apparatus employed, in its crudest form, would consist of two lead plates, submerged in dilute sulphuric acid, one connected to the positive pole, and the other to the negative pole, of a dynamo. On a current of electricity being passed from one plate to the other through the liquid, a chemical action is set up, which, on the charging current being stopped, will cause the cell, as it is termed, to give back the energy when called upon. A view of an accumulator-cell in common use is given in Fig. 620. A cell, when fully charged, will for a very short time give 2 volts, and when exhausted with working, 1.9 volts. To charge a cell, 25 volts are required. The inefficiency of the apparatus is at once apparent, and were it not that this is often overbalanced by the advantage of being able to obtain a small amount of current throughout a long period, just as economically as a large amount for a short period, accumulators would be avoided. As each cell gives only 1.9 volts when discharged, a large number in series are required to form a battery for an electric-lighting circuit.