In order to avoid confusion in the readers mind I have hitherto avoided mentioning any but continuous - current dynamos, i.e. those generating a current flowing continuously and uninterruptedly in one direction. There are, however, alternating-current dynamos, i.e. those generating a current flowing first one way and then the opposite way. The number of such alternations per second might be, for instance, fifty, so that one can imagine the pulsations of the curreut could, at such a "frequency" or "periodicity", as it is termed, be hardly noticed. By reducing the periodicity sufficiently, - say by having only ten alternations per second, - the pulsations would be, in many instances, dis-tiuctly noticeable to the naked eye. This will be readily understood by imagining an incandescent lamp lighted by a "continuous" current, which would give an uninterrupted light, and then imagining a similar lamp lighted by an alternating current. This alternating current would, as I have explained, first pass through the lamp in one direction and then in the other, and consequently, if the alternations were sufficiently few to the second, there would be a distinct pause at each reversal, and the lamp would at each such moment go out, or have a tendency to do so. For lighting purposes, then-ton-, a minimum periodicity must always be maintained

An alternating current dynamo is similar in principle to a continuous current dynamo, with the exception that the latter has a commutator which commutes the direction of the current, which would otherwise alternate Turning back to the armature previously described and illustrated in Fig. 614, page 239, we note that the wires on the periphery have current induced in them; and as these wires pass the north pole of the electro-magnet, the current is induced in one direction, and as they pass the south pole, in the other direction; thus, were it not for the commutator, the result would be the production of an alternating current.

The commutator, however, has the effect of connecting the wires to the brushes according to their direction, that is to say (turning back to our diagram of the shunt dynamo, Fig. 616, page 240, and calling the magnet pole on the right south, that on the left north, and imagining the armature to be revolving in tin-direction of the hands of a watch when looking at the face), the current in a wire coming, during a revolution, under the north pole, is induced in a direction away from the reader, and is, on reaching the brush, transmitted to the circuit in that direction; then the wire, passing on between the poles of the magnet, becomes electrically idle, and on passing under the south pole has a current induced in it in a direction facing the reader, and this current is transmitted by the second brush (which it has by this time reached) to the circuit in that direction, i.e. opposite to the former direction; this completes the production of a continuous current, since the current from the south-pole brush should flow outwards through the circuit, and thence back into the armature, entering at the north-pole brush, which, as just shown, the commutator has arranged shall have the current going inwards.

Continuous and alternating current dynamos have each their proper sphere of action. The former are by far the more convenient, as will he seen later, hut the latter have the distinct advantages of being adaptable for very high pressures, which would not be feasible with the continuous-current dynamo, owing to the difficulties of insulating and commuting currents of more than moderate pressures; they can also be used with transformers, which are apparatus to trans-m the nature of a current, as I will proceed to describe, without the necessity of having any moving part in such transformer.

I have previously mentioned that electrical energy can be generated at various pressures without altering the value as energy; for instance, one thousand watts can be represented by 500 volts and 2 amperes, by 2 volts and 500 amperes, 1000 volts and 1 ampere, and so on, all of them representing the same amount of electrical energy, namely, 1000 watts.

A transformer is merely an apparatus for changing energy in one form, to similar quantity of energy in another form. Every change so effected must result in some loss, but the loss in changing electrical values in transformers is very small, and so we speak of transformers being of very high efficiency. If we had a dynamo giving 5000 volts and 10 amperes, it would be an easy matter to build a transformer to receive the energy, and give it out in any form we might desire, say, 500 amperes and KM) volts. Such a facility is very useful in electric lighting, as it permit- of current being transmitted to a distant point at a high tension, and there changed, by passing it through a transformer, to a low tension.

To explain the advantage of transmitting energy at a high tension and then transforming it, I must first describe the method of conveying it, which 1 need harly say is by providing for it a metal path.

The various metals offer different resistances to electricity, and consequently what is required is a metal which offers only a reasonable amount of resistance combined with cheapness. Copper meets the requirements of the case generally better than any other metal, and is, for electric-lighting purposes, universally used. For the comparatively infinitesimal currents used in telegraphy, iron is frequently employed, although its resistance is many times as great as that of copper. Silver offers less resistance than copper, but of course the price is prohibitive. The greater the resistance which the metal offers to the current, the greater is the heat generated by the electrical energy endeavouring to overcome it, and this feature is often made use of, as I have already explained in the section on "Wanning and Cooking by Electricity".

As a standard of current-carrying capacity, a square-inch section of copper is recognized as capable of carrying a thousand amperes. This standard is used in all calculations to convey to one's mind the quantity of current, per section of metal, employed in any particular case. An electrician would know that the wires, in a room fitted with the electric light, would be sufficiently large if arranged on the basis of 1000 amperes to the square inch, but he would be at once dissatisfied if he were told that the wires were loaded with current to the extent of 4000 amperes to the square inch, because he would know that, upon such a basis, there would be sufficient heat generated in the wires to damage the insulation covering them, the heat arising from the energy absorbed in forcing the current through such a size of wire.