as required, by simply passing a current of electricity through it, such current in practice being produced by means of an engine and a dynamo. It may also be observed that the primary battery at the present moment is not a suitable means for producing electricity on a large scale, for, apart from expense, the necessity to replace new chemicals in the cells from time to time involves considerable trouble and inconvenience; whereas to replenish an accumulator by running an engine is a very simple matter.
To turn to the dynamo: no attempt can be made in an article of this kind to enter into a full description of this apparatus, which is made in an unlimited number of ways; but sufficient explanation will be given of its general principle to show how a current is produced from it,—the principle being the same in all types, no matter how the machine may be constructed.
Dynamos are of two kinds, one made to produce a continuous current and the other an alternating current. Although these currents are very different in their character, no difference is to be observed when they are employed to give light. A continuous current may be compared to a liquid wherein waves are continually progressing in one direction. The direction towards which these waves are moving is termed the positive direction, and that from whence they come the negative. Consequently, if a wire, with a current flowing in it, is attached to a lamp in such a manner that the waves move towards the point where the wire is attached to the lamp, it would be stated that that point is attached in such a way as to receive a positive current; and the other part of the lamp, where the current leaves it (since the waves will evidently move away from this point), is termed the negative. In this explanation the lamp must be regarded as a part of the wire modified in such a manner as to be capable of converting the current into light. Therefore, when a positive or a negative current is spoken of, it simply indicates direction. Notwithstanding this, positive electricity and negative electricity have a variety of phenomena peculiar to each.
The alternating current may be regarded as a fluid in which a wave proceeds first in one direction and then in the other, seesaw fashion; and in practice these changes of direction take place as frequently as from fifty to two hundred times per second. Instruments employed for continuous currents may contain iron and permanent magnets, but for the alternating currents the latter must be completely absent, and any iron employed in the construction of the instruments must be very soft and much subdivided, or it will become extremely hot.
The dynamo producing the continuous current consists of two parts, one stationary and one to be revolved. In the most usual type the stationary portion is a powerful electro-magnet, horseshoe in shape, the free ends of the horseshoe being termed poles. These poles are so arranged as to permit the armature to revolve between them. The axis of the armature consists of a spindle running in bearings, and is revolved by motive power, either being connected directly with the engine or indirectly by means of a belt, in which latter case the spindle carries a pulley. Upon the spindle is placed a suitable frame-work,
