be assumed that electricity can probably be produced only by chemical action, and that whenever any chemical action takes place electricity is produced. However, it is usual to speak of the electrical current being produced either by means of an engine of some kind, or from batteries, without entering into further refinements. The engines employed are usually steam or gas, and both are so familiar to every one that to describe them is unnecessary. The apparatus which is revolved by the engine and termed the dynamo, in which the current is actually or, more strictly speaking, the apparatus in which the power derived from the engine is converted into electrical energy, will claim our attention after a brief reference to batteries.
Batteries are divided into primary and secondary. The former are those wherein the materials, part or the whole of them, become exhausted, and at this stage fail to supply electricity unless some or all of the materials are renewed. The secondary battery is one which becomes exhausted in exactly the same way as the primary, but the chemical contents are of such a nature that it is merely necessary to pass a current of electricity through the battery in order to reinstate it in its original condition. To give a practical instance: every one is familiar with the primary cell, consisting of a pot of weak sulphuric acid, in which are immersed two plates, one of copper, one of zine, not touching each other in the liquid, but connected with each other outside the liquid by means of a wire the ends of which are joined to the plates respectively. Under these conditions the liquid bubbles like soda-water, indicates that some chemical action is proceeding and some of the fluid is being converted into gas; a current also flows through the wire. This is a simple illustration of chemical and elec- trical action proceeding at the same time, which, as already stated, must necessarily take place, although not always self-evident. On disconnecting one end of the wire from one of the plates, the bubbling ceases, indicating that the action has been arrested. In the course of the wire an electrical lamp, a motor, or apparatus for indicating the presence of a current, or any other apparatus, may have been inserted; and all the results, so well known, which can be produced by means of an electrical current, may have made themselves manifest. But of course no appreciable quantity of light or power would be produced from such an experimental cell; a large number of such cells coupled together would be necessary in order to be of practical service; and this combination is termed a battery. In a short time the zinc plate in the cell is dissolved, and the power of the liquid becomes exhausted, when the production of the electric current ceases. But supposing these two plates, instead of being copper and zinc, are one lead and the other double oxide of lead, then, although the current produced ceases after a time, instead of requiring a portion of the contents of the cell to be renewed, it is only necessary to pass through it a current pro- duced from some other source in order to renew its vitality. Such a combination is termed a secondary cell, and a number of these suitably connected form a secondary battery very commonly known under the name of an accumulator. An accumulator, therefore, is nothing more than a primary battery in which the chemicals can be renewed, as often
