The number of plates varies, but in each cell of those we use (as in that shown in Fig. 18) there are seven plates, three positive and four negative. The positive plates are of a dark chocolate colour, while the negative are of a slaty-grey. The dilute sulphuric acid in which they are immersed should be of a specific gravity of 1190, attained by adding 1 part of pure H2S04 to 5 parts of water. The dilution should, when cold, be tested by a hydrometer supplied for the purpose, and should be adjusted, if necessary, by addition of more H2S04 or water according as the indicated density is below or above the desired standard.

The box encasing the cells, and its divisions, are usually lined with lead, and little strips or buttons of wood or glass are placed between the plates, to prevent them coming in contact during transport. If adjoining plates were allowed to touch each other while the cell was charged, they would be rapidly destroyed by the strong discharge of electricity. Over each cell of a portable set is a vulcanite or wooden cover, with a blow-hole in the centre; which hole must be plugged when the cells are being moved, but left open when the cell is being charged. Each box has its terminals painted, the positive red, and the negative black.

The capacity of an accumulator depends mainly and directly upon the quantity of lead in its plates, and is ex- pressed in ampere-hours. Thus, * 60-ampere hours ‘ signifies that an accumulator can discharge 1 ampere for sixty hours, 2 amperes for thirty hours, etc.

Charging.—In charging an accumulator, the wire from the positive pole of the source must be connected to the positive terminal—that painted red—and the negative wire to the negative terminal—painted black.

It is of prime importance that no mistake be made in this connecting; for, if wrongly done, the accumulator will rapidly discharge, and may be totally destroyed.

A ready method of judging the relative polarities of the two wires leading from any source of supply is to lay them, about i inch apart, on a piece of moistened litmus-paper. At the positive pole is produced a red colour (acid), and at the negative pole a blue colour (alkaline).

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