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Rh nearly always present in dry batteries. For .these reasons the accumulator or storage cell is usually preferred.

Storage Batteries.—The great difference between a dry battery and a storage battery is that in the case of the latter when the battery is exhausted it can be completely re-charged in a few hours. The 'accumulator' or secondary battery may be briefly described as a number of prepared lead plates, immersed in a weak solution of sulphuric acid and water. These lead plates are alternately positive and negative, and are separated from one another by thin strips of ebonite, glass, or other non-conducting material. The whole is contained in a square cell of some suitable substance, which is unaffected by the acid. The positive plates, which are connected together, may be easily detected by their chocolate appearance. The negatives (slate colour) are also connected together, and convenient brass screws (terminals) are fixed, to which wires may be easily attached.

The capacity of each cell depends on the size and number of plates. The pressure of any cell may be taken at two volts when working. Two cells, coupled in series, are therefore required to make a battery of four volts.

Charging Storage Batteries.—Accumulators require before use to be charged with electricity. This can be done by means of a large primary battery, small dynamo, or preferably off any continuous current electric light curcuit. In any case it is absolutely essential that the positive pole of the cell should be connected to the positive pole of the generator, and the negative to the negative. The positive pole of an accumulator is usually painted red and marked thus, (+). An easy way of finding the poles of a generator or accumulator is by placing two small strips of lead connected to the battery wires in a tumbler of acidulated water, and after passing a current of electricity through them, the piece of lead connected to the (+) pole will become chocolate. 'Pole-finding' paper may be also used for this purpose.

How to Charge Storage Batteries from an Electric Light