Accumulators are often spoken of, somewhat loosely, as ‘ storage cells’ of electricity. This they are in effect, for, after receiving an appropriate * charge’ of electricity, they may be kept for some time, and thereafter a ‘ discharge’ of electricity obtained from them; but the term is misleading.

More properly they might be termed * energy transformers,’ or storage cells of energy. An electrical current, passed through an accumulator cell, produces chemical changes in the constituents of the cell, which thereafter stores energy in a potential form, represented by the force impelling the con- stituents to return to their former condition.

An external circuit being completed between the terminals of the cell so charged, its constituents more or less gradually resume their former condition, and the potential energy thus liberated appears in the form of electrical effects in the circuit.

The chemical changes referred to are somewhat compli- cated, and we do not here attempt a full explanation.

Each cell (as shewn in Fig. 18) contains several lead plates, connected alternately to the positive and negative terminals of the cell; and all are immersed in, and well covered by, dilute sulphuric acid.

During charging, a current of electricity is sent through the cell from a suitable source of supply. By electrolytic action there are produced from the acid quantities of free oxygen and hydrogen. The former appears on the plates connected with the positive pole, converting them gradually into peroxide of lead; whilst the hydrogen appears on the negative plates, reducing them to a porous, spongy mass of metallic lead. Those actions on the lead plates having pro- ceeded as far as possible, the gases escape in bubbles from the liquid, thereby indicating the completion of the charging process. In discharging, those processes of oxidation and reduction are reversed; the plates return to their previous condition, and the acid regains its original strength.

Thus the process may be repeated any number of times in the same cells, if due precautions be observed.

Some of those precautions we will now discuss, before describing the methods of charging accumulators from various sources of supply.

Accumulators for X-ray work are generally arranged with four cells in each box, as in Fig. 18. The cells are connected in series—that is, the negative pole of one is connected to the positive pole of the cell adjoining it, leaving at one end of the box a free positive terminal, and at the other a free negative terminal. The E.M.F. of each cell being fully 2 volts, such a box of four cells will give, when fully charged, at least 8 volts. Three such boxes are as a rule employed, thus obtaining an E.M.F. of 24 volts for use.

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