Using only one such cell or two in series, it will be readily seen that half of the current will really be lost, since only one of its two periods or phases is transmitted to the accu- mulator. Also the back electro-motive force of the phase suppressed will rapidly heat the electrolyte and wear down the aluminium. Fig. 24, however, shews a plan of connect- ing up two cells whereby both phases of the current may be utilised, the accumulators receiving alternately the single periods or phases as allowed to pass by the cells. By follow- ing out the connections as shewn in the sketch, and noted under it, the action will be more clearly understood than by verbal description. Besides utilising almost the full energy of the current supplied, this arrangement lessens the heating of the electrolyte and the wear of the aluminium electrode, since one or other path is always open to the current, and the back electro-motive force in each cell is thus minimised.

Each accumulator will be charged at half the rate corresponding to the power of the lamp used. Thus, with a 32 candle- power lamp in series, with a 100-volt supply, each will receive about i ampere.

The above arrangement assumes that two or more accumu- lators will be charged at one time. For a single accumulator an arrangement of four cells is possible, arranged after the manner of a Wheatstone bridge; but for X-ray work more than one accumulator will practically always be in use.

III. Primary Batteries.

Primary batteries are of little, if any, practical use in X-ray work. The requisite current can indeed be obtained if a sufficient number of cells be suitably connected; and for such a purpose bichromate cells, or Bunsen’s cells (each with an E.M.F. of about 2 volts), will probably be found most suitable. But it is hard to conceive of any set of circum- stances in which it would not be much more convenient and efficient to employ some other source of supply.

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