Where a plug is employed, it should be of a concentric type, thus making always the same connection with the main. The ordinary bayonet-catch type may be inserted in either of – two positions, and the polarity of the wires from it will differ according to the position. Hence, with such, the posi- tion must be indicated by corresponding marks on the plug and socket, or the polarity of the wires must be tested before each time of using.

Fig. 21 shews a permanent wall-board that would involve a minimum of trouble when once installed. The voltage of supply is there registered, as well as rate of charging, and there is also inserted an automatic cut-out.

(b) Alternating current cannot be used directly for charging accumulators, but some device must be interposed to render the current unidirectional. Many devices have been suggested and employed thus to * rectify’ the current.

Those are mentioned, and some described, earlier in this chapter (p. 27); but, for the purpose of charging accumu- lators, we need only describe the device known, and already referred to, as Nodon’s valve, aluminium cell, or electrolytic • rectifier. In Fig. 17, on p. 30, is illustrated a set of those cells as made for sale, and on the preceding page will be found a brief explanation of the action of the device.

Fig. 22 shows a more simple arrangement of cells connected to an accumulator.

But efficient cells maybe made from simple material by any one for his own use. Thus, a cell may be m ade from a large jam-jar containing a strong solution of neutral am- monium phosphate, and arranged as shewn roughly in accompanying sketch (Fig. 23).

Into the jar dips an electrode of aluminium of about the thickness of a pencil, and a second electrode of thin iron— such as hoop-iron—from 2 to 3 inches broad, each electrode being about 9 inches long. Each electrode is shown suspended in the solution by being passed through a bar of wood or other suitable insulating material which rests on the top of the jar, the aluminium being made firm by passing through a cork and the iron being wedged in. At the top of each is a screw electrode for connecting wires.

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