(c) Nodon’s valve, or aluminium cell, is a simple and valuable device for utilizing alternating currents for medical purposes, since it has no moving parts and requires little attention.

Such a cell consists of an active electrode of aluminium, immersed in a suitable solution (in some of common salt, in some of ammonium phosphate), along with an indifferent electrode of lead, carbon, or iron; the containing cell in many designs supplying the latter. The action of the cell depends upon the peculiarity of aluminium that as kathode it allows a current to pass freely, but as anode it offers high resistance. This resistance is due, doubtless, to polarisation, whereby a thin, insulating film of oxide is formed on the surface of the metal. The other phase or direction of current easily breaks through this, but up to a certain voltage the cell automatically allows only that phase to pass.

By suitable combination of cells, explained later, both phases may be utilised, and a pulsating unidirectional current obtained from an alternating supply.

Heating of the liquid will interfere with the efficiency of a cell if the temperature rises above 70° F.; but short of that those cells may do a lot of heavy work, especially if arranged as advised on p. 48, and shown in diagram there.

For X-ray work the current produced is not satisfactory, either led direct to the coil or passed through an interrupter.

For charging accumulators from an alternating main, however, these cells work very satisfactorily. In the note on accumulators already referred to will be found a description of the arrangement.

In our hospital work this was the original plan adopted, and it served fairly well for about two years, until pressure of work induced us to discard it for the less troublesome plan of a motor-transformer.

Fig. 17 represents a set of cells as manufactured for sale, but efficient cells may be made very cheaply by anyone willing to take a little trouble. For description of such a cell and other practical points, see p. 42.

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