We may repeat that the action of such a cell depends upon a property of aluminium (also magnesium), whereby as kathode it allows current to pass freely, but as anode it offers great resistance. Iron is a convenient material for the other electrode, since it is not acted upon by the electrolyte.

The solution should be a saturated one, and it is preferable to use distilled water in preparing it, as chlorides seem to hamper the action. By escape of ammonia the liquid be- comes acid, and crystals tend to form. To obviate this, a little weak ammonia solution should be added occasionally.

Periodically—each three or four months—the cells should be taken apart, the solution filtered and replenished, and any deposit on the plates or cell scraped off.

In action a certain amount of heat is generated in the cell, and if the temperature rise above 70° F., the efficiency falls.

Thus the size of cell and quantity of electrolyte should be proportionate to the quantity of current passed through the cell; but the aluminium should be kept small in area. The cell should be placed between the lamp-resistance and the accumulator, with the aluminium electrode connected to the positive terminal of the latter.

A cell of about the dimensions described should act well with a current of 100 volts passed through a 32 candle-power lamp. “Where current supplied is at a higher voltage, it will be profitable for regular use to pass it first through a trans- former, whereby pressure will be reduced to 50 volts, and the amperage correspondingly increased; otherwise two cells in series may be employed to deal with the heavier voltage, though this is not so efficient.

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