. The Röntgen rays in medical work . circuit, as shown at K; the direction from lead to aluminium is-indicated by the arrows. The alternating current is applied at Aand C. No current can pass in either direction, for it is opposed THE ELECTRICAL APPARATUS 67 by one or other of the aluminium cells; but if B and D beconnected a current will flow from B to D. When A is the posi-tive pole, the flow will be by A, B, D, C, and when the sign ofthe electromotive force changes, and B is positive, it will flow byC, B, D, A—always, therefore, from B to D, through any circuitjoining B and D. The system ma
. The Röntgen rays in medical work . circuit, as shown at K; the direction from lead to aluminium is-indicated by the arrows. The alternating current is applied at Aand C. No current can pass in either direction, for it is opposed THE ELECTRICAL APPARATUS 67 by one or other of the aluminium cells; but if B and D beconnected a current will flow from B to D. When A is the posi-tive pole, the flow will be by A, B, D, C, and when the sign ofthe electromotive force changes, and B is positive, it will flow byC, B, D, A—always, therefore, from B to D, through any circuitjoining B and D. The system may be regarded as a Wheatstonesbridge circuit, in which the resistances automatically change withthe changes of sign of the alternating electromotive force applied atA and C. The Nodon valve and Pollaks rectifier are practical arrangementsof aluminium cells on this principle, and they answer quite wellfor accumulator-charging, and may also be used to supply currentfor driving motor interrupters for coils, and for working the coils. -\AAAA Lampresistance Fig. 40.—Arrangement of Two Aluminium Cells for Accumulator Charging (H. K. Lewis). K, Aluminium cells. A, Accumulators to be charged. themselves. The current given out by these forms of rectifier isnot smooth, but for most practical applications it answers almostas well as if it were. A large number of solutions have been proposed for the electro-lyte. The choice among these will be partly governed by the natureof the metal used for the second pole of the cell, for it is desirablethat this should not be attacked through electrolytic the chemicals which have been suggested are alkalinesulphates, especially ammonium sulphate, ammonium phosphate,ammonium chromate, double tartrate of potassium and sodium(Rochelle salt), and many others. Nodon states that for magnesiumpoles the best effect is obtained with an electrolyte of ammoniumfluoride. In general the salts of potassium and sodium are morelikely to corrode the
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