. The Röntgen rays in medical work . foe, Charging (Schall). cord, and by combining lamps of different candle-powers we can getalmost any current we like. It hangs up, out of the way, from anylamp-socket. We only need to know how much current passes through eachkind of lamp, and this is most easily calculated from the figuresmarked on all good lamps. It is a great convenience to have the choice of the binding-screws, or of the bottom lamp-holder, for the attachment to what-ever connecting cord we may have to use.* * The Stella Lamp Resistance is to be obtained of Miss Emily Long, 85,Crouch Hil
. The Röntgen rays in medical work . foe, Charging (Schall). cord, and by combining lamps of different candle-powers we can getalmost any current we like. It hangs up, out of the way, from anylamp-socket. We only need to know how much current passes through eachkind of lamp, and this is most easily calculated from the figuresmarked on all good lamps. It is a great convenience to have the choice of the binding-screws, or of the bottom lamp-holder, for the attachment to what-ever connecting cord we may have to use.* * The Stella Lamp Resistance is to be obtained of Miss Emily Long, 85,Crouch Hill, 1ST. For tables of rates of charging of batteries from the mains,and much useful information, etc., see Portative Electricity,by J- T. Niblett(Briggs and Co., Salisbury Court, ). 60 THE RONTGEN RA YS IN MEDICAL WORK Charging from Alternating Mains.—Accumulators may becharged from alternating current mains by the aid of certaindevices, although alternating current cannot be used directly tocharge accumulators. For example :. Fig. 34.—Stella Lamp-Resistance (Dr. E. E. Maddox). 1. An alternating current motor may be used to drive a directcurrent dynamo. The machines may be coupled by a belt, or theymay be built upon one shaft. In this method there is a conversion of electrical into mechanicalenergy, and a reconversion of mechanical energy into electrical THE ELECTRICAL APPARATUS 61 again. The method allows of a free choice as to the pressureand magnitude of the generated current, within the limits of theenergy expended on the motor side. 2. A mechanical rectifier may be used, and by this is meant aninstrument which periodically makes contacts, closing the circuit atsuch intervals of time as to transmit the impulses passing in onedirection, while arresting or diverting those impulses which wouldtend to pass in the opposite direction. A rectifier, therefore, doesnot give a smooth flow of direct current, but a series of distinctimpulses which all pass in the same directio
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