250 Mr. E. H. M. Bosanquet on Practical Electricity. 



rightly. Now suppose that, after the accumulator has ac- 

 quired a sensible charge, we stop the dynamo machine with- 

 out breaking the connexions. The power of the machine to 

 drive back the battery-current was derived from its velocity, 

 which is gone. Consequently the battery-current discharges 

 itself through the machine in the opposite direction to the 

 proper current of the machine, and reverses the magnetism of 

 the machine. This change is recognized, as I have mentioned, 

 by the effects of the pole-pieces on a pocket-compass. When 

 the machine is " straight," the upper pole-piece attracts the 

 unmarked end of the needle. When it is reversed, it attracts 

 the marked end. 



If the machine be then set in motion again, the effect is to 

 strengthen the current in the magnets. But this is now the 

 discharge current of the battery. Consequently the machine 

 proceeds to pump the electricity out of the battery. 



This reversal is prevented in ordinary practice in two diffe- 

 rent ways. In charging by Gramme machines, in general, 

 two machines are employed, of which the one serves only to 

 drive a current through the magnets of the other. This cur- 

 rent is wholly independent of the charging current ; and the 

 arrangement is consequently not liable to reversal. 



The power which is employed in this case to maintain the 

 current in the magnets contributes nothing to the work; con- 

 sequently this arrangement is not economical. But in a large 

 establishment one small machine may be used to excite several 

 large ones, and the waste is reduced to a minimum. 



The best method for charging in general would appear to 

 be the use of machines of the Siemens type, with the field- 

 magnets in a shunt circuit of high resistance ; for then the 

 reaction of the battery-current seeking to pass backwards 

 into the machine conspires with the forward current from the 

 machine, so far as the supply of the field-magnets is con- 

 cerned. 



Diagram of Circuits in Shunt-Circuit Machine. 

 —Accumulator 1 



Magnets 



— Armature ' 



But as my machine is not thus arranged, I have had to 

 devise other means of doing the work. I have not seen any 

 account of these processes. 



