34 ELECTROLYSIS. 



The resistance of the metallic conductors can be estimated 

 at 0014 ohm ; it is that of a bar of copper 10 metres long 

 and 1 square centimetre in section. To illustrate a case, we 

 will suppose that the internal resistance of the dynamo is equal 

 to the tenth part of the total external resistance, and that its 

 mechanical efficiency is 90 per 100, a result which is often 

 realised in practice. 



The resistance K' created by the counter electromotive 

 force is 



The internal resistance of the dynamo, as well as that due 

 to friction, may be estimated at 0*0006 ohm. Then it can 

 be admitted that a resistance of 0*002 ohm is due to the 

 polarisation of the electrodes, and to other causes of a varied 

 nature which necessitate an expenditure of energy, and which 

 have not been calculated ; the total resistance of the circuit is 

 thus shown : 



E = 0-003 + 0-000512 + 0-0014 + 0-0006 + 0-002 

 = 0-007512 ohm. 



The number of amperes 833 remaining constant, the electro- 

 motive force required to overcome this total resistance will be : 



E = ^ = 833 x 0-007512 = 6-25 volts. 

 1C 



The proportion between the work actually required for 

 liberating the zinc and that absorbed in the electrolysis of the 

 chloride of zinc is therefore 



2-45 



6-25 



= 0-40. 



The primitive work being 2 ' 7 horse-power, the total work 



2'7 

 is in reality _ . = 6 * 75 horse-power. 



In order to extract one kilogramme of zinc per hour from a 

 bath of chloride of zinc, it will thus be necessary to have : 



(1) An engine of about 7 horse-power ; 



(2) A dynamo electric machine giving a current of 833 



