704 INDUSTRIAL APPLICATIONS. 



This is a machine which belongs to what we have called the 

 magnetic type. 



The value of Z at Paris being about 0-422 (C.G.S.), if a = i metre, 

 we have 



x 0-422 = ni -33. io 4 (C.G.S.), 



I0 



To obtain a volt, the number of turns per second should be 



io 4 - 

 - =7474- 



1260. The same apparatus may be transformed into an electro- 

 dynamic machine. Let us suppose that the circuit comprises a coil 

 DD', the mean plane of which passes through the conductor, and 

 the axis of which coincides with the axis of rotation. 



The mean action of the coil on the circle of radius a being F m 

 for unit current, the electromotive force for a current I, apart from 

 the action of 'the earth, will be 



Used as an electromotor, the machine can only produce a 



R 



current provided n > - . 

 2 *m 

 If the coil contains N turns of mean radius A, and we take as 



2 N7T RA 



approximate value the mean action P m = -- , we get n > - . 



A 2N7T 2 2 



The total length of the circuit is sensibly 2NwA, and if S is the 

 section of the wire, and a- its specific resistance, the resistance R is 



equal to - 2N?rA; it follows from this that n> -- -. 

 S 



This condition only depends on the ratio of the squares of the 

 radii A and a, and therefore on the ratio of the corresponding 

 surfaces. 



The specific resistance of copper is about 1600 ; if we use wires 

 whose section is a square centimetre, and assume A 2 = 27r# 2 , we get 



