516 CONSTANTS OF COILS. 



The first apparatus comprises an internal inducing coil with a 

 somewhat thick wire, and an external coil on which is wound a 

 tore formed of twenty insulated wires slightly twisted. The coeffi- 

 cient of mutual induction M between the internal coil and any 

 one of the twenty wires is the same. By means of a commutator 

 the induced current may be made to traverse 20 p wires in one 

 direction and / others in the opposite direction. The coefficient 

 of mutual induction between the internal coil and the twenty wires 

 thus joined is 2(10 ) M. The wires, moreover, being joined end 

 to end, the resistance of the external coil is constant. 



In the second apparatus the coefficient varies continuously. It 

 consists of an external inducing coil of thick wire in the form of 

 a cylinder, and of an internal coil of fine wire placed in the centre 

 of the first, and which can turn about an axis along a common 

 diameter to the two coils. If the external coil was infinitely long, 

 the internal field would be uniform, and the coefficient of mutual 

 induction will be strictly proportional to the cosine of the angle 

 of the two axes; but if the length of the great coil is ten times 

 that of the small one, the error may be neglected. 



The same condition will be approximately realised by placing 

 the centre of the induced coil in the mean plane of a coil with 

 any channel and of a much greater radius. An uniform internal 

 field would finally be obtained with a spherical coil, the successive 

 windings of which are arranged in equi-distant parallel planes. 



