MUTUAL INDUCTANCE 25 



fluctuations. Hence one effect due to the presence of mutual in- 

 ductance is equivalent to a lowering of the self-inductance of the first 

 circuit. Again, the currents induced in the second circuit will pro- 

 duce heat in it, and since the power corresponding to this rate of 

 heat generation conies from the first or inducing circuit, it follows 

 that the total power supplied to the first circuit will be greater than 

 the power taken by it at the same p.d. when the second circuit is 

 absent. A second effect due to mutual inductance is therefore 

 equivalent to an increase in the resistance of the first circuit. 



Let us next suppose that the first circuit is fixed, and the second 

 movable. If we assume the two circuits to be parallel to each other, 

 the second circuit will be repelled by the first, since the result of 

 such motion would be to reduce the amplitude of the flux fluctuations. 

 Thus, a ring of copper or aluminium slipped over the pole of an 

 alternate-current electromagnet will be projected upwards as soon 

 as a sufficiently strong current is sent through the coil of the electro- 

 magnet, and if provided with suitable guides the ring may even be 

 kept floating above the electromagnet, gravity being neutralized by 

 electromagnetic repulsion. Striking experiments of this nature have 

 been carried out by Prof. Elihu Thomson and Prof. Fleming. 



If the second circuit is prevented from having a motion of trans- 

 lation, but is free to rotate about an axis, rotation will take place 

 until the plane of the second circuit is parallel to the inducing field ; 

 for this is the position in which the flux fluctuations are completely 

 suppressed. Since action and reaction are always equal and opposite, 

 an equal and opposite couple will be experienced by the first circuit. 

 A coil of wire, for example, conveying an alternating current will, 

 when pivoted or suspended in front of a sheet of conducting material, 

 experience a couple tending to turn it into a position at right angles 

 to the conducting sheet. 



12. Electromagnetic Repulsion Motor 



The principles just explained find a practical application in the 

 construction of a certain class of alternate-current motor known as 

 the repulsion motor. There are various types of this class of motor, 

 but we shall here select for description one of the types originally 

 devised by Elihu Thomson, and termed by him the shaded-pole motor. 



In this form of motor, an alternating-current electromagnet is 

 made to act on two secondary short-circuited circuits. Let us suppose 

 that two rings of copper of the same size are suspended parallel to, 

 and nearly in contact with, each other in the field of such a magnet. 

 The currents induced in the rings by the alternating magnetic field 

 will be nearly in phase with each other (the rings being nearly in 



