288 Prof. Norton on Molecular Physics. 



magnetic field; but Faraday has shown that a current may also 

 be induced in the wire, by bending it into a curve and causing 

 it to revolve around the magnet, after one end has been brought 

 into contact with the equatorial part of the magnet, and the 

 other with a wire or rod leading out v . 



from the pole, as shown in fig. 11. ^' 



"A copper ring was fixed round and 

 in contact with the equatorial part, 

 and the wire e made to bear by 

 spring pressure against this ring, 

 and also against a ring on the axis." The direction of the current 

 changed with the direction of revolution. Corresponding cur- 

 rents were also obtained by rotating the magnet in the opposite 

 directions, the wire remaining fixed. To explain these currents 

 upon the principles now developed, we must first observe that 

 the impulsive force of the magnet will impart a transverse pola- 

 rization to the molecules of the wire. Now let a motion of revo- 

 lution be imparted to the wire in a direction opposite to that 

 of the circulation of the magnetic currents, and the relative velo- 

 city with which the sethereal impulses will fall upon the mole- 

 cules will be the sum of the velocity due to the impulse, and 

 that of the molecules themselves in the opposite direction. The 

 molecules at the end e of the wire will therefore take on a 

 higher polarization than is induced in the copper ring by the 

 magnet simply. This polarization should be attended with a 

 disturbance of the electric condition of the molecules in the 

 direction of the length of the wire. There should then be an 

 inequality in this disturbance at the point of contact e. This 

 inequality should originate a current that would pass around 

 the circuit. Let v denote the velocity answering to the magnetic 

 impulse, and v' the velocity of revolution of the molecule at e. 

 Then the effect due to the polarizing force at the end e of the 

 wire may be represented by m(v + v 1 )' 2 , and that induced in the 

 contiguous particles of the copper ring by mv 2 . The difference 

 is mv'ftv + v 1 ), which represents the electromotive force of the 

 current. If the wire were made to revolve around an unmag- 

 netized bar, the originating force of the current would be mv 12 . 

 The electromotive force just mentioned would exceed this nearly 

 in the ratio of 2v to v'. A very high velocity of revolution of 

 the wire would therefore be required to develope a sensible cur- 

 rent if the bar were unmagnetized. The above expression for 

 the electromotive force, viz. mv'C&v + vf), which is nearly equal 

 to 2mvv , i shows that this force is proportional to the velocity i/ 

 of revolution of the wire. The entire force developed in ten 

 revolutions of the wire should then remain the same if the velo- 

 city of revolution should be changed (as determined by Fara- 



