472 Dr. 0. Lodffe on an Electrostatic Field 



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And this is the momentum generated whenever the sole- 

 noidal magnetism is generated or destroyed. This therefore, 

 or some simple multiple of it, is what is observed in the ex- 

 periment. 



Now in calculating out a numerical value for this we shall 

 write 



1= -, — - =fjt,G times a length, 



and 



Q = SV = KV times a length. 



Hence in the product IQ occurs the product /aK, and this is 

 the reciprocal of the square of the velocity of light. 



This amply accounts for the smallness of the observed effect. 

 By various devices the geometrical parts of I and Q can 

 be increased somewhat to make the observation more easy, 

 but nothing can get over the fact that the velocity of light 

 squared occurs in the denominator of the expression. 



To return to the account of experiments devoted to finding 

 this effect. If there is any importance in the result, and I 

 hope there is, though I sometimes fear there is not, much of 

 the credit belongs to Mr. Chattock. The only reason why 

 this is not a joint paper is because he had left the laboratory 

 before a result was obtained. 



I must mention also that he showed me, somewhere about 

 this same time, an ingenious paper read before the Society of 

 Telegraph Engineers some two or three years back by Mr. 

 George Forbes, in which that gentleman, making free use of 

 the analogy between a magnetic circuit and a voltaic circuit, 

 predicted almost the precise arrangement which we have found 

 successful, as a magnetic analogue of a tangent-galvanometer. 



Some other predictions were also made on the strength of 

 the same analogy ; but I have made no experiments like them 

 at present. 



The experiment of which I have to speak may be described 

 as follows :^You buy at a shop a large hank of iron wire 

 weighing 28 lbs. or so : you cover it with tape and wind it 

 over with No. 14 copper wire connected to a reversing-key 

 and storage battery. At the centre of the ring you place a 

 glass vessel in which are delicately suspended two oppositely- 

 charged very light conductors, made, for instance, of aluminium 

 foil, connected to each other and to a mirror by a shellac arm. 

 You then work the reversing-key by hand in unison with the 

 natural period of vibration of the suspended arm, so as to keep 

 on reversing the magnetism in the ring, and you hope thus 



