produced hy varying Magnetic Induction. 471 



Liverpool to act as Demonstrator, and I found him imbued 

 with similar notions, and also with more practical ideas for 

 carrying them out. Especially was he convinced of the fact 

 that an actual magnetic field in which to suspend the charged 

 body was entirely superfluous, but that varying magnetic 

 induction in a region near the charged body would be equally 

 effective and easier to manage. He wished to use, in fact, a 

 closed magnetic circuit with all its lines enwrapped in itself, 

 and to hang the charged body near but outside the circuit, so 

 that no lines of magnetic force really ever passed through the 

 charge, at least when the magnetic induction was steady. 

 But when the magnetic induction was varied, say by reversing 

 a magnetizing current round the closed magnetic circuit or 

 ring solenoid, Mr. Chattock pictured lines flashing past the 

 charged body in enormous numbers and producing the desired 

 effect. 



I am bound to say that the idea of the closed magnetic 

 circuit was not natural to me at first, and I resisted the views 

 of the inventor of the magnetic potentiometer for the best part 

 of a week. At the end of that time my conversion had taken 

 place; and from that time to this some sort of closed magnetic 

 circuit, with one or other form of charged body inside it, has 

 been set up in my laboratory and experiments made with it 

 from time to time. 



The theory of the effect I have observed it may be useful 

 just to write down in the most elementary manner. 



The magnetic induction in a solenoid being I, the E.M.F. 

 induced in any complete circuit round it, when I changes, is 



dl , . 



'^di ^1> 



If an E.M.F. act on a statically charged body at a distance r 

 from centre of solenoid, the work done in driving it once 

 round a circle is 



cQ = 27rrF, (2) 



where F is the mechanical force exerted on the charged body. 

 Now if the E.M.F. in (2) is the same as in (1), and is caused 

 by the induction I being either generated or destroyed, it will 

 only last a very short time, and we must therefore consider 

 what is the v»'hole impulse of the force during the time it lasts, 

 viz. 



</> 



==rYdt=^ (3) 



Jo 27rr ^ ' 



