12 The Hon. J. W. Strutt on some Electromagnetic Phenomena 



an induced current can produce, after repetition, as the measure 

 of its magnetizing powers. 



The same kind of reasoning may be applied to more compli- 

 cated problems. As an example, we may recur to a former 

 combination, in which the primary current is excited in the 

 wire Aj, while the secondary circuit includes A 2 , B 1? and the 

 magnetizing spiral. The initial current y Q , on which, as we 

 have seen, the magnetizing power mainly depends, will be greatly 

 increased if the ends of the wire B 2 are joined so as to make a 

 tertiary circuit j for a current in B 2 is developed, which, being 

 equal and contrary to that in B lt neutralizes its action on the 

 magnetic field, and so allows the energy, immediately after the 

 sudden rise of the current x in A v to be vanishingly small, 

 exactly as when the secondary circuit consisted of A 2 alone. 

 The effect of closing B 2 is therefore to increase the current y 

 from — \ x to — #, and at the same time to produce a new cur- 

 rent denoted by + x in B 2 itself. The following were some of 

 the experimental results : — 



fA new needle, 

 i 



B 2 open< After 1 break-discharge, gave 7 \ per minute. 



L » ° >-> a " J) 



On closing B 2 we had, with the same needle, 

 After 1 discharge, 15 per minute. 

 )> 8 „ L7 „ 



A new needle gave, 



After 1 discharge, 17 per minute. 



Another new needle in the tertiary circuit gave, 

 After 1 discharge, 16 per minute. 



The magnetizing spiral was here removed from the secondary 

 to the tertiary circuit ; and although its resistance was by no 

 means relatively small, the results are none the less compara- 

 ble; for in this experiment resistances (within limits) are of 

 no account, and the self-induction of the spiral was quite in- 

 sensible. 



Had there been twenty coils ABCD similar to A B, 



with the wires B 2 C^ C 2 D l5 &c. connected, as in the experi- 

 ment just described, the magnetizing power of the current in 

 the last would not, I imagine, be much less than in the first ; 

 for the condition of minimum energy would still be fulfilled by 

 currents in the series of coils all equal in numerical value, and 

 alternately opposite in algebraic sign. On this subject much 



