430 The Hon. J. "W. Strutt on an Electromagnetic Experiment. 



complete vibrations in a minute observed. The numbers were : — 



Arrangement (a) . 4 J, 6, 6 : mean 5. 

 (b) . 21, 19, 19 : „ 20. 



The superior efficiency of (b) is very conspicuous. 



There is another way in which the subject may be investigated. 

 If the secondary current containing a galvanometer be broken so 

 quickly after the primary that the induced current has not time 

 during the interval sensibly to diminish, the deflection of the 

 needle may be considered to measure the initial value of the in- 

 duced current. To carry out this experiment properly would 

 require rather elaborate apparatus, on account of the necessity 

 of a constant interval of time between the breaks. The contri- 

 vance that I used was of home manufacture and very rough, and 

 acted by the almost simultaneous withdrawal of wires from two 

 mercury-cups. The secondary circuit in case a consisted of the 

 two wires of a large coil, A, joined consecutively, and of a short 

 wire galvanometer. Iron wires were inserted in A in order to 

 increase the duration of the induced current. In b only one 

 wire of A was used, the resistance being made up by the substi- 

 tution of another wire, whose self-induction might relatively be 

 neglected. The total currents in the two cases would be as 2 : 1, 

 and the initial currents as 1 : 2. The deflections of the galvano- 

 meter-needle were rather irregular; but the sum of ten throws 

 in case a was 317°, while in b it rose to 480°, so that there 

 could be no doubt as to the reality of the phenomenon. 



Returning to the experiments with steel needles, I thought it 

 desirable to compare the permanent magnetisms developed in 

 two cases where the initial currents were equal. With this object 

 the primary current (originating in a Grove cell) was passed 

 through two wires, Q„ Q 2 , of the quadruple coil combined for 

 self-induction. The induced circuit included Q 3 , Q 4 and the 

 magnetizing spiral. The arrangements in other respects being 

 as before, I obtained — 



13, 14, 15, ]6, mean 14 J vibrations per minute*. 



Q 2 was now removed from the primary circuit (the resistance 

 being made up to former value) and Q 4 from the induced circuit. 

 The numbers now were — 



11, 10, 12, 12: mean ll^. 



Q 9 being next replaced, but not Q 4 , there resulted, — 



35, 35, 33: mean 34. 



* The experiments were not made exactly in the order here adopted for 

 convenience, but were broken up. 



