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



in passing from one case to the other. The primary current was 

 excited by a Daniell cell in the two wires of A arranged collate- 

 rally, and was interrupted at a mercury-cup. The needle was 

 submitted to the break induction-currents only — although the 

 make currents had no perceptible magnetizing-power, on ac- 

 count of the relatively large time-constant of the primary cir- 

 cuit, and the consequent slow rise of its current to the maximum. 

 On actually submitting a new needle to the current a, I 

 obtained after one discharge 12 vibrations (complete) per 

 minute, a number raised after several discharges to 15. On 

 the other hand, a new needle after one discharge b gave 

 only 5 per minute, and was not much affected by repetition. 

 The last needle being now submitted to discharge a gave 8^, 

 and after several 12. Other trials having confirmed these 

 results, there seemed to be no doubt that the current a was 

 the most efficient magnetizer. There remained, however, some 

 uncertainty as to whether the time-constant, especially in b, 

 was sufficiently large relatively to the time for which the spark 

 at the mercury cup lasted to allow of the initial current being 

 formed undiminished by resistance. In order to make the fall 

 of the primary current more sudden, I connected the breaking- 

 points with the plates of a condenser belonging to a Ruhmkorff 

 coil, and now found but little difference between the magneti- 

 zing-powers of a and b. Seeing that the theoretical condi- 

 tion had not been properly fulfilled, I prepared another triple 

 coil of much thicker wire, and, for greater convenience, arranged 

 a mercury-cup commutator, by means of which it was possible 

 to pass at once from the one mode of connexion to the other. 

 The magnetizing spiral was still of fine wire coiled, without any 

 tube, closely over the needle, and its ends were soldered to the 

 thicker wire of the triple coil. 



The experiment was now completely successful. Out of the 

 large number of results obtained, the following are selected as 

 an example. A new needle was submitted to the break dis- 

 charge of arrangement b, and gave, 



After 1 discharge 



,19 



per 



minute. 



)) 



3 



}) 



23 





)> 



)) 



6 



3) 



24 





)) 



Another needle was now taken and magnetized by discharge a. 

 It gave, 



After 1 discharge, 11 per minute. ' 

 „ 3 „ 12 ,. 



On submitting this needle, which had received all the mag- 



