50 Mr. J. J. Thomson on some Electromagnetic 



The current through the primary circuit was produced 

 by three Grove cells connected in series. The primary cur- 

 rent was broken six times whilst the same needle was in the 

 magnetizing-spiral, in order to get rid of any irregularities in 

 the breaking of the contact. It Avas found to be a matter of 

 indifference whether the needle was left in the spiral whilst 

 contact was made in the primary circuit, or whether it was 

 taken out for each make : this is doubtless owing to the make 

 being a much more gradual operation than the break. The 

 intensity of the currents through the spiral was estimated by 

 the magnetization of the needle placed inside it, which was 

 determined by the number of times the needle oscillated in a 

 minute when suspended by a silk fibre. A new needle was, 

 of course, used for each experiment. 



If now the secondary coils be arranged according to the 

 scheme denoted by ABODE F, then in whatever part of the 

 secondary circuit the magnetizing-spiral be placed, no ap- 

 preciable magnetization of the needle occurs. If, however, 

 the secondaries were connected up according to ihe scheme 

 AB — DCEF, then a needle which had been inside the mag- 

 netizing-spiral vibrated in one case 29^ times a minute, in 

 another 28. If the circuit was closed by joining the ends A 

 and F, then a needle placed in the magnetizing-spiral vibrated 

 50 times a minute ; so that the magnetism produced in the 

 open circuit is quite comparable with that produced in the 

 closed. In some experiments to be described later the mag- 

 netism produced in the open circuit was actually greater 

 than the magnetism produced in the same circuit when closed. 

 If the coils were arranged according to the scheme ABDCEF — , 

 the order of the coils being the same as before, but the mag- 

 netizing-spiral being put at the free end of the secondary 

 instead of in the middle, then a needle placed inside the mag- 

 netizing-spiral vibrated only 3 times a minute. If, however, 

 every thing be kept the same as before, except that the ends 

 A and F are connected with the plates of a condenser of about 

 half a microfarad capacity, the magnetizing-spiral being 

 between the end F and a plate of the condenser, then a needle, 

 after being placed inside the magnetizing-spiral, vibrates 41 

 times per minute. 



These experiments show that in some cases there is a consi- 

 derable transference of electricity along some parts of the 

 coil, even although it be open. This is due to the fact that 

 the coil acts like a condenser; and the difference between the 

 two cases mentioned above can be easily explained. For if 

 we proceed along the secondary coil, the potential produced 

 by the break of the primary circuit will increase continuously 



