278 Sir W. Thomson on the Time-integral of a 



siderable excess in the time-integral (as shown by a ballistic 

 galvanometer) of the secondary current on the make above 

 that on the break. A magneto-static galvanometer in the 

 primary circuit showed the current before the break to be 

 always a little less than immediately after the make. This 

 difference was due to heating of the primary circuit, because 

 a potential galvanometer applied to the two terminals of the 

 voltaic element used, which was a large storage-cell, showed 

 no sensible drop of potential during the flow of the current 

 in the primary circuit. It was, however, insufficient to account 

 for the large differences found between the ballistic deflexions 

 produced by the induced currents on the make and on the 

 break. A rapid succession of makes and breaks has given 

 large irregular permanent deflexions of the ballistic galvano- 

 meter, sometimes in one direction, sometimes in the other, 

 which we have found to be chiefly due to magnetic sus- 

 ceptibility of the steel needle of the ballistic galvanometer : 

 and we find that this cause has probably vitiated the observa- 

 tions on the effects of single makes and breaks. I have there- 

 fore arranged to have experiments continued, with a coil of 

 very fine wire bifilarly suspended, instead of the steel needle 

 of the ballistic galvanometer ; a steady current through the 

 tine wire being maintained by an independent voltaic battery. 



Hitherto the make and break have been performed by hand, 

 dipping a wire into and lifting it out of a cup of mercury. 



Various well-known methods may be used to render either 

 the break or the make so gradual that we may be sure of the 

 induced current running practically full-lore through the 

 secondary. On the other hand, very sudden breaks may 

 be effected by separating two little balls or other convex 

 pieces of copper by the blow of a hammer. The experiment 

 may be varied by short-circuiting the ends of the inductor 

 and allowing the current from the battery to continue 

 flowing through electrodes of sufficient resistance not to 

 allow an injuriously great amount of current to flow. 

 These electrodes, between the voltaic element and the ends 

 of the inductor-wire, may have large self- inductance given to 

 them by coiling them round a closed magnetic circuit of soft 

 iron. The starting of the current in the inductor-wire will 

 thus be rendered much more sudden than the stopping ; and 

 the induced current in the secondary will no doubt be found 

 stronger on the stoppage than on the start. Mr. Tanakadate 

 is continuing the experiments with these modifications in 

 view. 



The mathematical foundation of the common opinion that 



