1895.] on the Effects of Electric Currents in Iron, &c. 



543 



coil occurs immediately upon reversal of the current, and it occurs 

 after some time in No. 2 coil, and after a much longer time in No. 1 

 coil, and also to observe the difference of the effects according as the 

 magnetising current is large or small. In all these experiments a 

 considerable battery power is used, and its excess of E.lVi.F. is taken 

 up with non-inductive resistances consisting of lamps ; that is in order 

 to confine the effects to the currents in the iron, and to get rid as far 

 as possible of the effects of induction in the copper coils themselves. 

 We will begin with a somewhat low force, 2 • 4. You notice that, as 

 shown in Fig. 3, the galvanometer of No. 3 coil is promptly deflected, 

 and as promptly returns to zero, because there is no depth of iron 

 outside it in which currents can circulate. No. 2 coil soon moves off 



Fig. 4. 



to a substantial deflection, and presently, after 50 seconds have 

 elapsed, it moves off to a greater deflection. The behaviour of No. 1 

 coil is not dissimilar, but it takes much longer. As in the case of 

 the transformer, the same thing can be shown as a curve (Fig. 3). 

 Observe the relation of currents in the exploring coils to time. No. 3 

 coil rises to a maximum, and at once ceases. No. 2 goes off fairly 

 rapidly, dwells for a time, and then rises to a full maximum at about 

 50 seconds ; whilst No. 1 coil goes through the same events, but more 

 slowly, as is shown by the increased abscissaa taking four minutes to 

 reach its maximum. Here is a curve (Fig. 4) for a lower force, 1 • 2, 

 half of the last : notice how great the difference — the maximum is 

 reached much more speedily and the currents drop more slowly. 

 Take a somewhat higher force, 6*0, to see its effect on the galvano- 



