546 



Dr. John Hophinson 



[April 26, 



meters. The sensibility of No. 3 galvanometer is much less than that 

 of Nos. 2 and 1, which are about the same. You see a new phenomenon 

 develop : there is a first maximum in Nos. 2 and 1 coils, followed by 

 a dwell and then a rise to a second maximum. Look at the correspond- 

 ing curves (Fig. 5). The maxima are occurring earlier than with the 

 force of 2*4 — that for No. 2 coil at 30 seconds instead of 50 seconds, 

 and of No. 1, the centre coil, at 130 seconds instead of 240. Again 

 take a higher force, this time 11 (Fig. 6) : see how marked the first 

 rise has become. On the curve notice that the maximum of No. 1 coil 

 is as great as No. 2 — with lower forces it has always been less. The 

 highest force I shall show you is 24 (Fig. 7), and it is really the 

 prettiest. The first maxima occur markedly at once. No. 2 coil 

 comes down to a small value, then rises to a substantial maximum in 



5 10 15 



1 Magnetising Force. S. 



Fig. 9. 



about 10 seconds, remains near its maximum value for 10 seconds, 

 and passes away. Whilst No. 2 has been at its maximum, No. 1 has 

 been quietly remaining undisturbed, and only when No. 2 has quite 

 finished does No. 1 suddenly move off to a maximum greater than 

 No. 2 at 35 seconds and then very suddenly disaj)pear. We must 

 not leave this part of the subject without showing one experiment 

 •with No. 4 coil, which is placed in the annular part of the magnet. 

 We will use the maximum force (24). The general character of the 

 changes is the same as before, but in this case the magnetism changes 

 at once in the interior of the ring, and later at parts nearer to the 

 outside. You see the same thing from the curve, Fig. 8. 



I am afraid I can only give you a very general explanation of the 

 peculiarities of these curves. I will first of all throw upon the screen 

 a diagram (Fig. 9) showing what are known as cyclic curves of 



