552 On the Effects of Electric Currents in Iron, &c. [April 26, 



induction has diminished, and we have, lastly, the curve c, where the 

 period, of the cycle is four-tenths of a second, and you will observe 

 that it has much more diminished. This is owing to the fact that 

 changes of induction in the centre of the iron hardly exist, and that 

 therefore the total effect is materially diminished. The diagram 

 (Fig. 15) is for steel. We have a the natural curve of induction ; 

 o the case when the period of the cycle is three seconds ; and c 

 when the period is six-tenths of a second. Curves such as these, of 

 course, very readily give you the average effects upon the whole 

 mass of the iron. The curves which I have shown you give the 

 particular effects of different parts of the mass. 



Fig. 15. — Magnetic Cubve-Tbaceb Ccbves of Steel. 



(a) Cycle performed slowly. 

 (h) Period of cycle 3 seconds, 

 (c) Period of cycle ■ G second. 



In conclusion, let us indulge in a little wild speculation, not 

 because it is probable that it is in any sense true, but because it is 

 interesting. [Suppose a magnet were made exactly like the one on 

 which we experimented, but of the size of the earth, and that some 

 mighty electrician generated such a current in its copper coils as 

 would give a magnetising force of 2*5, and then reversed it, it would 

 take some thousands of millions of years before the rate of dis- 

 turbance at the centre attained its maximum value. The speculation 

 I suggest is this : is it not conceivable that the magnetism of the 

 eaith may be due to currents in its material sustained by its 

 changing induction but slowly dying away ? 



[J. H.] 



