THE THEORY OF THE DYNAMO 



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due to the self induction of the coils which produce sparks in them. 

 I have requested Professor Fitzgerald to take up that point, and will 

 leave it for him to consider. 



There is another point with regard to the dynamo which can be 

 treated in this simple manner with no use of the calculus. This is 

 very simple reasoning if you only know the principles. I shall con- 

 sider two machines similar in all respects, except that one is larger than 

 the other, or rather consider one machine, and see what the effect will 

 be when that machine gradually changes in size. 



The point from which we start shall be that the magnetic field is con- 

 stant in the two machines. For, owing to the fact that there is a limit 

 in the magnetization of a magnet, we cannot have a field with more 



FIG. 10. 



than certain strength produced by iron, and I will suppose that the 

 strength is reasonably near that maximum for iron. It cannot be up 

 to the maximum strength, of course, but somewhere near it. I made 

 some experiments many years ago upon an ordinary magnet, the results 

 of which were published in Silliman's Journal, by means of what I call 

 the magnetic proof plane. (Am. J. Sci., vol. 10, 1875, p. 14.) It 

 applies beautifully to dynamo machines, and I obtained everything with 

 it that I have referred to here. If I remember right, I found in that 

 magnet about one-third of the field that an iron magnet could pos- 

 sibly have. 



It is theoretically possible to get a force equal to the magnetizability 

 of the iron, but practically, I suppose that instance is about the case 

 of the ordinary dynamo machine. We start, then, with the supposition 

 that the field of force in the two machines, one of which is larger than 



