Mr. R. H. M. Bosanquet on Electromagnets. 349 



Power of velocity 

 to which current 

 is proportional. 



Inductions in dynamo 



having electromagnets 



such as in fig. 2. 



12,500 

 13,640 

 15,320 



Magnetizing forces 

 required by "bars 

 with pole-pieces. 



x, 



3 



2 

 1-25 



39-4 

 45-5 

 72-9 



We cannot of course assume that the magnets of the Gramme 

 machine follow the same law as that of our bars; in fact they 

 do not do so at all approximately, probably in consequence of 

 the large amount of cast iron in the machine. But these 

 numbers are enough to illustrate the limited nature of the 

 variations of the induction which may be possible during the 

 working of a dynamo, while the current produced varies in a 

 ratio of more than 2 : 1. By carrying the magnetization 

 higher still, we get a further considerable range of current 

 with a small change of the induction. Looking at fig. 2, we 

 see that it would be possible to draw a secant through the 

 point of inflection, representing a Froliclr's law, and deviating 

 but little from the curve from about 12,500 up to 18,000. In 

 this way the law of magnetization would be approximately 

 represented by a Froliclr's law over a very wide range. 



A word as to the physical meaning of the quantity 7, which 



is connected with x by the relation = x. This 7 may be 



said to be what determines the dynamic action. The dynamic 

 action consists of the summation of an infinite number of ele- 

 ments, whether of magnetism, current, or E.M.F., which 

 originate in one small change of velocity. It is only where 

 these elements are successively less and less, and so form a 

 convergent series, that their sum is finite, and gives rise to a 

 definite behaviour of the machine, or to what we may call a 

 state of dynamic equilibrium. 



Let 7 be then the ratio of diminution of the successive 

 elements, due to the additional element of induced magnetism 

 being less than the element of inducing current. It is easy 

 to see that we may express the whole change of C, say, due 

 to a small change of velocity dn, thus : — 



AC n 2 . dn 



1 dn 



" 1 — y n 



