394 



REPOET 1892. 



are given in a curve showing the relation between the magnetising force, 

 H, and the resulting induction, B. 



' The same curve continued through a complete cycle of positive and 

 negative magnetisation gives the value of the hysteresis of the iron, and 

 the variation of this quantity with the induction is plotted in another 

 curve. 



Fig. 19. Fig. 20. 



Mz^vetisiTiglbrcc: 



.^toiactuM/. 



' These two curves give the whole magnetic properties of iron, and hence 

 are in constant use in all calculations of electro-magnetic machinery, the 

 more so as they have not been reduced to a convenient equation for 

 mathematical treatment. 



' One of the most important curves in connection with this is the 

 characteristic curve of a dynamo, as drawn by J. and E. Hopkinson.' 

 The relation between the difference of potential at the terminals of a 

 dynamo running at constant speed, and the exciting current round the 

 field magnets, is plotted as a curve (fig. 21), and they show how, from a 



Fig. 21. 



_; u ^ ■ ^.^^.^^--^-^^ r=^- — -> ,- 



I 1 ! I -_i^— — . I I ' 



I I 1^ ' I 1 ! 



I I ^^ I 1 I I I ' 



J 1 ^^_. + 4 -f -I , ^ J- 



I y< ' ' I ' ' ' 1 



I ,' / ' 1 ! I ' 1 1 

 ' / 'it I ' I 



I yC [ J j 1 -I ^ -I H- 



I /l 1 i I I 1 I ' I 



I / ' ] ' 1 I 1 I 1 ' 

 _|-/_|-.-^-_— I—- .; 4-— i ;.— |— -I 



./ I ' I 1 i I I 1 I 



/ ' I I I I ' ' I I 



4 1- 1 1 4 -I -r ^ + H- 



AmpeTTR tarns tvzzticZ' JFiisZol/ JtieiaTL&ttS. 



knowledge of the curve of magnetisation of the iron and the dimensions 

 of the machine, the shape of the curve may be foretold with considerable 

 accuracy. When developed in a third direction, representing the current 

 flowing through the armature, a surface is obtained showing the complete 

 behaviour of the dynamo. 



' Curves are of the highest importance in the manufacture of dynamos, 



' PUl. Tram., 1886. 



