I'll. I IOR 15 



brushes, and consequently make the reading >f the volt- 

 meter lees than the true imluced tension. 



If the speed be plotted along a horizontal axis and the 

 reading* of the voltmeter along a vertical axis, then if M 

 is constant for n current in the magneto, the locus 



f the observed points should be a straight line, and this 

 should pan through the axis if there be no residual 

 magnetisation. 



.rives the retmltfi of an experiment made mi th- 



arc light dynamo, previously described. In this experi- 



rlie rurrent w.-i* k.-pt constant, first at two and then 



at four amperes, and the speed varied, the tomion 



corresponding to each speed being observed and jltt. ! in 



t he figure. There was no em-rent in the armature. The 



md lie on straight linen passing through the 



i, showing that the induction factor i- imi.-|..-ii.|.-iii 



>r the speed. 



1 f a current is passing in the armature during a test, 

 the tension observed at the brushes will be less than the 

 induced tension by an amount CK volts, where C is the 

 current and If the resistance of the armature from brush 

 to brush. The product CK is called the ' heat drop.' 

 Hence to find M when a curn-nt is passing, we must 

 measure C 1 ; knowing /'. we can add the product i 

 the measured volt* at the brushes. 



Example 9. We wish to know the torque on til-- 

 shaft of n i u pled generator due to a current . 

 amperes pasting in the artnntut-- The speed <>f the 

 dynamo as indicated by a tachometer is 400 r.| m . 

 while the voltmeter reads 120 volU on open circuit, 

 ng the volts by the revolutions per second, we find 

 i..n factor to be 18; the torque thrown on the 



