1892.] 



Dynamo-Electric Machinery. 



57 



4*96 



112-2 sq. cm., and therefore an ordinate of 1 cm. = -Q2-2 x ^ 6 = 



4'42 x 10* lines in 10°. Hence, an ordinate of ] cm. represents an in- 

 4*42 



dnction of gg^ x 10 4 — 647 lines per sq. cm. The difference between 



ordinates at 323° and at 233° is 4*2 ; hence, the difference of induction 



2/cmC 3iX 104x12-3 

 is actually 2718. Theoretically, we have — ^ — = = 



2870, as against 2718 actually observed. 



At page 345 of the paper on Dynamo-Electric Machinery it is shown 

 that 



v — 1 A 3 / 4XwC 



1 + 4\mC -r = F Uirwc— 



v 2l 2 \ v 



where I = F(47rac) is the characteristic curve when = 0, and X is 

 the lead of the brushes. 



The following is an endeavour to verify this formula. The 

 potentials both upon the magnets and upon the brushes were taken 

 by a Siemens' voltmeter, and are rough. The speeds were taken by 

 a Buss tachometer, and there is some uncertainty about the precise 

 lead of the brushes, owing to the difficulty in determining the precise 

 position of the symmetrical position between the fields, and also to 

 the width of the contacts on the commutator. 



It was necessary, in order to obtain a marked effect of the armature 

 reaction, that the magnet field should be comparatively small, that 

 the current in the armature should be large, and the leads of the 

 brushes should be large. 



The two machines had their axles coupled so that No. 1 could be 



