346 
DRS. J. AND E. HOPKINSON ON DYNAMO-ELECTRIC MACHINERY. 
taking PR as ordinate of any point in tire normal characteristic, cut off QR equal to 
-—-4X?7iC^r, that is, draw OQ so that 
V 2 4 
tan QO,r= l —-j kirin —— 
V !24/ \ V7T J 
v —- 1 A 3 Xto 
z/ 2L 
2 tt)1 - 
v 
It is noteworthy that as the current C, and therefore OPt increases, PQ, the induction, 
wiil attain a maximum and afterwards diminish, vanish, and become negative. That 
in series wound machines the E.M.F. has a maximum value has been many times 
observed. The cause lies in the existence of a waste field not passing through the 
armature, and in the saturation of the magnet core. 
The effect of the current in the armature on the potential between the brushes of 
any machine is the same as that of an addition to the resistance of the armature pro¬ 
portional to the lead of the brushes, and to the ratio of the waste field to the total 
field, combined with that of taking the main current — times round the magnets in 
direction opposite to the current c. The preceding investigation tells the whole story 
of a dynamo machine, excepting only the relation of X to C in order that the brushes 
may be so placed as to avoid sparking. The only constant or function which has to be 
determined experimentally for any particular machine is v, the ratio of total to 
effective field, all the rest follows from the configuration of the iron and the known 
properties of the material. 
