Continuous-current Transformers. 161 



With respect to self-induction and its effects in such 

 machines, it may be remembered that, in an ordinary single- 

 wound dynamo, the effect of self-induction is to add to the 

 armature a spurious resistance, owing to the successive stop- 

 page and restarting twice in every revolution of the current 

 in each section. Now if matters were so arranged in the 

 double-wound machine that commutation of the current in 

 any one section of the primary should occur exactly at the 

 same instant as commutation of the current in that section of 

 the secondary wire which was wound over the same part of 

 the common core, it is clear that, as the currents in the pri- 

 mary and secondary circulate around the core in opposite 

 senses, the stopping of the current in the one would tend to 

 stop the (inverse) current in the other, and the starting again 

 of the current in the one would tend to start the inverse 

 current in the other. In other words, the mutual induction 

 between the two sections would tend to counteract in both 

 the effects of self-induction. In practice it is impossible to 

 fully reaHze this neutralizing effect ; but that it is very nearly 

 realized is evidenced by the almost complete absence of 

 sparking in such machines. 



In the above argument it has been assumed that the mag- 

 netism of the field -magnet was constant in amount ; but 

 incidentally it was noticed that the expressions were indepen- 

 dent of the magnetic field. The more powerful this is, the 

 slower need the armature run to generate the respective 

 electromotive forces. Assuming that the transformer is 

 supplied at constant potential at its primary terminals, and 

 that the internal resistance of the secondary winding is small, 

 it will be self-regulating, giving a constant potential at its 

 secondary terminals quite irrespective of the variations of its 

 speed with the load. 



In yet one other respect does the action of the motor- 

 generator resemble the alternating transformer: when supplied 

 at constant potential it is almost exactly self- regulating in 

 respect of its automatic action in adjusting the amount of the 

 inflowing primary current, in proportion to the outflowing 

 secondary current. When the secondary circuit is entirely 

 opened, the motor part runs just so fast that the back electro- 

 motive force Ej in the primary part increases and dams back 

 the primary current; only just so much flowing through as 

 will suffice to drive the machine against the reactive forces of 

 mechanical and magnetic friction. 



Precisely similar relations to those traced out for motor- 

 generators hold good in the commuting-transformers. The 

 reactions between the primary and secondary windings go on 



Phil. Mag. S. 5. Vol. 26. No. 159. Aug. 1888. M 



