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Dynamo-Electric Machines. [Apr. 15, 



equation may be thus expressed in words, if X be such that the coils at 

 commutation embrace the whole or nearly the whole induction. The 

 effect of the current in the armature upon the difference of potential 

 between the brushes of any machine, is the same as that of an 

 addition to the resistance of the armature proportional 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 



V7T 



magnets in a direction opposite to the current c. Many consequences 

 can be deduced, of which we may notice the following : — In a series 

 wound dynamo C is equal to c, and if c be increased beyond a certain 

 point, I must attain a maximum and then diminish; this has been 

 frequently observed. We now see that it depends upon the existence 

 of a waste field. Secondly, let the coils of the magnets be entirely 

 disconnected, and let X be the negative : if the armature be short 

 circuited through a small resistance and be run at a sufficient speed, 

 a large current may be produced in the armature. This latter deduc- 

 tion we have verified by direct experiment. 



The efficiency of the type of dynamo machine upon which the 

 experiments before indicated have been made, has been accurately 

 determined by the device of coupling two similar machines, both 

 mechanically and electrically, so that one should act as a generator of 

 electricity, driving the other electrically, whilst the latter acted 

 as a motor driving the former mechanically: the loss of power 

 required to keep the whole combination in movement being determined 

 by direct dynamometric measurement, and the power passing electri- 

 cally from the one machine to the other being measured by ordinary 

 electrical appliances. 



The whole of the experiments were carried out at the works of 

 Messrs. Mather and Piatt, to whom we are indebted for the ex- 

 ceptional opportunities we have enjoyed of putting theoretical conclu- 

 sions to the test of experiment on an engineering scale. 



The Society then adjourned over the Easter Recess to Thursday, 

 May 6th. 



