26 On the Dynamo as a Generator and as a Motor, 



motors thus far, the subject may be dismissed for the present 

 with the remark that a further consideration of the matter 

 serves only to reveal the fact that in almost every phase of their 

 actions the two machines afford further opportunities of a direct 

 comparison. 



Turning now to the question of efficiency alluded to above, 

 it is evident that there is some important cause of loss or waste 

 of energy in electric motors which is absent in generators. 

 This has always been a kind of lacuna in the explanations 

 of the action of motors ; and in order to localize the loss 

 and to ascertain its cause, the several possible sources of waste 

 were carefully considered. These are : — (a) Friction at the 

 bearings, air-friction, and friction of the brashes against the 

 commutator. (6) Loss of energy in heating the armature 

 and field-conductors, and a certain loss due to self-induction, 

 (c) Loss by the production of eddy-currents in the iron. 



Now it is evident, especially with a generator or motor 

 having the qualities sketched above, that at the same speed, 

 and working with the same currents in its conductors, the 

 losses under (a) and (b) must be identical, whether it be 

 working as a generator or as a motor. And as with such 

 conditions its efficiency as a motor is lower than as a generator, 

 the cause of the loss must be sought under (c), i. e. the eddy- 

 currents in a dynamo must be less than in a motor, all other 

 conditions being the same. And such is the case, the expla- 

 nation arrived at by the writer and Mr. Watson being a very 

 simple one. 



In a dynamo the rotation of the armature causes eddy- 

 currents to be generated in the iron core, in the same direction 

 as in the conductor proper with which the core is surrounded. 

 Of course, as the armature is always more or less subdivided 

 or laminated in a direction at right angles to the lines of force, 

 any circulation of currents round the core is avoided ; but local 

 currents, which are aptly called eddies, are set up, and, taken 

 as a whole, these eddy-currents on the outside of the core are 

 in the same direction as the current flowing in the copper 

 conductors. 



In an electric motor, however, the eddy-currents and the 

 currents in the copper conductor are in opposite directions ; 

 as, although the E.M.F. set up in the conductor is in the same 

 direction in a motor as in a dynamo, the current in the former 

 is forced through the armature in a direction contrary to the 

 E.M.F., or opposite to its course in a generator. According 

 to the laws of induction, therefore, it will be seen that while 

 in a dynamo the two sets of currents, those in the iron and 

 those in the conductor, tend to oppose and to reduce one 



