as a Generator and as a Motor. 25 



Calling the current C, and the resistance of the armature R, 

 we know that 



e 1 = CR; 



The resistance of the armature of the motor in question 

 was *027 ohm, which we will call '03 in order to make some 

 allowance for the effect of heating. 



In the first and last readings of the second set of tests 

 (curve B) we have therefore the following conditions : — 



Speed. e x . e. 



680 -87 99-13 



660 3-75 96-25 



Now 



680x96-25 _ QCM a 

 660 yU ■ 



From which it appears that the counter electromotive force was 

 exactly proportional to the speed, as is to be expected where 

 the field is constant and the magnetic distortion nil. 



The other cases do not work out with the same accuracy. 

 The results are, however, quite within the limits of error in- 

 separable from the rather rough conditions of these workshop 

 tests. 



One other fact may be pointed out in connection with these 

 tables of results and curves. In the case of dynamos working 

 with a constant field, the output with the same current is almost 

 exactly proportional to speed, as the E.M.F. is also simply 

 proportional to speed. So with the motor, the speed is pro- 

 portional to the E.M.F. of supply; and the work, with the 

 same current in the two cases, is simply proportional to speed, 

 therefore to E.M.F. 



With regard to the theory alluded to above, that in a motor 

 the fields should be much weaker and lighter than the arma- 

 ture, it may be pointed out that in this, as in the other points 

 mentioned, there is a real analogy between motors and gene- 

 rators, inasmuch as, according to the views briefly expressed 

 in this note, such a motor, although a bad one from most points 

 of view, would have a certain advantage in that for its weight 

 it would do a good deal of work. Similarly, dynamos may be, 

 and have been built, which, although far from efficient and 

 reliable, will give a large output for their weight. Their con- 

 struction is, however, scarcely to be recommended on this 

 ground, except, perhaps, under very exceptional circum- 

 stances; and they, as well as the motors constructed on the 

 same lines, appear to be instances of a mistaken apprehension 

 of elementary principles. 



Having briefly traced the analogies between generators and 



