472 Mr. W. E. feumpner on the Variation 



It is noteworthy that these ripples are dependent not so much 

 on the speed as on the value of the current. 



17. Plate III. fig. 4 shows an example worked out on the 

 basis of experiments actually made on the Kapp and Snell 

 transformer already referred to. The resistance of the circuit 

 is for simplicity assumed to be 1 ohm, so that the number 

 which represents the coefficient of self-induction L p in 

 secohms also represents the time-ratio in seconds. The 

 values of L p given above were obtained for the primary 

 coil when different currents A were traversing the secondary, 

 and should be plotted, not with the values of A, but with the 

 values of n 2 A/?i 1 , where n^n 2 is the ratio of the turns on the 

 primary to those on the secondary. This has not been done, 

 as it merely alters the scale of current. The values of L p 

 differ slightly for increasing and decreasing currents ; this 

 has been disregarded, as in the particular case taken the results 

 would have been but very little altered. The electromotive 

 force acting in the primary circuit (the secondary being open) 

 is supposed to be a simple sine wave having a period of 

 0*16 second, and is represented by the curve C . The current 

 curve obtained will depend for the first few alternations on 

 the initial circumstances. The curves C 1? C 2 , C 3 , and C 4 

 represent the first half-wave for different initial values of the 

 current. Of these, C 3 may be taken as the curve which 

 periodically repeats itself and to which all the others will 

 eventually come. It is not in appearance so markedly 

 different from a sine curve as Ci and C 2 . This is because 

 the impressed electromotive force is not sufficient to produce 

 a current capable of magnetizing the iron beyond the satura- 

 tion-point, at which the value of the coefficient of self-induction 

 begins to diminish. Otherwise the current curve would have 

 sharp peaks in it, as indicated in the curve C r This may have 

 something to do with the fact that it has been found best not 

 to allow the iron of transformers to be magnetized beyond the 

 saturation -p oint* . 



18. It is usual to assume that the electromotive force given 

 by alternating-current dynamos can be represented by a sine 

 function of the time with sufficient accuracy for practical 

 purposes. In the case of a Ferranti dynamo at the Central 

 Institution, however, the approximation to a sine wave is not 

 very good. 



The field-magnets were excited with a small current by 

 means of accumulators. A resistance of 100 ohms was 

 placed in the circuit to reduce the time-constant when the 



* See Mr. Gisbert Kapp's paper " On Alternate Current- Transformers," 

 Proc. Soc. Tel. Engineers, Feb. 1888. 



