of the Coefficients of Induction. 473 



current was made. The armature was used as a test-coil, and 

 was coupled in series with a D'Arsonval galvanometer and 

 suitable resistances. A pointer was attached to the spindle, 

 and a scale to the framework of the machine, so that the 

 phase of the armature could be accurately read. The test 

 swings were taken both on making and on breaking the exciting 

 current, and were found practically identical. The swings 

 were taken for many different phases of the armature for one 

 complete alternation. The results showed that the ordinates 

 of the curve of electromotive force gradually differed from 

 those of the most favourably drawn sine curve to the extent 

 of +5 per cent. 



The values of the maximum ordinates of the E.M.F. curve 

 were also compared for one complete revolution of the 

 armature. Five swings were taken in the neighbourhood of 

 each maximum in order to determine its amount. The 

 curious result came out that these maxima varied alternately 

 between values denoted by the numbers 302 and 333. The 

 eight positive maxima did not vary more than \ per cent, 

 from the value 333, nor did the eight negative maxima vary 

 more than this from the value 302. Since the area of the 

 negative part of the curve must apparently be equal to that 

 of the positive part, it follows that the curve of E.M.F. must 

 be distinctly different from a sine curve. The result found in 

 this case was not at all that which was looked for. It seemed 

 probable that the values of the different maxima would vary 

 gradually, and only repeat themselves after a complete revo- 

 lution of the machine. It was expected that the curve of 

 E.M.F. would be of the nature of a compound sine curve, 

 having as one term a sine of large amplitude and a period 

 corresponding with the current alternations, and having as a 

 second term a sine of small amplitude but long period corre- 

 sponding with the revolutions of the machine. Professor 

 Perry has pointed out that, if the wave of E.M.F. is a sine 

 curve marked with minor ripples caused by the presence 

 of subsidiary sine waves of smaller period, the effect of 

 induction in the circuit is to flatten out the minor ripples so 

 that the current produced is more nearly a true sine wave 

 than the electromotive-force wave which is producing it. 

 This is, however, only true on the assumption that the minor 

 ripples have the shorter period. It is very possible that in 

 some alternating-current dynamos there is, due to some want 

 of symmetry, a ripple of small amplitude but having a long 

 period corresponding with the revolutions of the dynamo. 

 Whenever this occurs, the action of induction is to magnify 

 the minor ripples in comparison with the main alternating 



