1893.] 



Magnetic Viscosity, 



357 



in which A, B are the poles of the transformer or generator ; C, D the 

 terminals of the non-inductive resistance R ; H the coil surrounding 

 the ring ; P and Q the studs of a reversing key connected to the 

 quadrant of a Thomson quadrant electrometer ; L a key by means of 

 which Q could be connected with C or E at will ; and K a revolving 

 contact maker, through which P was connected to D. A condenser 

 was connected to P and Q, in order to steady the electrometer read- 

 ings. The contact maker K was bolted on to the axle of the gener- 

 ator. It consists of a circular disc of ebonite, about 13 in. in 

 diameter, having a small slip of copper, about y 1 ^ in. wide, let into its 

 circumference. A small steel brush presses on the circumference, 

 and makes contact with the piece of copper once in every revolution. 

 The position of the brush can be read off on a graduated circle. The 

 quadrant electrometer thus gives the instantaneous value of the differ- 

 ence of potential between the points C and D, or the points D and B, 

 according to the direction of the key L." 



Frequencies of 5, 72, and 125 per second have been tried, two 

 values being given to the potential difference at the terminals of the 

 alternator in each of the frequencies 72 and 125, making in all 5 

 complete experiments. The curves so obtained are given in figs. 8, 

 9, 10, 11, and 12 respectively. From observations of the values of 

 the electromotive force between C and D (fig. 4) at different times 

 in the period, a curve A (in each experiment) was plotted, giving the 

 magnetising force in terms of the time ; a similar curve was plotted 

 for the electromotive force between D and E, which, when corrected 

 by subtracting the electromotive force due to the resistance of the 

 coil H, gives the potential or time rate of variation of the induction 

 in terms of the time. Hence the area of this curve (B) up to any 

 point, plus a constant, is proportional to the induction corresponding 

 to that point. This is shown in curve C, which is the integral of B. 

 In each of the five experiments the ring with the non-inductive 

 resistance was placed across the terminals of the alternator, and the 

 excess of potential taken up by a non-inductive resistance. 



In fig. 5 the hysteresis curves for frequencies of 5, 72, and 125 

 are compared with the ballistic curve. These curves are marked 5, 

 72L, and 125L respectively. The corresponding values for B and H, 

 from which these curves have been plotted, are given in Tables II, 

 III, V, which have been obtained from the curves in figs. 8, 9, and 

 11 respectively. 



The most noteworthy features in these curves are that the curve 

 with a frequency of 5 is very near the ballistic curve, if allowance 

 is made for difference in the magnetising current, and that the curves 

 with a frequency of 72 and 125 deviate very materially, particularly 

 in the part of the curve somewhat preceding the maximum induction. 

 Hence the time effect mainly develops with a greater frequency than 



