EuTHEKFOKD. — On Magnetic Viscosity. 187 



(The wires connecting the non-inductive resistance r to the 

 condenser were short, so that we may assume, without any 

 sensible error, that the diiierence of potential between the 

 coatings of the condenser at any instant is equal to the 

 E.M.F. between the terminals L and M of the resistance r.) 



When the lever C D is reached, the remainder of the 

 quantity of electricity required to charge up the condenser to 

 the steady difference of potential passes through the galvano- 

 meter. 



The throw of the galvanometer is therefore proportional 



to the value of 37 at that instant. 

 at 



By gradually increasing the distance between the levers by 



turning the screw we get a series of values corresponding to 



-^ for different values of time. 

 dt 



When the current has fully risen -tt = 0, so that we then 



get no throw in the galvanometer, as the whole quantity flows 



through the shunt. Since the value of -,- is known at any 



instant, the induction N through the iron for that instant may 

 be calculated, and, since the corresponding current is known, 

 we have all the data required to plot out the hysteresis curve 

 for a very rapid cycle. 



EXPEKIMENTAL VeEIFICATION. 



In order to see to what degree of accuracy the time- 

 apparatus could be depended on, the time of rise of the cur- 

 rent in a coil of known self -inductance was compared with the 

 theoretical time of rise as determined from the equation 



c=l(i-^~^') 



The coefficient of self-induction L was very accurately de- 

 termined. The mean value of L was found to be 2-315 x 10''cm. 

 The period of the ballistic galvanometer needle in this and all 

 succeeding experiments was 7 seconds. The sensitiveness of 

 the shunt-levers of the time-apparatus was tested, and they 

 were found to work perfectly, no correction having to be 

 made. 



The resistance of the whole circuit was 15"65 ohms, and a 

 battery of two Daniells's cells was used in this case. 



The following are the results of a series of observations of 

 the deflection of the galvanometer, and the number of turns 

 of the screw at which the deflections were observed. Each 

 observation is a mean of two experiments at least, and the 

 curves in manv cases were determined several times : — 



