KuTHERFORD. — Oil Magnetic Viscosity. 



185 



divided up into twenty divisions, and the apparatus was quite 

 delicate enough to show a difference for every division of the 

 screw-head when determining the times of rise of currents of 

 very short duration. 



The apparatus could therefore readily measure intervals of 

 time up to ^.j^^q of a second. 



Now, this gives the time-intervals as derived from theory. 

 In practice the time- intervals corresponding to one turn must 

 be slightly greater, due to the retardation of the falling weight. 

 The causes of the retardation are — (1) friction of the wire 

 against falling weight ; (2) the work done in knocking away 

 the first lever ; (3) friction of air, &c. As the wire was well 

 oiled and placed exactly vertical, cause (1) is very small ; as 

 the weight was very heavy compared with the lever A B, the 

 correction for (2) cannot be very great ; and (3) is quite insig- 

 nificant. 



Later, experimental verification will be given that the cal- 

 culated values are very nearly the same as the true values. 



For the success of the experiments it was not necessary 

 that the absolute va\\\&% of the time-intervals should be known, 

 but only that successive turns of the screw should correspond 

 to equal intervals of time, and this, from the nature of the 

 instrument, is very nearly true. 



In order to determine the hysteresis curve for soft iron 

 and steel when the current varied very rapidly, the time of 

 rise of the magnetizing current for soft iron and steel rings 

 was obtained by use of the time-apparatus. 



x\rrangement of Experiment. 

 A battery of five Grove cells was connected to the binding- 

 screws A and B of the time-apparatus, k wire led from B 



Balllshc Ca/ti 



CommtiM/a 



Condenser 



Iron Rinff 



through a non-inductive resistance r, thence round the iron 

 ring which is to be experimented on, and back through a 

 resistance-box E to the binding-screw A. 



