EuTHEKFORD. — Oil Magnetic Viscosity. 183 



hysteresis at periods from 2 seconds to ^-Iq ^^ ^ second. 

 Hopkinsou had obtained quite a marked difference between a 

 slow and a rapid cycle, and had conclusively shown that the 

 difference observed was not due to any time effect on the 

 ballistic needle (Proc. Eoy. Soc, 20th April, 1893). As the 

 subject of the dissipation of energy due to magnetic hysteresis 

 with varying periods is one of considerable interest, I deter- 

 mined to continue my experiments on the subject, especially 

 as I was enabled to deal with intervals of time much shorter 

 than those in Hopkinson's experiments. 



In order to carry out these experiments a special form of 

 apparatus for measuring short intervals of time was designed. 

 It was necessary for the research to be able to measure the 

 times of rise of currents in circuits whose self-induction was 

 chiefly due to the amount of iron in the circuit. The " time- 

 apparatus " was found to work very satisfactorily, and by its 

 means time-intervals of less than yooWo ^^ ^ second could be 

 with certainty determined. 



Description of the Time-apparatus. 



A B, C D were two solid copper levers, pivoted at A and 



D respectively. The lever A B was kept pressed against a 



copper rod F by means of the spring H. The lever C D 



was kept pressed against the point E of a screw S by means 



f.g [ 



of the spring K. A vertical nickel wire W G, of length 6ft., 

 passed between the extremities B, C of the levers, and was 

 tightly stretched. A falling weight L M slid freely on this 

 vertical wire. The shape of this weight is shown on the 

 right-hand side of Fig. 1. A hole passed longitudinally 

 through the falling weight, and, in order to prevent undue 

 friction, the hole in the centre of the mass of metal was larger 

 than at the ends, so that the wire could only touch the metal 

 at the extremities L M. 



In order to hold up the falling weight at any height on the 

 vertical wire an electro-magnet was made to slide on the 

 wire, and was held in position at any point by a screw. On 

 turning off the current the weight fell instantly without com- 

 nmnicatmg any movement to the wire. 



When the ends of the levers B, C were exactly in the 

 same horizontal plane the falling weight knocked the levers 



