120 Mr. R. L. Wills on the Effect of 



The energy dissipated in hysteresis for any induction can 

 therefore be determined by making two measurements of U 

 for different values of R, E being adjusted so as to keep the 

 primary current constant. 



The induction was measured by the ballistic method used 

 by Professor Ewing and Miss Klaassen *, and the temperature 

 was measured by a platinum wire as in the previous 

 experiments. 



The general arrangement of the apparatus is shown in 

 fig. 1. In the key F, which causes a gradual reversal of the 

 current, ab is an arm of insulating material free to turn about 

 its centre over a circular ring consisting of metal strips c, d y 

 e,f, g, insulated from each other. Resistances rr are placed 

 between the strips/, g and c, d respectively. 



The terminals of the battery circuit containing the small 

 variable resistance R 2 and the Weston amperemeter C are 

 connected to the ends of the arm ab which make sliding- 

 contact with the strips. In the figure the arm is shown in 

 one of its extreme positions ; as it is turned to the other 

 extreme position the current is first diminished by an amount 

 depending on the value of v. When the end b rests on the 

 insulated strip e, the strips c and g are brought into contact 

 by means of the contact-piece attached to the end a of the 

 arm. As the arm is turned further, the current through the 

 magnetizing coil increases again but is reversed in direction, 

 and reaches its original maximum value when the end b rests 

 on the stripy. Before the contact-piece attached to the end a 

 of the arm leaves the strip g, the" end b rests on the strip /• 

 it will thus be seen that the primary circuit is never broken 

 during the reversal of the current. A choking-coil L having- 

 a very large coefficient of self-induction is inserted in the 

 primary circuit to prevent the current varying too rapidly. 

 The key M is a modification of an ordinary reversing-key, 

 and was used for taking cyclic B-H curves when desired. 

 The terminals e and k are connected together, and an adjust- 

 able resistance Ri is iuserted between the terminals h and Jc. 

 By suitably adjusting the resistance R : the magnetizing 

 current can be reversed in direction and reduced to any 

 value by a single motion of the rocker. 



The reversing-key N is connected to the key M in order 

 that when the resistance R : is in the circuit the current may 

 be reversed without altering its value. 



The primary current flows through the fixed coils A of the 

 electrodynamometer and passes through the keys M, N to the 



* Ewing 'Magnetic Induction in Iron and other Metals/ 3rd ed. 

 revised pp. 356-370. 



