1895.] loss of energy in Hysteresis. 5 



determined by simple observation of the "throw" in each case. 

 If it is desired to investigate these effects fur a given maximum 

 value of B, the magnetic induction, the process is a little more 

 complicated, because the value of B has to be found by the bal- 

 listic galvanometer method, and the strength of the magnetizing 

 current so adjusted on each occasion that the value of B may be 

 kept constant under the varying conditions imposed upon the iron. 



In the electrodynamometer which I have constructed for this 

 experiment, the suspended coil is a long narrow coil of 120 turns, 

 about 8 cm. long and 1"5 cm. wide, suspended by a silver wire 

 y^oq inch in diameter. The fixed coils have 500 turns and are 

 designed so as to fit closely round the suspended coil. With 1285 

 turns in the secondary coil and with the resistance in the secondary 

 circuit about 100 ohms, the instrument is sensitive enough to 

 measure the hysteresis loss in the case of an iron wire 02 cm. 2 

 in cross section. I have made the instrument sufficiently sensitive 

 to measure W for so thin a wire with the view of investigating 

 the effects of tensile and torsional stress upon the hysteresis. 



Up to the present (3 Dec. 1895) I have not been able to make 

 any accurate determinations owing to the fact that I have so far 

 used an ordinary rocking mercury key for reversing the primary 

 current. With this key, on breaking contact, the primary current 

 is stopped very quickly, and when contact is re-established the 

 current very quickly rises to its full strength, and each of these 

 changes takes place in a time not very large compared with the 

 time-constant of the secondary circuit. On account of this the 

 instrument has given results about 100 °/ too large, as judged by 

 the value of W obtained by a B-H cycle. When, however, a coil 

 with a large coefficient of Self-induction was inserted in the 

 primary circuit so that in spite of the key the primary current 

 could not vary rapidly, the discrepancy was reduced to about 

 5 °/o- But I have designed a key (somewhat after the fashion of 

 Prof. Ewing's "Sine Function" key) which will allow the current 

 to be quite gradually reversed, and some preliminary observations 

 have led me to expect that with this key the instrument may 

 work quite satisfactorily. 



Note added 18 Dec. 1895. The following method of deter- 

 mining the constant of the instrument is much more convenient 

 than that described in the paper. Let a constant current C flow 

 in the primary circuit, and while this current is flowing suddenly 

 produce a known change P in the number of lines Qf magnetic 

 force passing through the secondary circuit. This is conveniently 

 done by suddenly inverting an " earth inductor " which has been 

 placed in the secondary circuit. The time-integral of the current 



