Bkown — -The Subsidence of Torsional Oscillations. 523 



letters A. C. wlien alternating magnetic fields were employed at the different 

 frequencies. The wire iu this hard state shows very little damping of the 

 torsional oscillations. There is a decrease in the amplitude of only 40 m.m. 

 in the D. C. fields, and the greatest damping with the A. C. fields is obtained 

 with a frequency of 50. There is practically no difference for frequencies 

 150 and 250 per second. 



The wire was now removed from the solenoid, and was suspended 

 vertically under its own weiglit. It was heated tioice to a dull red heat from 

 the top downwards by means of a broad Bunsen flame, and when it was 

 cold and cleaned up, its rigidity was again measured and found to be about 

 790 X 10'' grammes per sq. cm. It was then replaced inside the solenoid 

 under the same conditions as before, and tested first for fatigue, and secondly 

 for Bubsideuce of torsional oscillations, for each value of the frequency of 

 alternating magnetic field employed. To explain shortly the metliod of 

 observing the fatigue : — With the direct current round the solenoid to give 

 a longitudinal magnetic field of 20 c.g.s. units, and with one ampere through 

 the wire, the lower free end of the wire twisted through a small angle which 

 was read off as a deflection of the light-spot on the scale = D. The direct 

 currents were then switched off both the wire and the solenoid circuits, and 

 an equivalent alternating current sent round the solenoid so as to give an 

 alternating magnetic field of 20 c.g.s. units for, say, one minute. This 

 alternating current was then switched off, and separate direct currents of the 

 same values as before were sent round the solenoid, and thi-ough the wire, 

 and the deflection again read off on the scale = d, and so on until by the 

 repetition oftliese processes the deflection (^ was no longer diminished, tlieu — 



F = the fatigue of the wire. 



D = the unfatigued deflection. 



d = the fatigued deflection. 



„ n- d 



F = 



D 



The observations for the fatigue for an alternating magnetic field of given 

 frequency having been taken, the subsidence of torsional oscillations was then 

 taken for that magnetic field, and it was found by experiment that the 

 damping curve or curves of torsional subsidence were practically the same 

 whether they were obtained before or after the fatigue tests. In every case 

 the observations for the fatigue were made first. Now, having obtained the 

 fatigue of the wire and the torsional damping for a magnetic field of a certain 

 frequency, the wire has to be quite recovered from its fatigue before a magnetic 

 field of another frequency may be tried. If the wire is left alone for a long 

 time, it recovers slowly, but it can l)e restored rapidly as follows: — Ease the 



4 o 2 



