STRAIN ON THi: PHYSICAL PHOPKHT1KS OF MATTKi: 



Hitherto the magnetic stress had been maintained during the whole period 

 between one reversal and the next, but it was presently ascertained that if the 

 magnetic stress be removed immediately after each reversal the damping is much 

 increased. 



Experiment VII. 



A reversing key was used, such that the magnetising circuit could be opened the 

 instant after each reversal ; * the magnetising stress was 4278 C.G.S. units ; the 

 vibration-period of the wire- was 4'725 seconds, and with this the reversals were 

 made to synchronise in one of the trials which were made. 



Interval in I 

 between connccutive reversal* 

 of the magnetising sire*. 



4-000 

 4725 

 5-000 



Logarithmic decrement due 

 to internal friction. 



007986 

 017416 

 008759 



On comparing these last results with those of Experiments V. and VI., it will be 

 seen that when the battery circuit is opened instantly after each reversal the effect 

 on the internal friction is twice as great as when the magnetic stress is maintained 

 between each reversal and the next. 



Nothing has been said as yet respecting the phase of the torsional vibrations of the 

 wire at which the reversals of the magnetic stress were made. When the reversals 

 synchronised with the torsional vibrations the former were made when the wire was 

 nearly^ at the end of its swing on one side or the other, but it was soon discovered 

 that the effect of reversing the magnetic stress was greatest when the reversals took 

 place exactly at the end of each swing, and least when they occurred at each instant 

 the wire was passing its position of equilibrium. 



* The time during which the circuit remained closed after each reversal must have been only a small 

 fraction of a second, as on reversing a sharp tap was given to the key, the spring of which was rather 

 strong, and the finger removed as quickly as possible. 



f That is, no particular pains had been taken to ensure that the reversals should take place exactly 

 at the end of each swing. 



MIX-a'KXXXYlII. A. 



