STRAIN ON THE PHYSICAL PROPERTIES OF MATTER. 



11 



It is evident that, under the above circumstances, even when the magnetic stress is 

 reduced to an amount which begins to be comparable with that of the earth's 

 magnetic stress, there is a very appreciable damping' due to the reversals. 



By subtracting '000782 from the numbers in the second column of the last Table, we 

 get the increase of the logarithmic decrement due to the reversals in each case, and it 

 is interesting to compare this increase with the stress producing it ; this comparison 

 is made below. 



The increase of the logarithmic decrement due to the reversals of the magnetic stress 

 is, for small stresses, proportional to the latter. When the magnetic stress reaches 

 1'800 the friction increases in greater proportion than the stress, until the latter ex- 

 ceeds 12'935, when the ratio B : A begins to decline.* This is what might be expected 

 from the behaviour of the magnetic permeability as the stress is gradually increased : 

 in fact, with this particular specimen of iron, the permeability was found to increase 

 very rapidly as the magnetic stress rose from 3 to 5 C.G.S. units, and again fell very 

 rapidly when the stress rose from 12 upwards. 



The damping effect of magnetic stress, when applied as above, is noteworthy for its 

 magnitude ; for the logarithmic decrement when the wire is subjected to reversals of 

 a magnetic stress of 28 '5 40 is actually between twenty and thirty times as great as 

 when the wire is free from stress or under the influence of a sustained magnetising 

 stress.t 



Experiment X. 



In this experiment the effects of reversals and of applying and removing the mag- 

 netic stress always in the same direction were compared together, the magnetic stress 

 being in each case 42775 C.G.S. units, and the vibration-period of the wire 4725 

 seconds. In both cases the circuit was broken instantly after it was closed, and the 

 reversals or closings were made to synchronise with the vibrations of the wire. 



* See also Experiment VI. 

 f Experiment II. 



c 2 



