STRAIN ON THE PHYSICAL PROPERTIES OF MATTER. 



incuts, conducted m..-,t r.-m-rnllv. l.-d t,, vimil.-ii- rwniti :nnl :il'Uh>!:nit Iv 

 the above-mentioned independence. 



When the magnetising current is interrupted, or when it is reversed whilst the wire 

 is vibrating, there is for the larger magnetic stresses an increase of the internal 

 friction which may become very considerable.* 



Experiments II. and III. seem to prove that when the number of interruptions or 

 reversals in a given time exceed a certain limit the effect produced by them on the 

 logarithmic decrement begins to decline, but it would appear from Experiment II. 

 that even when the interruptions occupy only a small fraction of a second t their 

 effect in increasing the molecular friction is very sensible. 



Experiment V. was made partly with a view of ascertaining how far the interrupted 

 or reversed magnetic stress might be diminished before it ceased to exercise any 

 perceptible influence on the internal friction. For the rather rapid interruptions 

 produced by the clockwork it appears that when the magnetising stress is diminished 

 to 4'615 its effect on the friction is nearly nil. This, however, is by no means the 

 case when the current is reversed every two seconds, the value of the logarithmic 

 decrement being then more than three times as great as it is when the wire is not 

 magnetised; and even when the stress has been reduced to 0*104 it still exerts a 

 sensible influence. 



The experiments also show clearly that, at any rate for short periods of time, the 

 longer the time of action of the magnetising stress, the greater is the effect on the 

 internal friction, for otherwise in Experiment III. there would be a greater logarithmic 

 decrement when the vibrations are 16 in 16 seconds than when the vibrations are 8 in 

 16 seconds, whereas the contrary is the case. Nevertheless, this increased effect on 

 the internal friction which accompanies increased time of action does not extend 

 beyond a period of a few seconds, for the increase of logarithmic decrement is 

 considerably greater when the vibrations are 8 in 16 seconds than eight times the 

 increase of logarithmic decrement when the vibrations are 1 in 16 seconds. } 



[Added Sept. 29<A, 1 887. Experiment III. has shown that there is a definite 

 frequency of reversal of magnetising stress for which the damping effect is a 

 maximum, and it appeared to be of interest to ascertain more exactly what is the 

 frequency producing the greatest effect with the particular wire examined, and also 



* See Experiments II. and III. 



f The clockwork arrangement must have interrupted the current at least ton times in one second. 

 [Later experiments showed that the diminution of the effect of an interrupted magnetic stress as the 

 interruption-frequency increased arose from the fact that the difference between the vibration-frequency 

 of the wire and that of the interruptions increased.] 



J The increases of the logarithmic decrement are '00346 and '00024 in the two cases respectively. 

 [September 25, 1887. It was afterwards found that the results of Experiment III. are to be attributed 

 almost entirely to differences between the vibration-frequency t>f the wire and the interruption- 

 frequency.] 



