820 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS AND 
cylinders are shifted along the bar without any alteration of mass; * they also, as I 
shall show in a future paper, can be imitated by means of magnetic stress. 
Attention should also be directed to the fourth column of the experiment, where it 
will be seen that not only is the loss of energy increased by change of temperature, 
but so also is the vibration-period ; the latter is, however, so much less altered than 
the former that it requires careful observation to detect any such similarity between 
them. 
In consequence of the above-mentioned effect of fluctuations of temperature on the 
internal friction, it was necessary to choose a day, or time of day, when the tempera¬ 
ture was fairly steady, and all the results which will be recorded in the following 
portion of this memoir were obtained when this was the case, for with all the metals 
which I have examined the phenomenon is more or less pronounced. 
Since it is impossible, with the arrangements which I have described, to obtain a 
perfectly steady temperature, it seems probable that most of the values of the logarithmic 
decrements previously recorded are slightly in excess of what they would have been, 
could we have secured greater uniformity in this respect. 
The question next arises : will any change of temperature, however small, introduce 
increase of internal friction ? I think not; for not only in some instances have I 
obtained values for the logarithmic decrements, which were as constant as the limits 
of errors of observation would allow, and yet where during the experiments the 
temperature did change several times, and in an irregular manner, though by small 
amounts, but I shall in another paper bring forward instances where the molecules 
were slightly twisted backwards and forwards by small magnetic stress without 
appreciably affecting the value of the logarithmic decrement. 
The Effect of Loading on the Internal Friction. 
We have seen that according to both Sir William Thomson and Professor G. 
Wiedemann, when the load on the wire is increased, there is at first an increase in 
the internal friction, but this increase diminishes with time, so that the logarithmic 
decrement tends to become the same as with the lighter load. Neither of the above 
observers has, however, attempted to ascertain vdrether or not the internal friction 
is entirely independent of the load, provided sufficient time be allowed, and sufficient 
account be taken of the effect of the resistance of the air in diminishing the amplitude 
of vibration. The following arrangements were therefore made for the purpose of 
settling this point by experiment :— 
The brass block, into which was clamped the bar Y V of the vibrator (fig. 2), 
* This is one of the reasons why the logarithmic decrement is at first increased when the mass of the 
vibrator is decreased or increased, even thongh the change of mass produce no perceptible permanent 
alteration whatever in the length of the wire, and why it is prudent to allow the wire to oscillate for 
some time after shifting the cylindei’S, as in Experiments III., IV., V. 
