AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
73 
In these experiments, the divided wire was of German-silver, each division being 
equal to •00021105 ohm at the temperature of the room, and since 100 ohms were 
used at each end of the wire, the alteration of resistance represented by moving the 
sliding-piece over one division of the wire would equal • oooo oToXs' °f the whole, the 
effect of one-tenth of a division was readily perceptible, and the second place of 
decimals in the above columns were got by taking the mean of several observations 
at each pressure. From the above results it was calculated that the decrease of resis¬ 
tance per unit produced by an increase of fluid-pressure of 1 grm. per square centimetre 
would be 27854 X 10 -13 . 
On showing these results, however, to Professor G. G. Stokes, he suggested that 
sufficient account had not been taken of the fact that the alteration of pressure might 
affect the resistance by altering the melting-point temperature of the ice, though, as 
I thought, the ice would not be sufficiently wet in the inner cylinder, as the tempera¬ 
ture of the room at the time was frequently almost at 0° C., and the cylinder well 
surrounded by ice on the outside to affect the result in this way. Nevertheless, 
acting on this suggestion, I carefully determined the increase of resistance at 0° C. 
for a rise of 1° C., in the manner adopted by Matthiessen, and found this to be 
•0038587 per unit of resistance. Now, J. Thomson was led by theoretical considera¬ 
tions" to the conclusion that the melting-point temperature of water would be lowered 
•0075° C. by an increase of one atmosphere of pressure, and the matter was put to 
experimental test by W. Thomson, who arrived at results agreeing almost exactly with 
this conclusion. If we assume one atmosphere to be equal to 76 centims. of mercury, 
or 1034 grms. per square centimetre, the lowering of temperature produced by a 
pressure of 1 grm. per square centimetre on the melting-point of ice should be 
*00000725° C. Now, if the alteration of resistance was entirely due to change of 
temperature in the melting-point of the ice, my experiments would give a lowering of 
temperature for a pressure of 1 grm. per square centimetre equal to 
to -00000723° C. 
278-54 xlO" 10 
•003859 5 
i.e., 
This result then led me to believe that either there was no change of resistance 
produced by the pressure merely, or, as subsequently proved to be the case, that the 
alteration of pressure was too small to produce an appreciable effect even with the 
very delicate galvanometer which I was using. I also remembered that, when water 
had been employed, as in the previous experiment, as has been before mentioned, there 
was no appreciable effect immediately after the air had been let into the vessel. 
Moreover, I called to mind that on two occasions when the temperature of the room 
was almost exactly 0° C. there was no effect, when, as in the last experiments, ice had 
been used, even for some 10 or 20 minutes after using the pump; but in this case 
I had attributed the absence of effect to the regelation of the ice over the cylinder 
* Trans. Roy. Soc. Edinburgh, Jan., 1849. Cambridge and Dublin Math. Journal, Nov. 1850. 
MDCCCLXXXIII. L 
