AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
75 
water, the cover with the wire suspended from it was put on and secured by means of 
three screws and nuts, two of which, H, H, are shown in the figure, the cover having 
been made to fit water-tight under the highest pressures employed, by means of a 
gutta-percha collar. The wire X and the comparison-wire were then connected up 
in the usual way, as shown in the figure,* and the whole well-covered with baize. 
The first wire which was tried was a piece of the copper wire used in the last 
experiments, and a day having been selected at which the temperature was about 
4° C., pressure was put upon the wire by means of a force-pump which formed part of 
a hydraulic press.! The pump communicated with the vessel by means of the tube 
F, and was capable of supplying a pressure of upwards of 5000 lbs. on the square 
inch. The pressure was measured by a strong spring pressure-gauge, divided so as to 
read to a pressure of 100 lbs. on the square inch. 
On working the pump so as to increase the pressure there was, after a short time, a 
very decided decrease of resistance perceptible, and on removing the pressure the 
resistance returned to almost its former value. Several attempts were made with 
greater and greater pressures, which were carried up to 4000 lbs. on the square inch, 
and all gave indications in the same direction ; but unfortunately, on raising the pres¬ 
sure to 5000 lbs. on the square inch, the insulating substance round one of the stout 
brass wires cracked, and allowed the water to come through the cover. As these trials 
were only intended to be preliminary, no measurements had been taken of the alteration 
of resistance produced, but there was now no question that in the case of copper wire 
increase of pressure produced decrease of electrical resistance. 
These experiments could not be renewed till the following winter, when the flaw 
mentioned above having been repaired, some fresh trials were made with iron wire 
and the following measurements taken :—- 
Experiment XX. 
Pressure in lbs. per 
square inch. 
Number of divisions of the 
platino-iridium wire through which 
it was necessary to move the 
sliding piece in order to restore 
the balance. 
900 
20-0 
1100 
37-7 
1300 
25-0 
Mean 1100 
Mean 27'6 
* In the actual experiments X was secured to the vessel by string, the connexions having been well 
insulated by wrapping them up in paper. 
f This pump was lent to me by the Rev. T. A. Cock, of King’s College, who kindly had it put into 
complete working order before I used it. 
L 2 
