BRIDGMAN. — MERCURY UNDER PRESSURE. 



413 



of volume is evidently incorrect by the thermal expansion at the partic- 

 ular pressure of the total quantity of kerosene in the upper cylinder. 

 The temperature changes were usually very slight. They were made 

 still smaller at the cylinder itself by enveloping this in a large mass of 

 cotton batting. The temperature of the cylinder was read with a 

 thermometer directly in contact with it. The correction demands an 

 approximate knowledge of the quantity of kerosene in the upper cylin- 

 der, which was obtained by measurement of the position of the piston. 

 The thermal dilatation of the kerosene at any given temperature was 

 found from the correction curve already described, making the justi- 

 fiable assumption that at constant pressure the dilatation is sensibly 

 constant over the temperature range. The maximum correction was at 

 —20°, where the dilatation is a maximum, and where the temperature 

 variation, 0°.7, happened also to be a maximum. The correction here 

 was 0.5 per cent. Above 10° the correction was entirely negligible. 



Possible Change of State of the Transmitting Liquid. — The effect 

 of any freezing of the kerosene with contraction during the freezing 

 of the mercury would be to make the change of volume appear too 

 large. This possibility was entirely eliminated, however, incidentally 

 by the measurements determining the thermal dilatation of the 

 kerosene. At any given temperature these measurements extended 

 considerably beyond the freezing pressure of the mercury, and no evi- 

 dence of a change of state with change of volume was found. 



Measurements of the Piston Displacement. — These measurements were 

 made with a Brown and Sharpe micrometer reading to 0.0001 inches. 

 The measurements were made between a point on the head of the 

 advancing ram which drives the piston and a fixed point on the frame 

 of the press against which the cylinder is pressed. There are two pos- 

 sibilities of error here. One, the most serious, is warping of the entire 

 frame of the press. Such an effect was detected and may give rise to 

 discrepancies of 0.002 inch over the entire stroke of 4 inches. It was 

 obviated by making four measurements of every displacement between 

 four pairs of points at the four corners of the frame of the press. The 

 second error may come from elastic deformation of the parts of the 

 press between the head of the ram and the fixed point on. the frame, so 

 that the measured change of length does not give the actual piston 

 displacement. The effect, which is in any event small, was obviated 

 by taking advantage of the fact that there is considerable friction in 

 the packings. In virtue of this it is possible to vary the pressure on 

 the low pressure end of the ram over a considerable range without pro- 

 ducing motion of the piston, and so without altering the high pressure. 

 Now it is the pressure on the low pressure end which produces distor- 



