BRIDGMAX. MEASUREMENT OF HYDROSTATIC PRESSURES. 337 



The apparatus for producing and measuring pressure consisted of 

 two cylinders connected by a tube of nickel steel. In the lower cylin- 

 der the pressure was produced and measured. The absolute gauge of 

 the first part of this paper was screwed directly into the side of this 

 C34inder. The cylinder itself was of Krupp chrome nickel steel, 8 inches 

 outside diameter, 1 1/8 inches inside diameter. It was placed in a 

 hydraulic press of 200 tons capacity, and the pressure produced by a 

 1 1/8-inch piston forced into the cylinder by the ram of the press. 

 The upper cylinder, also of Krupp chrome nickel steel, 4 1/2 inches 

 outside and 9/16 inch inside diameter, contained the manganin wire to 

 be tested. The connecting tube to the lower cylinder passed through 

 the bottom of the tank of a thermostat, which surrounded the upper 

 cylinder, and with which the temperature could be kept constant to 

 0.01°. No such temperature precaution was necessary for the absolute 

 gauge. The lower cylinder was filled with the mixture of glucose and 

 glycerine needed to secure tightness of the piston of the absolute gauge, 

 and the upper cylinder was filled initially with either kerosene or gaso- 

 lene, in which the manganin coil was directly immersed. The action 

 of pressure was to compress the kerosene, glucose passing fi-om the 

 lower cylinder to the lower jmrt of the upper cylinder. The compres- 

 sion was never sufiicient, however, to bring the glucose into contact 

 with the manganin. Although kerosene or gasolene are somewhat incon- 

 venient because of their high compressibility, still their use was made 

 necessary by the fact that a heavier oil freezes under pressure, so that 

 it does not transmit pressure hydrostatically to all parts of the wire. 

 The kerosene is also known to become stiff like vaseline at say 10'^ 

 and 8000 kgm., but the viscosity is not so great as to introduce irregu- 

 larities. When either kerosene or gasolene is used, the insulating 

 qualities of the plug are practically perfect without requiring any spe- 

 cial precautions. The insulation resistance was always over 10 megohms, 

 which was the limit of the measuring device conveniently at hand. 

 Formerly, in working with mercury, pressure was transmitted by a 

 mixture of water and glycerine. It was necessary to specially protect 

 the separate parts, and even then the insulation resistance was never 

 greater than several hundred thousand ohms. 



The electrical measurements were made by the same null method 

 and on the same Carey Foster bridge as those described in the former 

 paper. 



In making the calibration and in using the gauge, there is one fact 

 to be borne in mind which has its analogy in the mercury resistance. 

 This is the seasoning effect of pressure ; the gauge does not respond 

 to the first application of pressure in the same way that it does to the 



VOL. XLVII. — 22 



