Whence the change of area, A^ = 



408 PROCEEDINGS OF THE AMERICAN ACADEMY. 



where h is the internal and a the external radius, /x the shearing modu- 

 lus, and K the compressibility modulus. 



a was measured directly from the outside of the cylinder and was 

 3.536 cm.; b was found from a measurement of the plug at the end of 

 the piston after this plug had been forced to conform to the hole by 

 the application of high pressure. It has been already stated that no 

 change in b could be detected during the course of the experiments of 

 so much as 0.00004 cm. b was found to be 0.7183 cm. /a and k were 

 found by the ordinary equations of elasticity from Young's modulus, 

 taken as 2 X 10® kgm./cm.^, and Poisson's ratio, taken as 0.28. These 

 quantities vary only slightly in different grades of steel. The values 

 of /u. and K calculated in this way were found to be 7.81 X 10^ and 

 15.2 X 10^ respectively. Substituting in the formula gives directly 



A^. = 0.00491 cm. for 10,000 kgm./cm.^. 



2 X 0.00401 

 .7183 

 = 1.35 per cent for 10,000 kgm./cm.^. 



Temperature Corrections. — There are a variety of temperature cor- 

 rections to be considered ; those which are to be met in any physical 

 measurement, and those which were peculiar to this particular work. 

 The ordinary corrections concern only the reading of the temperatures 

 and may be dismissed with a few words. Temperature about the lower 

 cylinder was maintained constant to 0°.01 with a thermostat. Above 

 zero this temperature was read with a Tonnelot thermometer calibrated 

 at the Bureau of Weights and Measures in Paris. The correction for 

 the zero was determined, but it was not necessary to apply any of the 

 other corrections. Readings on this are probably accurate to 0°.01. 

 Below zero, readings were made on a toluol thermometer, graduated to 

 0°.l, and calibrated at the Reichsanstalt. In addition to the correc- 

 tions of the Eeichsanstalt, the zero correction was applied. This is 

 accurate to 0°.l, which corresponds to the accuracy of the pressure 

 measurements. 



Of the corrections peculiar to this work, the temperature effect on 

 the pressure readings was the simplest. The manganin resistance coil 

 with which pressure was measured was placed in the lower cylinder 

 with the mercury, and so was exposed to the same changes of tempera- 

 ture. It has already been shown in the paper on the gauge, that 

 within the temperature limits of this work the pressure coefficient of 

 resistance is constant. The only temperature effect is on the total 

 resistance of the coil, which, therefore, merely displaces the zero. This 



