COMPRESSIBILITY OF METALS. 171 



effect of pressure on resistance, and needs no further description. 

 The sensitiveness of the electrical arrangements was such that a 

 motion of the wire of 1.5 X 10 -6 cm. could be detected. Of course 

 there were always extraneous disturbances, so that an accuracy cor- 

 responding to this sensitiveness could not be obtained, but it was 

 nevertheless possible to measure very small motions with a highly 

 gratifying accuracy; these measurements would have been entirely 

 impossible without some such method. This method of measuring 

 small displacements would seem to be applicable in a number of other 

 places. 



The external change of length of the cylinder was measured some- 

 what differently from formerly. This was done previously with a 

 microscope, and there were difficulties at the higher temperature (50° 

 in the former work). The arrangement of the pressure producing 

 part of the apparatus was such that it was now much more convenient 

 to use the rocking mirror device familiar to many engineers. A mirror 

 was attached to a diamond sectioned staff, which could rock between 

 two bars, one attached to the upper end and the other to the lower end 

 of the cylinder. The motion of the mirror was read with a telescope 

 and scale, and the elongation of the cylinder could at once be calcu- 

 lated in terms of the dimensions of the various parts. The conven- 

 tional devices of three point contacts and springs to ensure freedom 

 from back-lash, etc., were used, and the apparatus functioned satis- 

 factorily in every particular. Two measuring devices were used, 

 attached at opposite sides of the cylinder, so that any error due to any 

 slight Bourdon spring action of the cylinder under pressure (there 

 usually is such action in appreciable amount) was eliminated. The 

 method of mounting the mirrors at a distance of 30 cm. or so from 

 the cylinder and transmitting the motion to them by bars allowed the 

 cylinder to be placed in a temperature bath, and the temperature to be 

 controlled by a thermostat and stirrer, as in all these experiments. 

 The stirring of the water produced only a slight mechanical shaking of 

 the mirrors, too slight to introduce perceptible difficulty in making 

 the readings. 



It is obvious that there are various corrections to be applied to the 

 readings as directly obtained. The most important of these is for the 

 effect of pressure on the resistance of the nichrome wire B. This was 

 determined by direct experiment, and is very low; in fact this, as well 

 as the high specific resistance, was the reason for choosing this material 

 for the wire. The resistance between terminals fixed to the wire was 

 found to decrease linearly with pressure, the decrease for 12000 kg/ cm 2 



