BRIDGMAN. — A DETERMINATION OF COMPRESSIBILITIES. 257 



is applied. The rod shortens, therefore, under the uniform external 

 pressure, while the cylinder lengthens under the interior pressure. 

 The lengthening of the cylinder is very much less than the shortening 

 of the rod. In the present experiment it was only 5 per cent. The 

 strain in the cylinder is complicated, consisting of a radial displace- 

 ment away from the centre, and of a longitudinal extension which 

 may produce warping of the originally plain sections. This warping 

 is greatest at the ends, and must vanish at the mid section if the 

 cylinder is symmetrical at the two ends. The warping cannot be 

 easily calculated, and was neglected in the present work. It can in 

 any event constitute only a correction for the above 5 per cent correc- 

 tion term. The method consists, therefore, in subtracting from the 





Figure 1. Apparatus for measuring the linear compressibility of rods. 

 The rod to be measured is indicated by the shading. The stop D is held per- 

 manently against the shoulder B by the spring C, which is kept compressed 

 by the pump connections, not shown. The brass ring F is kept in contact 

 with the shoulder G during increase of pressure by the spring E, which pushes 

 the shortening rod through the ring F, so as always to be in contact with the 

 stop D. When pressure is released the ring comes back with the rod and the 

 displacement is measured. The rod is removed through the end E to make 

 these measurements; the connections at A to the pressure pump are not dis- 

 turbed during the measurements. The elongation of the cylinder is measured 

 externally at the scratches H and I. 



relative change of length of the rod and the cylinder the increase of 

 length of the cylinder as obtained from the measurement of external 

 change of length under pressure. The result is the linear compressi- 

 bility of the rod, from which the cubic compressibility is calculated. 



The cylinder used is shown in Figure 1. It is made of annealed 

 tool steel, 18 in. (45.7 cm.) long, and 2 in. (5.1 cm.) in diameter. It 

 is pierced through the entire length by a reamed § in. (0.95 cm.) hole, 

 in which the rod to be tested is placed. At either end the f in. hole is 

 enlarged in several steps in the manner indicated, in order to afford 

 room for the various connections. The enlargements of the holes 

 are precisely alike at the two ends, so as to insure symmetrical warp- 

 ing of the cylinder. The rod to be tested is indicated by the shading. 

 It is carefully turned so as to slide without lateral play into the reamed 

 hole. Three shallow grooves, milled the entire length of this rod, 

 vol. xliv. — 17 



