COMPRESSIBILITY OF METALS. 169 



The Absolute Compressibility of Iron. 



The method adopted is in many respects like that previously used. 

 An iron rod approximately 30 cm. long is placed inside a heavy cylinder 

 and exposed to hydrostatic pressure exerted by a fluid filling the 

 cylinder. The relative change of length of the rod and the cylinder 

 is measured, and at the same time the change of length of the cylinder 

 is measured at outside points. From these two the absolute change 

 of length of the iron rod under pressure may be computed. The only 

 assumption is that the change of length of the cylinder externally, 

 where it can be measured, is the same as that internally, where it is in 

 contact with the rod. This assumption is entirely justified if the 

 cylinder extends some distance beyond the end of the rod, so that 

 warping of the cross section due to end effects is negligible, and in 

 any event the assumption is of little importance because the correction 

 for the change of length of the cylinder is only a few per cent of the 

 entire effect. 



The essential difference between the old and the new method is in 

 the means of measuring the relative change of length of the rod and 

 the cylinder. Previously the rod carried a collar which bore against a 

 fixed shoulder in the cylinder and whose relative displacement on the 

 rod was measured after every application of pressure. This involved 

 a complete setting up and disassembly of the apparatus for a single 

 reading at any pressure. The present method is a continuous reading 

 one. Attached to the end of the rod is a fine wire of high resistance 

 alloy. This wire slides over a contact fixed to the cylinder. The 

 sliding contact is made one potential terminal and another potential 

 terminal is attached to a point of the wire. Current is passed through 

 the wire independently of the potential terminals, entering the wire 

 at one end and leaving at the other. The difference of potential 

 between the fixed and the sliding point on the wire is measured for a 

 given current, so that the resistance and hence the length of the wire 

 between terminals may be calculated, and hence the relative motion 

 of the rod and the cylinder. The reading is continuous, and after 

 every change of pressure a new reading may be obtained in a few min- 

 utes, or as soon as the heat of compression is dissipated. Further, 

 there is no error from the constant possibility of introducing minute 

 particles of dirt every time the apparatus is opened, as there was 

 previously. 



The essentials of the apparatus are shown in Figure 1. The high 



