BRIDGMAN. — A DETERMINATION OF COMPRESSIBILITIES. 259 



up the ring, which stays in its extreme position. The rod is then taken 

 out by unscrewing the plug at the end I and the distance between the 

 scratches L and M measured. The increase of distance over the zero 

 position gives the relative change of length of rod and cylinder. There 

 is here a small source of error in finding the effective length of the 

 rod, which terminates at some unknown place within the brass ring. 

 The effective length used was the length from the fixed end to the 

 middle of the ring when in the zero position. As the breadth of the 

 bearing surface of the ring was only about 2 mm., and the length of 

 the rod was 30 cm., the maximum error here is only 1/300. 



It is at once obvious that any slight error in replacing the rod after 

 each measurement in exactly its former position will produce consid- 

 erable error in the result, since the change of length produced by 

 pressure is small. In the form used, in which the rod is 30 cm. long, 

 the change of length for 1000 kgm. is only 0.05 mm. Slight particles 

 of grit are likely, therefore, to produce considerable irregularities. By 

 working with some care it was found possible, however, to secure 

 fairly uniform results. Particular attention must be given to washing 

 out the cylinder after each application of pressure. The effect of 

 pressure is, of course, to flood the interior of the cylinder with the 

 pump liquid, in this case glycerine and water, which may carry con- 

 siderable grit in suspension. After each measurement the cylinder 

 was thoroughly washed several times by a jet of water violently ex- 

 pelled from a glass tube reaching into the cylinder as far as the stop D. 

 No cloth or other substance must be used for wiping out the hole. 

 The rod to be tested was also carefully washed under the tap after 

 each measurement, again taking care not to wipe with a cloth or to 

 bring into contact with any possible source of grit. It was found that 

 by decreasing the diameter of the rod for a short distance at the end B, 

 there is less tendency for grit to collect between the end of the rod and 

 the stop D when the rod is replaced in the hole after each measurement. 



The change of length of the steel cylinder was not measured at the 

 same time as the relative change of length of rod and cylinder, but was, 

 instead, determined independently as a function of the pressure. 

 Three determinations of this extension were made, one preliminary 

 to, one in the course of, and one after the series of compressibility 

 measurements. The last two agreed within the limits of error; the 

 first was slightly different, as has always been found to be the case 

 when the deformation of a metal is measured on the first application 

 of pressure. In making these measurements, the cylinder was clamped 

 to a heavy comparator bed, which carried two microscopes. The 

 cylinder was clamped at only one point, the middle, so as to avoid 



