BRIDGMAJ^. — MEASUREMENT OF HYDROSTATIC PRESSURES. 341 



from 5/8 inch to 1 1/8 inches, but even this was not sufficient to raise 

 the limit permanently to over 15,000 kgm. 



The compactness of the manganin gauge makes it particularly 

 adapted for working with the highest pressures where everything 

 must be in one piece because of the impossibility of making con- 

 necting tubes. The gauge has been so used in a number of experi- 

 ments on the freezing of water under pressure. The gauge was 

 screwed into one end of a large steel cylinder, the plunger was 

 pushed in from the other end, and the water was in the space be- 

 tween. The dimensions were kept down so that the entire block, 

 together with a part of the hydraulic press, could be placed in a 

 thermostat. 



The manganin gauge may be used by extrapolation to measure pres- 

 sures beyond the reach of the absolute gauge. It has been so used in 

 investigating the freezing of water up to an indicated pressure of 

 20,500 kgm., and this limit could without doubt be exceeded. The 

 limit is not in the manganin itself, but in the hardened steel parts, 

 which have a tendency to stretch too much at pressures as high or 

 higher than 20,000 kgm. Of course the use of any standard by extra- 

 polation is undesirable, but at present any means of measuring these 

 very high pressures with probable accuracy is welcome. In any event, 

 the extrapolation from 12,000 to 20,000 is very much less than the 

 extrapolation from the previous maximum of 4000 to 12,000, which is 

 here shown by actual experiment to be justified. What is more, it 

 will be an easy matter to translate high-pressure readings in terms of a 

 manganin gauge into absolute pressures, if at any time the direct cali- 

 bration is extended from 13,000 to 20,000, and proves that a linear ex- 

 trapolation is not sufficiently accurate. 



The only serious disadvantage in the manganin gauge as thus far 

 described, when compared with either the mercury gauge or the abso- 

 lute gauge, is the fact that it is not readily reproducible, so that each 

 new coil of wire must be calibrated against an absolute gauge. This 

 disadvantage may be obviated by the use of fixed pressures of refer- 

 ence analogous to the melting points of the metals used as points of 

 reference in thermometry. The linearity of the relation between resist- 

 ance and pressure having been established by the work of this paper, 

 it is necessary to know for each coil only the change of resistance cor- 

 responding to a single known pressure in order to fix completely the 

 behaviour of the coil. 



Such pressures of reference are given very conveniently by the points 

 of transition between the various kinds of ice and water. For low 

 pressures such a pressure of reference is given by the pressure of transi- 



