HIGH PRESSURE— BRIDGMAN. 189 



one experiment a soft rubber washer became so brittle that it 

 cracked like glass, and a soft steel washer in contact with the rub- 

 ber was forced by the pressure in ridges into the cracks in the rub- 

 ber, thus showing that the rubber had become harder than the steel. 

 Another all-important task in the preliminary experiments, in 

 addition to that of finding what pressures the steel vessels could 



•"and, was to devise some way of accurately measuring the pressure, 

 he very simplest method that can be conceived proved to be the 

 \st in this case. It consists in inserting a steel piston through a 



iOle in the wall of the cylinder and measuring the force necessary to 

 prevent it from being blown out by the pressure within. There are 

 many mechanical difficulties in realizing such a method as this, the 



lost obvious being to overcome leakage. To do this the piston must 



it the hole tightly, but at the same time must fit so freely that there 

 is not enough friction to destroy the accuracy of the readings obtained 

 by its means. It was found possible, by using a small-diameter pis- 

 ton fitting into a comparatively long hole, to take care of both 

 these factors. With the gauge as finally constructed, pressures up 

 to 13,000 kilograms per square centimeter were measured with an 

 accuracy of one-tenth per cent. After high pressures had been suc- 

 cessfully measured with such a gauge, it was found possible to con- 

 struct gauges of a much more convenient form for actual use, and 

 to calibrate them against this, which became an " absolute " gauge. 

 One gauge that I have used in most of my later work is a manganin 

 resistance gauge, which consists of a coil of manganin wire placed 

 in the pressure cylinder and connected through insulated leads with 

 apparatus for measuring the resistance. The electrical resistance 

 of this coil is found to change directly proportionally with changes 

 of pressure in the cylinder. Manganin is a very much more conven- 

 ient material to use than any pure metal, since the resistance of all 

 pure metals decreases as the pressure increases, and the decrease is 

 moreover, not proportional to the increase of pressure. 



After the completion of the preliminary work, in which the meth- 

 ods of producing and accurately measuring high pressures had been 

 decided upon, it was decided to obtain, first of all, measurements of 

 compressibilities. The first substance chosen for the measurement 

 of compressibility was water, chiefly as it is so common a substance, 

 and because many measurements had been made on it previously at 

 low pressures. Water is not absolutely incompressible, as is com- 

 monly supposed, but its volume may be very appreciably diminished 

 by the application of sufficiently high pressures. Under 12,000 at- 

 mospheres a decrease of volume of about 20 per cent is produced. 

 The measurements of the compressibility of water by the new method 

 were found to be satisfactory at comparatively low pressures, but at 



