BRIDGMAN. — MEASUREMENT OF HYDROSTATIC PRESSURES. 327 



The functioning of the gauge at high pressures is therefore prevented 

 by two effects, — increased viscosity of the liquid, and closing of the 

 crack. In view of the fact that the gauge still worked at 13,500 when 

 both these effects were operative, the estimate made above that the 

 functioning would cease at 15,000 by the closing of the crack only 

 would seem to be amply low. The maximum value set on the correction 

 above is probably, therefore, too high. 



The calibration of the piston and cylinder at low pressures to de- 

 termine the effective cross-section was carried out by the method used 

 in the previous paper. Some such indirect method of calibration was 

 made necessary by the fact that the dimensions of the small piston are 

 so small as to make accurate direct measurements of its effective diam- 

 eter impossible. The method consisted in hanging weights on the piston 

 to be calibrated and on a larger piston of known area, in such a propor- 

 tion that the pressure produced by the two pistons should be the same. 

 This is done most simply by connecting the two freely moving pistons 

 to the same pressure chamber, keeping the weights on one piston invari- 

 able, and changing those on the other until neither rises or falls. The 

 details of the method were the same as that described before, the same 

 comparison piece of apparatus being used. 



The results of the comparison showed an effective diameter for the 

 piston of 0.06250 inches. The measured diameter was only 0.0623 

 inches at the larger end and 0.0622 inches at the center, showing a crack 

 between piston and cylinder 0.0003 inches wide at the center, 0.00025 

 inches at the lower end, and 0.00035 inches at the upper end, as used in 

 the calculation above. 



In the earlier measurement of high pressures, the thrust was found 

 by hanging weights directly on the piston, and determining by trial 

 that weight which produced neither rise nor fall of the piston. This 

 has the advantage of ideal accuracy, but has several serious disad- 

 vantages of manipulation. Flexibility of design in the apparatus is 

 sacrificed because the gauge must be kept vertical. The scale pan and 

 weights become increasingly cumbersome at high pressures, so that an 

 assistant is needed. And worst of all, it requires considerable time to 

 make a reading. This is a fatal objection where the pressure must be 

 read instantaneously, as in experiments to be described in a following 

 paper on the freezing of mercury by the method of electrical resistance. 



The present gauge was made direct reading and instantaneous by 

 causing the thrust to produce a measurable deflection in a stiff spring. 

 The new process is related to the old exactly as weighing with a spring 

 balance is to weighing with separate weights. The spring balance is 

 less accurate, but very much more convenient. The accuracy obtain- 



