BRIDGMAN. — MEASUREMENT OF HYDROSTATIC PRESSURES. 339 



To show within what limits different pieces from the same spool 

 of wire give the same results, three coils were made from the ends 

 and the middle of a length of wire of 70 m. The constants for 

 the separate coils at were .O523OI, .052307, and .052325, in the 

 order of the coils, a variation of one per cent. The temperature 

 effect was found by measuring these same coils again at 50°. These 

 same three coils gave 2295, 2319, and 2320 respectively. One of 

 the coils measured at —12° showed no measurable difference between 

 -12° and 0°. 



The maximum pressure to which these calibrations were made varied 

 somewhat with the temperature, because at the lower temperatures the 

 mixture of glucose and glycerine used with the absolute gauge became 

 viscous so rapidly with increasing pressure as to transmit pressure very 

 slowly. At the low temperatures the pressure was increased until this 

 limit was reached. The slow flow of glucose from the lower to the 

 upper cylinder might occupy an hour or more before the equilibrium 

 was complete. The fact that there was such a process of flow was 

 definitely shown by the slow fall of pressure in the lower cylinder as 

 indicated by the absolute gauge, with a simultaneous slow rise of 

 pressure in the upper cylinder, as indicated by the manganin resistance. 

 At 0° the maximum reached was in one case 11,000 kgm. This was 

 probably too high for complete equalization of pressure, for the change 

 of resistance was 1/2 per cent too low at this maximum. The measure- 

 ments at 0° were not usually carried as far as this, 9500 being the 

 more usual limit. Slight differences in the composition of the glucose 

 mixture made very pronounced differences in the viscosity at high 

 pressure. At 50° the highest reached was 12,000. Even here the 

 viscosity was very considerable. On one occasion pressure was pushed 

 to 13,000 at 50°. There was the same slow equalization of pressure, 

 extending over about half an hour, as was found at 0°. At the end of 

 this time the resistance had nearly acquired the value given by a 

 linear relation, when the experiment was terminated by an explosion. 

 The final reading below this at 11,500, at which there was also some 

 viscous yield, completely satisfied the linear relation. 



We may conclude, therefore, that over the temperature range 0°-5o° 

 the pressure resistance relation is linear within 1/10 per cent of the 

 change of resistance, up to 13,000 kgm. This was proved by actual 

 experiment at 50° to 12,000, and to 9500 at 0°. The extrapolation to 

 13,000 at 0° is comparatively slight, and is made all the more probable 

 by the fact that our usual experience would lead us to expect greater 

 departure from linearity at higher temperatures, and no such depar- 

 ture was found. 



