BRIDGMAN. — MEASUREMENT OF HYDROSTATIC PRESSURE^. 329 



tions in the width of the ground strip. A pair of springs so made will 

 support indefinitely at the center a weight of 1350 pounds without 

 permanent set. The deflection under this load is 2 or 3 mm. The 

 actual working pressure did not exceed 650 lbs. 



The small motion of the springs was magnified by a simple mirror 

 device, and observed with a telescope and scale rigidly attached to the 

 frame holding the springs. The scale distance was only 30 cm., but it 

 was nevertheless possible to obtain a magnification of over 1500 times 

 with perfect consistency and freedom from back lash or tremor. The 

 magnification was doubled by reflecting twice from the moving mirror. 

 The size of the piston, sensitiveness of the springs, and optical mag- 

 nification were altogether such that 8 kgm./cm.** on the gauge produced 

 a deflection of 0.1 mm. at the observing telescope. This gives 1/10 

 per cent as the accuracy of the pressure readings at 8000 kgm./cm.^ ; 

 proportionally more at the higher pressures. 



All of the parts connecting together the springs and the mirrors were 

 made of steel. This has the advantage of avoiding any motion of the 

 mirrors which might be produced by changes of temperature of the sur- 

 rounding atmosphere. The only temperature effect is that due to the 

 change in the elastic constants of the steel spring with variations of 

 temperature. This was so small that no appreciable error is introduced 

 under the ordinary conditions of use. 



Any device for measuring the magnitude of a stress by the deflection 

 of a spring must be subjected to pretty careful scrutiny before the 

 measurements can be accepted as accurate, because there are disturbing 

 effects, such as elastic after-working and hysteresis, which complicate 

 matters. It was hoped to reduce these eff'ects to a negligible value by 

 using as the working stress less than half the stress at the elastic limit 

 as mentioned above. But even with this precaution it seemed desira- 

 ble to calibrate carefully the springs under working conditions. 



In order to facilitate the comparison, the springs, multiplying mech- 

 anism, and telescope and scale were rigidly connected in one piece. 

 This could be screwed either to the end of the absolute gauge for the 

 purpose of measuring the thrust on the piston, or to the calibrating de- 

 vice. The calibration was eff'ected at first by hanging weights on a 

 stirrup, but this process, always discontinuous and sometimes as compli- 

 cated as applying two, removing one, applying two, etc., was so unlike 

 the process of loading during actual use that another method was seen 

 to be necessary. Two freely moving pistons were used, as when finding 

 the area of the 1/16-inch piston, both communicating with a Cailletet 

 pressure pump of the Socit^td Genevoise. One piston, 1/4 inch in di- 

 ameter, was the same as that used iu the previous work. Weights were 



