Ductile Materials under Combined Stress, 85 



thin, evidence of a bending-moment (small, however) was 

 evinced in some of the tests. 



This could have been avoided in the tension-internal pres- 

 sure tests by generating the fluid pressure in a reservoir 

 attached to the specimen, the plunger being connected to the 

 jaw of the testing machine, thus transmitting the pull and 

 producing the fluid pressure at the same time. This method 

 however, only permits of a certain ratio of fluid pressure 

 to tensional load for a given plunger, is inapplicable to tor- 

 sion-internal pressure tests, and is subject to other objections. 

 Accordingly the former method, despite the risk incurred of 

 an undesired bending-moment, was adopted. 



Up to 1000 lbs. per sq. in. the pressure was taken from 

 the system which worked the hydraulic straining-gear of an 

 Emery-Festing machine ; beyond that pressure a small in- 

 tensifier was employed to further increase the pressure. 



30. Measurement of the Fluid Pressure. — To measure the 

 pressure I invented a special form of gauge, but although 

 this proved useful for calibrating purposes, it was replaced 

 in the tests by a large commercial gauge, which could be 

 read by means of a telescope from the controlling-valve 

 of the Emery pressure-system, which valve was at a con- 

 siderable distance from the Wickstead machine. This com- 

 mercial gauge was calibrated by comparison with the volume 

 of air compressed into a uniform tube by the pressure and 

 by comparison with the special gauge, the constant of which 

 was determined at a low load by means of a Crosby gauge- 

 tester. The air-gauge consisted of a glass tube of about 

 1 mm. diameter of bore and 1 metre long, sealed hermetically 

 into a glass bulb holding about 20 c.c. ; below this a tube 

 projected downwards, and was then bent upwards for con- 

 venience in handling. The capillary tube was selected as 

 having the most uniform bore of a large number of such 

 tubes, and was so even as to render correction for conicallity 

 unnecessary. The bulb of the glass apparatus was contained 

 in a iron cylinder partially filled with mercury, to the top of 

 which the oil of the pressure-system had access. The iron 

 cylinder was closed with a screw plug, through which the 

 capillary tube projected ; the packing used was of the U-type. 

 The comparison of the volume of air under pressure with the 

 reading of the commercial gauge was conducted so slowly 

 that the changes of volume were practically isothermal. 

 The volume of the tube and of the bulb, and the quantity of 

 air contained, had been carefully measured, but a large 

 quantity of air was syphoned out by the action of the mercury; 

 and in preference to running the risk of an unnoticeable 



