BRIDGMAN. — A SIMPLE PRIMARY GAUGE. 207 



about 1.25 per cent carbon tool steel. The drilling of the hole in which 

 the piston moves demanded care. This was drilled first with a drill 

 about 0.002 in. (0.05 mm.) under A in. (1.59 mm.), and then en- 

 larged to full size with a two-lipped tV in. twist drill. The hole made 

 in this manner proved round, uniform, and satisfactory in every par- 

 ticular. It is a matter of common experience that a two-lipped twist 

 drill hugs the hole very tightly when used as a following drill. For 

 this reason, care is necessary not to push the drill too hard, as other- 

 wise the sharp cutting corners are quickly blunted. After turning and 

 drilling, the cylinder was hardened in water, and the temper drawn 

 below a blue. Drawing the temper is a necessary precaution in the 

 interests of safety, as glass hard steel proved itself, very treacherous. 

 The cylinder is so small that with the exercise of a little care in heat- 

 ing and quenching it is not distorted appreciably by the hardening. 



The piston was a piece of T V in. (1.59 mm.) " Crescent" drill rod, 

 hardened in oil, the temper not being drawn further. This drill rod 

 was found to be remarkably round and uniform in diameter, varia- 

 tions of so much as 0.0001 in. (0.0025 mm.) being rare from end to 

 end of the same length. Different pieces, however, of nominally the 

 same size rod may differ by 0.0005 in. (0.0125 mm.) in diameter. It 

 was merely necessary, then, to select from several lengths of drill rod a 

 piece fitting the hole in the cylinder. No grinding whatever was 

 necessary, either on the cylinder or the piston, except rubbing with 

 the finest emery paper to remove the film of oxide after hardening. 

 In fact, it is the salvation of this device that no grinding is necessary, 

 accurate grinding of a piston so small as rV in. being out of the ques- 

 tion, to say nothing of the t*b in. hole in the cylinder. Because of its 

 slenderness, considerable care is necessary in hardening the piston 

 without warping. Several attempts were usually necessary before a 

 perfectly straight piston was obtained. This, however, is a matter of 

 no consequence, because a piston can be made in a few minutes. The 

 writer has himself made two cylinders and pistons complete in one 

 day. 



Leak was reduced to a very low value by using a liquid of great 

 viscosity to transmit pressure to the piston. A mixture of molasses 

 and glycerine proved suitable. The viscosity can be given any de- 

 sired value by boiling away enough water from the molasses before 

 adding the glycerine. Besides increasing the viscosity, the glycerine 

 serves the useful purpose of preventing the molasses from drying 

 where it leaks out between piston and cylinder. The liquid used to 

 transmit pressure from the high pressure pump to the gauge was a 

 mixture of two parts glycerine to one part water. This was prevented 



