BRIDGMAN. — HIGH PRESSURE EXPERIMENTING. 



631 



my experience that the new high speed steels are not so good for this 

 purpose as the old fashion carbon steels. It is the hardness that 

 counts — and brittleness is no disadvantage. After prolonged use, 

 the pistons may be expected to show longitudinal cracks. I have 

 never had a piston fail in actual use, although I have used pistons a 

 number of times after the appearance of the cracks. The piston must 

 not be too long, or it will buckle. About 4 inches is a good length for 

 a diameter of | inch. In use, of course, the piston is partially sup- 

 ported by the cylinder when high pressures are reached. A cylinder 

 I inch in diameter may easily support 25000 kgm. with an unsupported 

 length of 1| inches, provided that it is properly centered. 



Some sort of face plate must intervene between the hardened piston 

 and the soft steel piston of the press. The form shown in Figure 4 is 

 convenient. This is made of tool steel, hardened, and drawn to a 

 blue. If not drawn, it is quite likely that it will crack, or even that 



F 



T)~4 



f 

 A 



-E 



-C 

 -B 



Fig. 4. Fig. 5. 



Figure 4. Shows the face plate of hardened steel between the half inch 

 high pressure piston and the piston of soft steel, P, of the hydraulic press. 

 The scale of the diagram is \ actual size. 



Figure 5. The packing plug on the movable piston for use up to the high- 

 est pressures in the finally seasoned apparatus. The scale of the diagram is 

 % actual size. 



the center will be sheared through. A thin (0.01 inch) copper washer 

 at A ensures uniform seating of the piston. The piston, it may be 

 mentioned, is most likely to crack longitudinally when the ends press 

 against something soft that may flow out laterally, carrying the piston 

 with it by friction. It is well, therefore, to make the center hole in 

 the face plate a good fit for the piston. 



The principle of the packing plug at the end of the piston has been 

 described, but there are a number of details of construction. This 

 plug may have two forms, one for use in the apparatus as finally set 



