OP ARTS AND SCIENCES. 99 



Pressure Measurement. 



13. Tait Gauge.* Adjustment. — The gauge is inserted at iV (Fig. 

 4), and is shown at qq, tt, ss. The essential part is the steel tube qq, 

 closed at one end by a close-fitting tinned bolt, r, soldered in place, and 

 connecting at the other end with the cup-shaped junction N. It is not 

 advisable to cut a deep thread in the tube 177. It may, however, be 

 fastened into JV by a fine shallow thread and solder ; and the deep 

 thread cut in the thick-walled tube of N, which is then tinned and 

 screwed into the barrel. 



To measure the expansion of qq under pressure, it is surrounded by 

 a close-fitting glass tube ttit, one end of which is joined to the steel 

 tube qq by a layer of marine glue. To do this the tubes qq and tt 

 are clamped in position vertically, with the open end uppermost. A 

 sheet of thin copper foil is wrapped tightly around the end of tt, and 

 fastened with wire so as to project somewhat above it, forming a small 

 annular trough. Marine glue is put into this, and the burner applied 

 cautiously from without. The fused marine glue slowly flows down 

 into the space between the glass tube and steel tube, and the heat is 

 withdrawn when the layer is an inch or two in length. When cold, 

 fusible metal is poured into the copper trough in the space left by the 

 glue. The joint thus formed is not only tight, but reliably rigid.f 

 Space must be left for the elongation of the steel tube. 



The other end of the glass tube tt communicates with the capillary 

 tube ss by which the expansions are measured. In Figure 4 this is 

 shown vertically; but it is expedient to bend it horizontally, parallel 

 and close to the jacket uu, to which it is rigidly fastened directly over 

 a suitable millimeter scale. In this way the capillary, though 70 cm. 

 or 80 cm. in length, is protected against injury. I may state, in pass- 

 ing, that pressure on the outside of the tube tt appreciably displaces 

 the meniscus in ss, an error which must be guarded against. It is 

 well, however, to bring slight and constant air pressure to bear on 

 the meniscus, by attaching a U-tuhe containing mercury to the end 

 of the capillary. 



* The form of steel high pressure gauge based on Hooke's law recommended 

 but not constructed by Professor Tait (loc. cit.), is somewhat different from the 

 above. I tliink my form lias the practical advantage of greater simplicity, be- 

 ing essentially a single-walled tube. The method of computation is due to 

 Professor Tait. 



t It was my purpose to make the tube tt of brass, so that the heat of com- 

 pression would be more quickly dissipated. But I failed to obtain tubing of the 

 proper bore and strength. 



