434 



REPOET — 1881. 



Fig. 2. 



about :j-incli in 



boiling water, and was maintained by means of steam. The bars were 

 immersed in a bath of melted paraffin wax, the interior surface of the 

 bath being lined throcighout with convolutions of ^ inch ' compo ' gas- 

 tubincr through which steam was caused to flow. 



The lower temperature was about that of the air, the bath just 

 described was again used ; but paraffin oil was substituted for the wax, 

 and cold water fi'om the main was made to circulate in the pipe instead 

 of steam. In both cases the paraffin oil or wax was kept continually 

 stirred. The standard was also immersed in paraffin oil, kept at a 

 uniform temperature by the circulation of water from the main, thi'ough 

 tubing in the containing vessel. 



The paraffin oil used possesses remarkably high insulating qualities, 

 bobbins of silk-covered wire, even of many thousand ohms resistance, 

 could be immersed in it, without the least sensible leakage, from spire to 

 spire. It is sold under the name of ' Strange's Crystal Oil.' 



The apparatus for maintaining the desired temperature when the 

 alloy is in the form of wire is far more convenient than that described 

 above for the bars. It resembles the instrument used for testing the 

 boiling point of thermometers. 



It is shown in section in the accompanying sketch, fig. 2. 



a is an open tube 

 diameter, surrounded by a second tube, 

 h, closed at the bottom and opening 

 at the top into the outer tube, c, 

 which is closed at both ends. The 

 supply pipe leads into the annular 

 space between a and h, near the bot- 

 tom, and the exhaust, or escape, pipe 

 from the space between h and c. 



When steam is allowed to enter by 

 the supply pipe, it completely en- 

 velopes, in its passage, the inner tube 

 a, and external influences, such as sud- 

 den draughts of air, are effisctnally cut 

 off by the steam jacket between h and c. 

 Tor observations at ordinary tempera- 

 tures water is allowed to flow into the 

 supply pipe instead of steam, and a 

 convenient sj'stem of pipes and cocks 

 allows the change to be made from 

 steam to water, or vice versa, with 

 facility. When steam is used, a mer- 

 curial gauge is provided, to indicate 

 the difference of pressure between the 

 inside of the tube and the atmosphere. 

 The temperature calculated from 

 the reading of this gauge, and that of 

 the barometer, was found to agree 

 within -jV degree with the indication 

 of the thermometer inserted in the 

 "* inner tube a. 



The bottom of the inner tube a is closed by an ebonite stopper, 

 through which pass two stout copper rods of semicircular section, so as 



I 

 I 

 I 

 i 



£SCAP£ PIP£ 



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