508 Johnston and Adams — Influence of Pressure on the 



wire may be made somewhat larger than the outer part.* 



The latter method lias been adopted in the case of large wires, 

 such as those of the heating circuit. The former method 

 was used for the thermocouple and other small wires. The 

 thermocouple wires are insulated in the horizontal portion of 

 their path through the ping G by sections of glass tube, which 

 are kept in place by short pings of fibre. The heating current 

 wire, after it has passed out through the soapstone, is likewise 

 insulated from the steel plug II by the insertion of a thin 

 cylinder of fibre. 



The material to be investigated is contained in a graphite 

 crucible of the form depicted in fig. 3. The crucible is held 

 rigidly in position by small steel rods, which screw into the 

 ends of larger steel rods, attached firmly to the plug G. The 

 crucible lid, also of graphite, is held in place by means of a 

 small screw-clip (not shown in the figure) fastened to one of 

 the supporting wires. To the crucible lid is fastened by two 

 steel pins the thermoelement jacket which is a porcelain tube, 

 7 mm in diameter. The thermocouple wires are separated from 

 one another, as usual, by means of a small porcelain tube 

 slipped over one of them. 



By these methods of construction, motion of the thermo- 

 element, or of its jacket, with respect to the charge, is abso- 

 lutely prevented, so that we could always be sure that the 

 junction was in the proper position with respect to the crucible, 

 viz., located axially and about 6 mm from the bottom of the 

 charge. 



Temperature Measurement. 



To determine the temperatures, thermoelements of cop- 

 per-constantan Avere employed. Three lengths were cut off 

 a reel of No.. 30 (B and S) constantan wire (0 - 25 mm ) and were 

 joined separately to lengths of copper wire of the same diam- 

 eter. Two of these elements were preserved as standards, and 

 gave readings at all temperatures not more than 2 microvolts 

 apart ; the third element was fixed in the plunger G in the 

 way already described. The differences between this element 

 and the standards amounted to 20-30 microvolts, probably 

 owing to strains set up in it while it was being made pressure 

 tight through G. 



Before proceeding to the measurements, it was, however, nec- 

 essary to calibrate the standard elements, since so far as we are 

 aware, no satisfactory calibration of such elements over the 

 temperature range 0° to 400° has yet been made. For this 



* An equivalent method which may he used with some wires is to tie a 

 knot which shall he too large to pass through the small hole drilled in the 

 soapstone block. 



