46 REPORT— 1903. 



these pairs was looped upon itself to form the compensator, and the other 

 cut in half for attachment to the ends of the ' bulb ' wire. Several kinds 

 of mica from different sources were tested as to their suitability for use as 

 insulating material for the frame and washers to support the wires, and 

 it was found that considerable discrimination was necessary in the selec- 

 tion of the mica for this purpose. Certain qualities which were colourless 

 before heating became on exposure to only 800° to 850° of a marked 

 brown tint, and it was found in one case this was due to organic material 

 having been used to fasten together several sheets to build up the neces- 

 sary thickness, the carbonaceous matter leading to a fall in insulating 

 power several hundred degrees below the temperature at which good mica 

 begins to appreciably conduct, which ought not to be lower than 1150° C. 

 In another case, a specimen which showed the characteristic silvery white 

 lustre after several hours' exposure to 1100° C, had lost so much of its 

 mechanical strength as to be almost unusable. A specimen which before 

 heating was of slightly green tint was finally selected, and of this tlie 

 whole of the mica frames and washers were constructed. The copper wires 

 connecting the platinum leads to the fusible -metal caps were silver- 

 soldered to the platinum, and for extra safety against possible strain the 

 wires were screwed into the caps as well as hard soldered. In order to be 

 protected as far as possible from unsymmetrical heating, which often gives 

 rise to thermo-electric effects in certain types of thermometer, these joints 

 between platinum and copper are arranged so as to be well inside the brass 

 tube into which the glass or porcelain protection tube is fastened. The 

 thermometer heads are of ebonite, and are of the design described by 

 Harker and Chappuis in 'Phil. Trans.' 194, p. 52. They are practically 

 airtight, and will stand vacuum or pressure for a considerable time. By a 

 small tap, which is generally kept closed, communication can be made with 

 a convenient appai-atus for exhausting and letting in dry air while the 

 thermometer is suitably heated. The effect of electric leakage in lowering 

 the apparent resistance of a platinum thermometer when damp is much 

 more easily traced on thermometers of 5 or 10 ohms FI than on the usual 

 1 ohm pattern used for high temperatures. "With the thermometers here 

 described, having the enclosed form of head, none of the determinations of 

 fixed points have been found to be vitiated by moisture, care having been 

 taken not to expose any portion of the interior to prolonged contact with 

 the outside air, after once being thoroughly dried out at a high tempera- 

 ture. 



The mica cross, having serrated edges with teeth of 1 mm. pitch, being 

 attached to the leads and compensator, the joints between the ' bulb ' 

 wire are made in the strongly oxidising fiame of a very small oxy-coa.l-gas 

 blowpipe without admixture of foreign material of any description. Auto- 

 genous soldering of this kind is not very difficult, even for very fine wires, 

 and is essential if the thermometers are intended for use to the highest 

 temperatures safely measurable, namely, 1150° C, as the copper and 

 silver contained in any solder which might be employed give off vapour 

 sufficient to injuriously affect the platinum on prolonged exposure to a 

 temperature considerably below this. The ' bulb ' wire when fastened to 

 the leads is then wound, not too tightly, upon the mica frame, and the 

 thermometer is then inserted into its protecting tube of very thin glass or 

 of porcelain, which must be glazed on the exterior, and if the thermometer 

 is not intended for use above about 1000° C, may with advantage be 

 glazed both inside and out. A measurement is then taken of the funda- 



