A. L. Day and J. K. Clement — Gas Thermometer. 419 



parison with a saturated cadmium cell. The cell was one of a 

 series described in a previous paper * which has been compared 

 from time to time with the standard cells of the National 

 Bureau of Standards and has never been found to contain an 

 error greater than one or two parts in 100,000. The galva- 

 nometer was a Siemens and Halske instrument of the usual 

 moving-coil type. With the help of a small rheostat in series 

 with the galvanometer, the sensibility was maintained at a con- 

 stant value such that one scale division in the telescope (distant 

 l'5 m from the galvanometer) corresponded exactly to two micro- 

 volts in the thermoelement reading, which is roughly equivalent 

 to one-fifth of 1°. In this galvanometer the wandering of the 

 needle from its zero position was slight and never amounted to 

 more than -2 or -3 of a scale division. It was also almost abso- 

 lutely dead beat with a period of about five seconds, so that 

 adjustments for a temperature reading could be made with 

 extraordinary rapidity and with an accuracy out of all propor- 

 tion to the needs of the experiment. 



The only error to which the thermoelectric observations 

 were subject was the contamination arising from the iridium 

 contained in the bulb. During the first year in which these 

 observations were begun the furnace coil also contained ten 

 per cent of iridium, but at that time the contaminating effect 

 of this metal upon a thermoelement was not well understood. 

 Later on, this coil was exchanged for a coil of pure platinum 

 made especially for this purpose by Dr. Herseus, which was 

 guaranteed to contain no more than 0*05 per cent iridium and 

 which was found upon analysis to contain considerably less 

 than this quantity. Inasmuch as the furnace coil is always the 

 hottest part of the system, this afforded considerable relief, but 

 the position of the elements in contact with the bulb made it 

 impossible to prevent some contamination above 900°, so long 

 as the bulb remained bare. An attempt was made to reduce 

 this difficulty still further by the use of a glaze made from 

 melted mineral albite, which was appreciably soft at tempera- 

 tures of 1100° but which appeared to prevent the sublimation 

 of iridium so long as the coating remained continuous. The 

 viscous material, however, showed a persistent tendency to 

 gather together into globules, leaving bare spots on the bulb 

 which were not wet by the glaze, so that this protection was 

 not complete. Porcelain insulating tubes open at the end 

 afford little or no protection. We were accordingly driven to 

 the conclusion that for the higher temperatures iridium must 

 be banished from the furnace completely before consistent 

 observations can be obtained. This is the chief reason why 



*Dav and Allen, loc. cit. p. 26. 



