350 DR. J. A. BARKER ON THE HIGH-TEMPERATURE STANDARDS 



VI. Filling of the Reservoir. 



The experiments of CHAPPUIS and the author, ' Phil. Trans.,' vol. 194, p. 93, 

 showed how important it is that previous to and during the tilling of the reservoir 

 with gas the whole should he heated to as high a temperature as possihle indeed 

 that with a reservoir of verre dur it was impossihle to obtain an accurate value for a 

 temperature in the neighbourhood of the boiling-point of sulphur unless it had been 

 previously heated for a considerable period to a temperature considerably higher than 

 this. Although very little is known as to the behaviour with change of temperature 

 and pressure of the films of condensed gas,- which by their surface tension adhere 

 closely to the walls of a glass or porcelain vessel, and how they influence the 

 coefficient of expansion of any gas enclosed in it, it seems at any rate advisable to 

 remove the existing film in any reservoir to be used for high-temperature work as far 

 as possible by repeated heating and evacuation. Accordingly for each new filling 

 during this work the procedure was to heat the reservoir very strongly, exhausting it 

 and the connecting tubes leading to the pump so thoroughly that an electrodeless 

 vacuum-tube attached by a side tube showed strong green fluorescence both when the 

 reservoir was hot and the next dav after cooling. This was only attained after much 



, n / 



labour by taking special care in the manipulation of all the taps and joints in the 

 circuit, and by blowing together into one piece the whole of the long canal leading 

 from the gas thermometer in the main laboratory to the pump and gas preparation 

 apparatus in a small room on an upper floor. After the apparatus had stood this test 

 the gas was filled into the reservoir to 100 to 200 millims. pressure at least three 

 times iu each case before the final filling and pressure adjustment were made, the 

 bulb being heated meanwhile as strongly as expedient in the particular case. 



VII. Dilfttatio-n of the Porcelain Reservoir. 



In 1898 and 1899, OHAITUIS and HARKEK, in the absence of precise data as to the 

 expansion at high temperatures of the reservoirs of Berlin porcelain they employed in 

 their gas thermometer, were obliged to content themselves with extrapolation of the 

 expression for the expansion obtained by them from determinations made in the 

 Benoit-Fizeau dilatometer between the limits and 100, confirmed by further 

 experiments made by a weight thermometer method. The values for the expression 

 they employed seemed at the time to be amply confirmed by the measurements of 

 HOLBORN and WIEN, made in 1892, which gave for the mean coefficient of linear 

 expansion between the limits and 600 the value 4400 X 10~ 9 , the value calculated 

 by CHAPPUIS and HARKER for the same temperature limits being 4484 X 10~ 9 . On 

 the other hand, the later experiments of HOLBOKN and DAY made on Berlin porcelain 

 rods heated horizontally in an electric furnace show fairly conclusively that the 

 expansion of porcelain cannot be represented by any simple function for more than a 



