OF THE NATIONAL PHYSICAL LABORATORY. 345 



give erroneous results at temperatures well below the maximum to which the 

 materials used in the construction of a platinum thermometer can be subjected 

 without injury. The investigations dealt with in the present paper have been carried 

 out at the National Physical Laboratory during the past two years, and consist 

 mainly of a continuation of the work of CHAPPUIS and the author on the platinum 

 thermometer, testing up to 1000 C. the validity of the difference formula for two 

 thermometers made of representative platinum wires of high purity, by comparison of 

 these instruments with the constant volume gas thermometer. With these instruments 

 were also compared simultaneously standard thermojunctions, whose electromotive 

 force at a series of temperatures had been determined with special care at the 

 Reichsanstalt at Charlottenburg. 



o 



II. The Gas Thennometer. 



The gas thermometer employed for this work is a duplicate of the one used bv 

 HOLBORN and DAY at the Reichsanstalt. It was obtained from the same maker, 

 FUESS, of Berlin, and was presented to the laboratorv by Sir ANDREW NOBLE. For 

 this munificent gift and for the kindly assistance and advice rendered by the 

 President of the Reichsanstalt, Dr. KoHLRAUSCH, and by Dr. HOLBORN in procuring 

 for us the gas thermometer, thermocouple, wire and materials for the construction of 

 electric furnaces, the laboratory is greatly indebted. 



The instrument is specially designed for rapid work at high temperatures, and was 

 arranged so that measurements could be made with any desired initial pressure and 

 with bulbs of different materials. The principle employed by CHAPPUIS, in the two 

 gas thermometers at Sevres, oi making all the measurements depend upon the 

 determination of a single length, though undoubtedly capable of giving by far the 

 most accurate results, becomes somewhat inconvenient when great changes of pressure 

 are needed. For this reason, therefore, in the present apparatus the manometer is 

 arranged so as to measure directly the difference of height between the level of a very 

 short metal point, to which the mercury in the closed limb A, fig. I, is adjusted, and 

 the mercury surface in the long tube B, which during the measurements communicates 

 with the atmosphere by the tap H.* 



The tubes A and B communicate by means of cone joints with the lower part of a 

 closed iron reservoir in the base plate of the apparatus, into which mercury can enter 

 from the upper reservoir G by means of the long tube C and steel tap D. The fine 

 adjustment of the height of the mercury to the point in A is made by a steel screw 

 with capstan-shaped head projecting from the bottom of the apparatus and working 

 on a thin steel diaphragm let into the bottom of the reservoir. 



* In the original form of the apparatus, tube A was joined to the reservoir below by a large three-way 

 glass tap, through the side tube of which the filling of the gas into the reservoir was made. It was found, 

 however, that this tap was a source of danger in the measurements, the results of one set of comparisons 

 YOL. CCIII. A. 2 Y 



