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



of the measurements, the other to increase their range. The 

 application of the gas laws is no longer subject to serious 

 question. The progress of recent years has given us electric 

 heating in place of gas and the consequent possibility of con- 

 trolling the temperature with great certainty and exactness. 

 It has also given us the metal bulb with a definite and measur- 

 able expansion coefficient and capable of holding the expand- 

 ing gas without loss. It has discovered a gas which does 

 not diffuse through the bulb or react with it chemically, which 

 does not dissociate within the limits of practicable measure- 

 ment, and of which the expansion can be expressed with 

 reasonable certainty in terms of the Kelvin thermodynamic 

 scale. It has discovered the source of the errors in the ther- 

 moelements and a way to avoid them. 



In 1904, Prof. Holborn of the Eeichsanstalt increased the 

 range of this scale as far as 1600° C, the probable error of the 

 new portion being 10°. Simultaneously with this eff or r, work was 

 begun at the Geophysical Laboratory in Washington with a view 

 to increasing the accuracy of the scale, first over the existing 

 range (to 1150°), and later, as much beyond this point as it 

 should prove possible to go. Temperature measurements 

 between 250° and 1150° have now been made and form the sub- 

 ject of the present paper. The particular points to which we 

 have given the most attention are the following : (1) To provide 

 a uniform temperature along the bulb by a suitable arrange- 

 ment of the heating coils. (2) To enclose the furnace in a 

 gas-tight bomb in which the pressure outside the bulb can be 

 maintained equal to that within for all temperatures. This 

 offers three distinct advantages : It provides against the 

 deformation of the bulb through differences of pressure within 

 and without in the region where the bulb material becomes 

 softer. By using the same gas within and without, there is 

 no tendency to diffuse through the bulb wall. It enables the 

 initial pressure to be varied within considerable limits, thereby 

 increasing both the scope and sensitiveness of the manometer. 

 The sensitiveness in our instrument with this arrangement was 

 about three times that of the Reichsanstalt. (3) The expan- 

 sion of the bulb material was determined with great care and 

 is probably accurate within 1/2 per cent. (4) The unheated 

 space between the bulb and manometer has been reduced until 

 the total correction in this hitherto uncertain region amounts 

 to less than 5° at 1100°. An error of 5 per cent in the deter- 

 mination of its volume or temperature distribution is, therefore, 

 practically negligible. It is probable that these changes serve 

 to reduce the aggregate error of the gas thermometer in the 

 region of 1100° to about one-tenth the magnitude which existed 

 at the time of the establishment of the present scale. 



Furthermore, and most important of all, these refinements 

 are not limited to this temperature region. It is therefore 



