106 Professor H. L. Callendar [March 10, 



ordinary liquid in glass thermometers, but the apparatus is more 

 cumbrous and difficult to use on account of the necessity of observing 

 both the volume and the pressure of the gas. This method is very 

 accurate at moderate temperatures, but the difficulties increase very 

 rapidly above 1000° C. Above 1200° C., it is doubtful whether such 

 measurements are of any greater value than those obtained by extra- 

 polation. Apart from the difficulty (which is common to nearly all 

 methods at high temperatures) of maintaining a uniform and steady 

 temperature, the bulb-method of gas-thermometry is liable to the 

 following special sources of error : — 



(1 ) Changes in volume of the bulb. 



(2) Leakage and porosity. 



(3) Occlusion or dissociation. 



In order to investigate these sources of error a special form of 

 porcelain air-thermometer (Fig. 3) was designed by the writer, and 

 was constructed in Paris, in December 1886, under the supervision of 



Porcelain Pyrometer 



r, A B 



< 



Settunvccb 

 C orD 



A 



PkUuuwv Wires inside* 

 Fig. 3. 



W. N. Shaw, F.E.S., of Emmanuel College, Cambridge. A figure 

 and description of this instrument were published in the Phil. Trans. 

 A., 1887. The same form has since been adopted by MM. Holborn 

 and Wien in their experiments on the measurement of high tempera- 

 tures at the Eeichsanstalt. Thick tubes of 3 sq. mm. cross section, 

 marked AC, bd in Fig. 3, were connected at each end of the cylindrical 

 bulb ba. The length CD could be directly observed at any time with 

 reading microscopes, and the linear expansion of the bulb could be 

 deduced. The volume of the bulb could also be gauged at any time 

 with air, and the mean temperatures of the separate portions ab, ac, 

 bd, could be determined by means of platinum wires extending along 

 the axis of the instrument. This was a more essential part of the 

 apparatus, as the wires afforded a means of accurately reproducing 

 any given set of conditions, and of testing the performance of the 



