17-i Holborn and Day — Gas Thermometer at 



illuminated from below with incandescent lamps. After the 

 temperature has reached a bright red heat where the bar itself 

 gives out light, the divisions still show very sharply by con- 

 trast with the polished metal surface as bright marks upon a 

 dark ground. 



But with the metals which oxidise and even with platin- 

 iridium which tarnishes before reaching 1000°, it is better to 

 illuminate the glowing rod from outside also. The divisions 

 then appear as dark lines upon a bright ground exactly as at 

 the lower temperatures. 



The temperature of the bar thus heated was measured at 

 nine different points with a thermo-element, both wires being 

 insulated with thin porcelain tubes so that the junction could 

 be brought to any desired point along its length. 



Platin-iridium (80 Pt, 20 Ir). — The following values were 

 obtained for the expansion \ s of the platin-iridium bar, which 

 together with a similar bar of pure platinum mentioned below, 

 was kindly loaned by the firm of Heraus in Hanau : 



A 5 (mm) 



t 



250° 



500 



750 



1000 



The mean length included between the two groups of five 

 divisions each at the two ends of the bar was 483 , 07 mm at 0°. 

 From these figures the expansion X of a bar of unit length at 

 0° may be represented by the formula 



X = [8198 t + l-418f] 10 . 



Platinum. — We had no bar of the alloy 90 Pt, 10 Ir, of 

 which the second bulb was made, but one of pure platinum was 

 measured under the same conditions and yielded the following 

 results, which we combined with the foregoing and used the 

 mean for correcting the gas thermometer measurements made 

 with the bulb of the lower alloy : 



l s (mm) 



t 



250° 



500 



750 



1000 



The length at 0° was 483*52 mm and the corresponding formula 

 for a unit bar takes the form 



X = [88892 + 1-274 f~\ 10- 9 



Observed. 



Calculated. 



1-033 



1-033 



2-150 



2-151 



3-357 



3-355 



4-645 



4 645 



Observed. 



Calculated. 



1-114 



1-113 



2-309 



2-304 



3 570 



3-571 



4-909 



4-914 



