14 Mr. C. Barus on the Fu 



sion 



yet be looked to for close results. The data, crude as they 

 are, however, show that /3 is determinable by this method 

 with the same degree of accuracy with which it is to be 

 applied, or that the stem error and /3 are determinable in 

 terms of each other reciprocally. In this respect the constant- 

 pressure method of high-temperature air-tliermometry is unique, 

 since it admits of easy modifications, by which the zero volume 

 of the bulb, its coefficient of expansion, as well as all permanent 

 changes of volume, may be evaluated without extra appliances. 



15. Tables. — In making extensive comparisons between the 

 thermocouple and the air-thermometer, I had three objects 

 chiefly in view : — (1) To find out whether the temperature 

 indications of the platinum-iridium thermocouple were 

 regular and free from serious anomalies, in other words to 

 calibrate the couple ; (2) To compare the indications of 

 different non-inglazed porcelain bulbs, for which purpose I 

 compared tw r o bulbs (Nos. 1 and 2) with the same thermo- 

 couple under identical conditions ; (3) To find whether the 

 flow of air between bulb and manometer was seriously 

 retarded by the interposed lengths of metallic capillary tubes. 

 This was discernible by comparing the data obtained when 

 the furnace temperature gradually increased, with the data 

 for decreasing furnace temperature (cooling). 



Accordingly, I have two sets of results in hand, the first 

 obtained with bulb No. 1, and consisting of four series of 

 data obtained on different days ; the second set obtained with 

 bulb No. 2, and consisting of five series of results. Of these 

 I will only reproduce the latter (Bulb 2) here, inasmuch as 

 these data are in many respects the more accurate. Correc- 

 tions for the permanent volume-contraction of the bulb 

 obtained from volumetric measurements made before and 

 after each series of heatings are applied. 



In Tables II., III., and IV., " No/' refers to the thermo- 

 couple used, e 2 o is the thermoelectromotive force when the 

 cold junction is at 20° C. and the hot junction at the air- 

 thermometer temperature T. 



