Thermal Radiation in Absolute Measure. 



545 



Two exactly similar tubes were employed, as shown in 

 fig. 2. They were connected together by a side tube as 

 shown; and by means of a branch tube, attached to. this side 

 tube and connected to a Sprengel pump, the air was withdrawn 

 from both tubes at the same time. By this arrangement it 

 was provided that the vacuum in the two tubes should be at 

 all times precisely the same. 



Current 



Potential, GaUvC 



im r x rs t s tu w eu TiTmnu*- 



In one of the tubes the platinum wire was brightly polished 

 and perfectly smooth just as it came from the maker's hands. 

 The other tube contained a platinum wire cut from the same 

 hank, but with the surface covered with an excessively 

 fine coating of soot. The soot was put on by passing the 

 wire carefully through the upper part of a clear paraffin- 

 flame*. 



The usual arrangements were made for drying the vacuum 

 of the tube, and of the pump, by means of phosphorus pent- 

 oxide; and the vacuum was measured by means of the Gim- 

 mingham modification of the McLeod gauge. 



The wires were heated, as in the former experiments, by 



* The texture of the soot depends greatty on the source from which it 

 is obtained, and on the way in which it is applied to the -wire. Some 

 preliminary experiments have been made with various coatings of soot, 

 and comparisons have been attempted between surfaces finely coated with 

 soot, and surfaces prepared with platinum-black and with a fine coating 

 of black oxide of copper chemically applied to the wire (cf. J. T. Bottomlev, 

 Phil. Trans. 1887, p. 4-19). 



