﻿Thermionic Currents from Tungsten. 805 



thermionic circuit, and the quadrants were connected to the 

 ends of these resistances in turn. For currents greater than 

 1 microampere a unipivot galvanometer with variable shunts 

 was used. With these arrangements, it was possible to 

 measure the emission over a range from 10~ 13 amp. up to 

 1 ampere. Owing to the high melting-point of tungsten, 

 it was possible to obtain thermionic currents of the order of 

 magnitude of the heating currents. In what follows, the 

 thermionic current will usually be expressed in terms of unit 

 area, otherwise it is to be taken as the total current from the 

 filament. 



Measurement of the Filament Temperatures, 



Through the kindness of Dr. Irving Langmuir, a curve 

 was obtained with the wire, showing the temperature of the 

 filament as a function of the current carried by it. This 

 curve was determined by photometric measurements on a 

 special lamp, using a piece of the same wire that was used 

 in these experiments. As a check on the temperature deter- 

 minations some small lamps were made from these filaments, 

 and were used in an optical pyrometer of the Holborn- 

 Kurlbaum type, constructed in this laboratory. These lamps 

 were calibrated by observations on a black-body furnace at 

 the melting-points of copper and nickel. The results of the 

 two methods were in satisfactory agreement. In the earlier 

 experiments it was customary to determine the resistance 

 each time an observation was taken, since the bridge oalva- 

 nometer was of course much more sensitive than the 

 ammeter to small variations in the temperature of the 

 filament. 



The effects produced by a thermionic current in the 

 •ordinary Wheatstone's bridge circuit have been considered 

 by llichardson and Cooke *. As far as the present experi- 

 ments are concerned these effects are of no importance, so 

 long as the thermionic current is small in comparison with 

 the heating current. For very high temperatures, however, 

 the large thermionic currents cause the temperature estima- 

 tions to be much too high. In fact, even if the bridge is 

 not used at all, the temperature of the filament will be over- 

 estimated if a large thermionic current is flowing, and. 

 furthermore, the two ends will be unequally heated. For a 

 given current per square centimetre these disturbing effects 

 increase with the length of the filament. 



* Phil. Mag-, vol. xx. p. 173 (1910). 



