during the Absorption of Electrons by Platinum. 175 



we are glad to be able to take this opportunity of expressing 

 our thanks. Osmium is a very refractory substance, and 

 when a filament of it is heated it emits a copious supply of 

 negative electrons which can very easily be regulated by 

 adjusting the heating current through it. A long narrow 

 strip of thin platinum foil was wound in the form of a grid 

 on glass supports so that the different strands were nearly in 

 one plane. Two osmium filaments (see below) were suitably 

 supported, one on each side of the grid, and insulated from it. 

 The filaments were heated by means of an electric current, 

 and an adjustable difference of potential could be applied 

 between the filaments and the grid, causing the electrons 

 emitted by the filament to flow into the grid. The ensuing- 

 rise in temperature of the platinum grid was determined by 

 measuring its resistance by a Wheatstone's bridge method. 

 The present arrangement differs from the usual Wheatstone's 

 bridge problem, however, in one important respect. In 

 addition to the usual battery current flowing through the 

 four arms, we have also the thermionic current, which flows 

 from the filament into the strip which forms the other arm 

 Unless the effect of this is compensated, there will be a 

 deflexion of the galvanometer, even when the resistances are 

 in the proper proportion for a balance to be obtained under 

 the usual conditions. This compensation was effected by 

 introducing two auxiliary resistances into the bridge in the 

 manner described below. 



In addition to measuring the change in the resistance of 

 the strip produced by the electrons, it was also necessary to 

 measure the thermionic current which produced it. This 

 was done by means of a micro-ammeter suitably inserted 

 between the bridge system and the positive terminal of the 

 cells which were used to maintain the applied difference of 

 potential between the heated filaments and the strip. 



The rest of the arrangement will be made clear by referring 

 to the accompanying diagram (fig. 1, p. 176). The electric 

 current which heated the osmium filament, F, was supplied 

 by the battery B 2 and could be regulated by means of the 

 rheostat J. The voltmeter V 2 served to measure the potential 

 drop along the filament. The strip, indicated by S, forms one 

 arm of the Wheatstone's bridge of which 11^21^3 are the other 

 arms. G is the galvanometer, and D, Ci, C 2 the battery arm, 

 R 4 and E 5 are the compensating resistances which are respec- 

 tively connected to the ends of the galvanometer arms .The 

 junction between R 4 and R 5 is connected through a switch 

 to the micro-ammeter N, which serves to measure the 

 thermionic current. The other terminal of this was connected 



