204 Mr. H. H. Lester : Determination of Work Function 

 Theory of Measurements. 



The methods consisted chiefly in comparing the loss of 

 teat due to turning on the thermionic current with the gain 

 in heat when a known increment of current was supplied 

 to the filament. The comparison was effected by assuming 

 that BH, the change in heat supplied to the filament, was 

 in each case proportional to BR, the change in resistance o£ 

 the filament. The increment of resistance due to an increase 

 in the current was accomplished by shunting x with y. The 

 final expression for the change in the supply of energy is 

 given by the expression 



w 



here 



dE = (l + 9 )V(2R.^.i + BE) ... (2) 



x-\-y v 



3R.=9xlO-».^i2(G + DE(l + j+J^)D., . . (3) 



where 



BE = increment of the rate of supply of energy ; 

 q = fractional part of bridge-current through the 



high-resistance arms ; 

 i x — current through the bridge ; 

 R = resistance of the filament ; 

 G= resistance of the galvanometer — 225 ohms; 

 x = shunted portion of external resistance — 1 ohm. 

 y = shunt about x — 100 ohms ; 

 BR e = change in resistance of the filament due to 

 shunting x ; 

 D e = galvanometer deflexion due to shunting x ; 

 DE and CD = resistances in the arms DE and CD 

 respectively. 

 The change in the resistance due to starting the ther- 

 mionic current is not so easily determined. The presence 

 of the thermionic current in the bridge system disturbs the 

 galvanometer balance, and there is a change in the resistance 

 of the filament due to the Joule heating effect of the ther- 

 mionic current. The first effect is minimized for the 

 " standard shunted " method by introducing the thermionic 

 current at the middle points of the two lower arms. With 

 this arrangement the current should distribute itself sym- 

 metrically around the network and should not influence the 

 galvanometer. For the " filament shunted " arrangement 

 the current theoretically departs from and returns to the 

 same point in the filament, and is not present as a disturbing 

 factor in the bridge network. As a matter of fact, perfect 

 compensation is not achieved in either case. In the first 



