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



where 



c)H = rate of loss of heat ; 



^R^ = change in resistance of the filament due to 

 turning on the thermionic current; 

 R = the resistance of the filament ; 

 R = the resistance of the circuit of battery B ; 

 j — the thermionic current flowing into unit length 

 of the filament ; 

 # 0? x = coordinates along the filament measured from 

 the centre of distribution of the thermionic 

 current ; 

 p = the resistance per unit length of the filament 



with T off ; 

 T = the thermionic current. 



The other symbols have the same significance as in (2) and 

 (3j. Expression (4) divided by expression (2), the change 

 in the rate ot supply of energy due to shunting x with y, 

 * gives the value of the ratio BH/dE, which is equal to 

 §IVdR e , or, 



B|[ _ BR* _ Dr-pf 



BE~BR*~ D e ' W 



where p^r is the deflexion of the galvanometer due to the 

 uncompensated part of the disturbance produced by the 

 thermionic current in the bridge circuit. Expressions 

 similar to (4) and (5) are obtained for the bridge current 

 reversed, except that — i is substituted for +i, and — p-\jr 

 for +pyjr. Substituting (4) in (5) for the two cases, adding, 

 and simplifying, the expression for the cooling effect is 

 obtained in the form 



4 + -(0i-Oo)- ( T + T')D.V(«+y) V + ^J 9R " (b) 

 where the primed letters refer to the values with current 

 reversed, and BR* is given by (3). 



Mounting and Preparation of Filaments, 

 The filaments were mounted axially in glass tubes as illus- 

 trated in fig. 2. The tungsten and tantalum filaments were 

 welded to copper leads, which were in turn welded to stout 

 platinum leads that were sealed into the ends of the tubes. 

 The molybdenum was silver-soldered to the copper leads, 

 and the carbon was attached by means o* a paste which was 

 supplied for the purpose by the Edison Electric Lamp Com- 

 pany of Harrison, New Jersey. The anode was made from 

 ^clean copper gauze bent into a cylinder to fit the glass tube 



