during Absorption of Electrons hij Different Metals. 405 



observations on strips of the following metals and alloys : — 

 gold, nickel, copper, silver, palladium, aluminium, phosphor 

 bronze, and iron. 



The method of experimenting was practically identical 

 with that already described (loc. cit.). The only important 

 change made was in the method of balancing the disturbing 

 effect of the thermionic current. This was made simpler, 

 both in theory and practice, by the following arrangement. 

 The resistance R 3 (fig. 1, loc. cit.) was replaced by a constantan 

 wire of resistance 4*7 ohms wound on a circular drum and 

 placed in an oil-bath. A movable contact-maker enabled the 

 thermionic current to be tapped off at any point of the wire. 

 The contact-maker w T as connected directly to the point E in 

 the figure referred to and the resistances R 4 and R 5 were 

 done away with. From the principles of the method of 

 balancing the thermionic current already described,, it is clear 

 that if it is tapped off at the right point of the resistance R 3 

 it w T ill produce no effect on the galvanometer G. To balance 

 the effect of the thermionic current it was also necessary first 

 to oppose the two batteries Ci and C 2 so as to destroy the 

 electromotive force in the Wheatstone's bridge circuit, and 

 rotate the contact-arm until the galvanometer spot w T as in 

 the same position with the thermionic current " on " or " off/' 

 The reliability of the method was tested by dummy expe- 

 riments, using the electromotive force from a battery in a 

 manner precisely similar to that used in testing the original 

 method, and it was found quite satisfactory. 



Effect of Pressure* 



In the former paper it was suggested that the results- might 

 perhaps be affected by the pressure of the residual gas 

 present in the apparatus. At that time experiments were 

 made to test the point. So far as they went they seemed to 

 indicate that, if the pressures, such as occurred, exerted an 

 influence on the results, it was an unimportant one. It was 

 felt, however, that these experiments were not very satis- 

 factory; so fresh experiments have been made under better 

 conditions. In these experiments an iron grid was used r 

 and the osmium filaments had been heated continuously for 

 a long time. The conditions were generally very steady. 



On account of the continuous evolution of gas by the hot 

 metal, it was necessary so use a continuous pump in these 

 experiments. This made it somewhat difficult to control the 

 pressure. The obvious way of producing a desired change 

 in the pressure is to vary the speed of the pump. This was 



