296 Royal Society ;— Prof. W. G. Adams and Mr. E. E. Day 



was increased. Also it was found that the electrical resistance of 

 the rod of selenium was different for currents going through it in 

 opposite directions. Thus if two platinum wires be melted into 

 the selenium at two points, A and B, and the resistance of the 

 selenium be balanced by the "Wheatstone's-bridge arrangement, the 

 positive pole of the battery being connected to the electrode A, 

 then, on reversing the current so that the negative pole of the 

 battery was now connected to the electrode A, the numerical value 

 of the balancing resistance was always found to be different from 

 that previously obtained. 



If the electrical conductivity of selenium followed the ordinary 

 law of metallic conduction, this would not be the case ; and hence 

 it seemed probable that a careful investigation of these points 

 would lead to some important results. 



In the experiments recorded in this paper, the objects we have 

 had especially in view have been : — 



(i) To examine the character of the electrical conductivity of 

 selenium when kept in the dark. 



(ii) To determine whether light could actually generate an elec- 

 tric current in the selenium. 



Several pieces of selenium were prepared as follows : — A small 

 piece varying from a quarter of an inch to an inch in length was 

 broken off a stick of vitreous selenium. A platinum wire was 

 then taken and bent round into a small ring at one end, and the 

 remainder of the wire turned up at right angles to the plane of 

 this ring. The rings of two such wires were then heated in the 

 flame of a spirit-lamp, and pressed into the ends of the little 

 cylinder of selenium, thus forming platinum electrodes. The 

 whole was then annealed. After annealing, copper wires were 

 soldered on to the platinum electrodes, and the selenium was then 

 inclosed in a piece of glass tube, the electrodes being passed through 

 corks fixed at the ends of the tube. A numbered label was then 

 attached to one of the electrodes ; and this was then always de- 

 scribed as the " marked " electrode. 



The method of annealing which we have found to give the best 

 results is very simple. A large iron ball is heated to a bright red 

 heat, and then placed in a large iron bowl of sand ; the sand is then 

 heaped up all over the ball, and left for an hour. The ball is 

 then taken out, and the selenium, wrapped up in paper, is put 

 into the hot sand and left there for twenty-four hours. On re- 

 moving it from the sand its appearance has generally changed from 

 a bright glassy character to a dull slate-coloured one ; and when 

 this is the case its conductivity is generally very good. 



In most of our experiments it was important to know what 

 was the direction of the current in any particular case ; and we 

 therefore decided to call those currents direct or positive cur- 

 rents when the positive electrode of the batteiy was connected 

 with the marked electrode of the selenium plate under examina- 

 tion. In order to be able to reverse the current with respect to 

 the selenium without affecting any other portion of the circuit, the 



