114 



Prof. W. Gr. Adams and Mr. R. E. Day on [June 15, 



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 

 alwaj^s described 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 reinoving 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 pcsitive currents when the positive 

 electrode of the battery was connected with the marl-ed electrode of the 

 selenium plate under examination. In order to be able to reverse the 

 current with respect to the selenium without affecting any other portion 

 of the circuit, the ends of the wire electrodes of the selenium were made 

 to dip into two little mercury cups fixed on a plate of ebonite, and then 

 were connected to the binding-screws of the Wheatstone bridge arrange- 

 ment. Thus by reversing the position of the electrodes the direction of 

 the current through the selenium was reversed. The positive direction 

 of the current was always determined at the commencement of each 

 series of experiments by means of a delicately suspended magnetic 

 needle. 



A few preliminary experiments were made to determine whether the 

 change of resistance with change of direction of the current had any 

 connexion with the position of the selenium or the direction of the cur- 

 rent with respect to the magnetic meridian. Xo such connexion was 

 found to exist. 



From the results obtained from a great many experiments made to 

 determine the diminution of resistance with increased battery-power, and 

 the change of resistance with a change of the direction of the current, 

 the following conclusions were drawn : — 



(1) That on the whole there is a general diminution of resistance in 

 the selenium as the battery-power is increased. 



(2) The first current through the selenium, if a strong one, causes a 

 permanent set of the molecules, in consequence of which the passage of 

 the current through the selenium during the remainder of the experi- 

 ments is more resisted in that direction than it is when passing in the 

 opposite direction. 



(3) The passage of the current in any direction produces a set of the 



