378 Mr. G. W. White on the 



pointed out by Graham Bell *, and for obtaining a wire 

 ' ' bridge " of low resistance he recommended the use of 

 brass in preference to platinum. The author pressed 

 electrodes of different metals on the opposite faces of 

 selenium cubes, and marked differences in resistance were 

 obtained. Table III. shows the results of observations on 

 two cubes. The direction of the current in the selenium 

 was kept constant in the case of each cube. 



Table III. 



Tin Foil. ' Copper Foil. Copper Gauze. 



1... 21,400 74,000 98,000 



2... 46,000 88,000 157,000 



This fact may have its explanation in the change of the 

 resistance at the contact of electrode and selenium. 



The resistance seemed also to depend to a large extent on 

 the nature of the mould used in crystallizing the selenium 

 block. The light-sensitiveness of the surface also depended 

 on this condition. For instance, the change in resistance 

 was 25 per cent, on illuminating a wire-gauze electrode 

 pressed on a selenium surface crystallized in contact with 

 brass ; the surface crystallized in contact with glass had a 

 light-sensitiveness of J 5 per cent, and the "air" surface 11 

 per cent. 



The selenium blocks showed the usual characteristics of 

 the selenium bridge, such as variation of resistance with 

 voltage, pressure, and duration of current ; and it is 

 significant that all these properties together with that of 

 dependence of resistance on direction of current, appear in 

 connexion with " crystal contacts.''' Brown t has called 

 attention to this similarity. It is also noteworthy that 

 Streintz f has explained the " apparent resistance " of 

 crystals and its dependence upon the current, pressure of 

 electrodes, temperature. &c, by the existence of a resistance 

 at the contact of the crystal and the electrode. 



Now the ordinary Bidwell wire bridges (in which the 

 selenium is crystallized firmly on the electrodes) show 

 the same "crystal contact"" properties. Also the effect of 



* Amer. Assoc. 1880 ; ' Nature,' vol. xxii. p. 500. 

 + " The Similarity of Electrical Properties of Selenium to those in 

 certain Crystal Contacts," Phys. Key. 1913, p. 245. 

 X Phys.Zeit. 1912. 



