with Crystalline Selenium. 



405 



Temp. 



Resistance of 



T , . « T i.- o i Selenium be- 

 Junction 2. Junction 3. ! tween2and3 . 



1 



o 



7-5 0. 

 15 5 

 210 



32-2 



megohm. 

 •0198 

 •0254 

 0273 

 0309 



megohm. 

 •0269 

 •0354 

 •0410 

 0457 



megohm. 

 0979 

 •1298 

 •1385 

 1663 



It appears from this, that the resistances of junctions and 

 selenium are both affected by variations of temperature in 

 nearly an equal ratio. 



Resistance of Selenium altered by the inversion of the Current. 

 — The fact that the current strength in the circuit of a bar of 

 selenium and a battery is subject to change when the direction 

 of the current is reversed has been pointed out by Professor 

 Adams and Mr. Day. I made an attempt to determine 

 whether this change is due to electromotive force or to re- 

 sistance, by carefully measuring the change on inversion 

 while the current in the selenium was kept constant ; but the 

 total resistance of the circuit and the battery were increased 

 in equal ratios. By this means the change, whatever it is, 

 remains constant while all the other factors are different ; and 

 when the change is sufficient, there should be no difficulty 

 in calculating it both as resistance and as electromotive force 

 and discriminating between the two suppositions. Unfortu- 

 nately, however, the majority of specimens of crystalline sele- 

 nium did not alter sufficiently to afford definite evidence ; and 

 those recently prepared specimens which showed a considerable 

 change, generally gave unsteady readings. 



Seat of the Change. — The inquiry naturally occurs whether 

 the seat of the change is in the selenium or at the junctions. 

 To determine this, it is only necessary to ascertain the resist- 

 ance of the junctions and of the selenium separately with two 

 different battery powers to find which agrees best in the two 

 measurements. 



A plate of selenium was carefully 

 measuredbetween its four wires (marked 

 a, b, c, and d) inserted in a Wheatstone 

 bridge with an intervening commutator, 

 so that the selenium could be inverted 

 whilst the other members of the bridge 

 remained unaltered. The side contain- 

 ing the selenium (s),fig. 2, was also fur- 

 nished with a reflecting galvanometer 



Rs. 2. 



