404 Mr. R. Sabine on some Electrical Experiments 



Junction 

 number. 



2 . 



3 . 



4 . 



5 . 



6 . 



Resistance of 

 junction. 

 •0333 meghom) 

 ■0553 „ { 

 •0328 „ | 

 •0199 „ 

 •0285 „ [ 



Resistance of selenium 

 between junctions. 



0-1914 megohm. 

 0-1045 „ 

 0-1233 „ 

 0-1084 



It is clear from these measurements that a large portion of 

 the observed resistance of a so-called selenium resistance may, 

 and frequently does, reside in the junctions and not in the 

 selenium. Therefore the larger we make the surface of con- 

 tact between the platinum and the selenium, the less likely 

 are we to find an otherwise sensitive piece of selenium ren- 

 dered comparatively insensitive by the introduction of high 

 junction resistance. In this respect the form of selenium plate 

 designed by Dr. Werner Siemens, in which the platinum wires 

 form gratings or interlying spirals, is unquestionably the best 

 form to employ when the object in view is to obtain a high 

 sensitiveness to light. 



Steady variations of Temperature. — The following experi- 

 ment had for its object to ascertain whether the alteration of 

 resistance accompanying a steady* variation of temperature 

 is confined to the selenium or to the junctions, or is partici- 

 pated by both. The following data were obtained with a plate 

 furnished with four platinum wires (Nos. 1 to 4), annealed at 

 200° C, measured in a Wheatstone-bridge with + current of 

 2 Daniell cells, after it had been at rest in the dark for several 

 hours at each temperature. 





Measured resistance between wires. 



Temp. 

















1 and 2. 



1 and 3. 



2 and 3. 



2 and 4. 



3 and 4. 



o 



meg. 



meg. 



meg. 



meg. 



meg. 



75 C. 



1641 



•2691 



1446 



•2429 



1521 



15-5 



•2168 



•3565 



•1905 



•3201 



•2003 



210 



•2343 



•3865 



•2068 



•3473 



•2225 



32-2 



•2739 



•4549 



•2429 



•4092 



■2577 



The several resistances of junctions and selenium are there- 

 fore as follows : — 



* Unsteady and sudden variations of temperature gave very anomalous 

 results. One plate of selenium, the resistance of which increased with a 

 steady increase of temperature, was found to decrease in resistance for a 

 few seconds by a sudden application of heat. 



