264 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1910. 



is overcome. You will first see that if 1 take ixwixy the screen so as 

 to alloAv light to fall on the selenium cell, current passes into the 

 galvanometer, and the needle sloAvly deflects several degrees. Noav, I 

 quickly shut off the light by intercepting it with the screen, and the 

 needle comes slowly back^vard. Such sluggish movement w^ould be 

 impossible for the purposes of photo-telegraphy, where at least half 

 a dozen changes per second are required to be recorded abruptly 

 even in transmitting the simple portraits to which the selenium 

 process is limited. 



NoAV, using two cells of different characteristics and a Wheatstone 

 bridge arrangement, I will once more allow light to fall suddenly 

 on the two cells simultaneousl}^, and you will see that the galvanom- 

 eter needle records the change in resistance of the combination (]uite 

 quickly; the combination is even more noticeable when the light is 

 suddenly shut off again, the needle returning to zero with great rapid- 

 ity. This compensated arrangement of selenium cells at once renders 

 their use of practical value for various physical and optical measure- 

 ments. Prof. Korn has found that for an increase in the illumina- 



tion ^I, the current obtained is given by the equation y = o^.ol.e 

 where y is the current, a the sensitiveness of the cell, /3 and m its 

 inertia constants, and e the basis of Naperian logarithms. For two 

 cells to be combined to the gi-eatest advantage we must have them 

 such that if their equations are respectively 



_i 



and 



_i 



-p,,« t 



then 



d{yx-y^ _r. 

 dt ^ 

 This makes the condition for good compensation that 



m is usually almost constant, and with suitable Giltay cells is about f . 



In practical language, the condition for compensation is that the 

 principal cell should have great sensitiveness and a small inei-fia con- 

 stant, the compensation cell low sensitiveness and a high inertia con- 

 stant, the product of sensitiveness and inertia constant being the same 

 in the case of both cells. 



The physical properties of selenium are of such importance that 

 I feel I may be allow^ed to digress for a few moments to show one 



