354 ELEMENTARY LESSONS ON [CHAP. ix. | 



389. Photo-voltaic Properties of Selenium. 

 In 1875 Willoughby Smith discovered that the metal 

 selenium possesses the abnormal property of changing 

 its electric resistance under the influence of light. 

 Ordinary fused or vitreous selenium is a very bad 

 conductor ; its resistance being nearly forty -thousand- 

 million (3-8 x io 10 ) times as great as that of copper. 

 When carefully annealed (by keeping for some hours at 

 a temperature of about 22OC, just below its fusing 

 point, and subsequent slow cooling), it assumes a crystal- 

 line condition, in which its electric resistance is consider- 

 ably reduced. In the latter condition, especially, it is 

 sensitive to light. Prof. W. G. Adams found that green- 

 ish-yellow rays were the most effective. He also showed 

 that the change of electric resistance varies directly as 

 the square root of the illumination, and that the resist- 

 ance is less with a high electromotive-force than a 

 low one. Lately, Prof. Graham Bell and Mr. Sumner 

 Tainter have devised forms of " selenium cells," in which 

 the selenium is formed into narrow strips between the 

 edges of broad conducting plates of brass, thus securing 

 both a reduction of the transverse resistance and a large 

 amount of surface-exposure to light. Thus a cell, whose 

 resistance in the dark was 300 ohms, when exposed to 

 sunlight had a resistance of but 150 ohms. This pro- 

 perty of selenium the latter experimenters have applied 

 in the construction of the Photophone, an instrument 

 which transmits sounds to a distance by means of a 

 beam of light reflected to a distant spot from a thin 

 mirror thrown into vibrations by the voice ; the beam 

 falling, consequently, with varying intensity upon a re- 

 ceiver of selenium connected in circuit with a small 

 battery and a Bell telephone (Art. 435) in which the 

 sounds are reproduced by the variations of the current. 



Similar properties are possessed, to a smaller degree, 

 by tellurium. 



About the middle of the present century Becquerel 

 showed that when two plates of silver, coated with 



