Physiology. — ''On the Photo-electricüi/ 0/ Ge/s." By Prof. H. 

 Zwaardemaker ami F. Hogewind. 



(Communicated in the meeting of February 22. 1919). 



When light is allowed lo fail on a metal disc, possessing a negative 

 charge, the disc will be discharged. Not, howevei-, when the disc 

 is charged positively. 



This phenomenon was tiist discovered and described by W. 

 Hai.lwachs^) in 1888 and is known as "Hallwachs effect". 



Afterwards the discharge appeared to be dne to an emission of 

 electrons under the influence of light, notably ultraviolet light. For 

 this reason a luuiiuou.s source rich in ultraviolet rays, is to be preferred 

 for this experiment, e.g. an arc-lamp carbon, whose light falls on 

 tlie metal disc either directly or with the aid of quarlz-lenses. The 

 lamp was com[)letely insulated l)y an amber i-od and connected with 

 an earthed electroscope, which had been charged negatively. 



With a \ iew to ward off the iidiibitory influence of the electrons 

 taken up in the air in front of the disc, a wire-work of oxidized 

 iron-gauze, which is only slightly sensitive, is placed before the metal 

 disc at a distance of about 1^ cm. This wirework is charged positively 

 up to a potential of from 50 to 100 volts, and immediately catches 

 up the electrons emitted. 



The light is transmitted through the network to the disc. The 

 electroscope at once begins to discharge itself. The rate of this 

 discharge, measured l>y the so-called iialf-way time, furnishes an 

 index for the photo-electrical sensitiveness of the disc. 



According to Hali,wachs the following materials are responsive 

 to photo-electricity : 



1. Metals. Most of all alkalimetals. Then follow Al, Mg, Zn, etc. 



'2. Many metallic compounds: oxides, chlorides, bromides, etc. 



3. Many minerals. 



4. Many dyes, e.g. anilin-dyes; also their aqueous solutions. 



5. Some insulators, such as sulphur, solid rubber. 



Gases become active through the extreme ultraviolet ra3'8. 

 Among the materials that proved to be inactive are water, stone, 

 granite, wood. 



') Mabx, Handb. der Radiologip. Hallwachs, Lichtelektricitat 1915. Bd. lllb. 

 S. 245. 



