DEVELOPMENTS IN ELEGTEOMAGNETISM BLOCH. 239 



VI. PIIOTOELECTmC EFFECT. (HERTZ AND LENARD EFFECTS.) 



Light, and especially ultra-violet light, discharges negatively elec- 

 trified bodies with the production of rays of the same nature as 

 cathode rays. Under certain circumstances it can directly ionize 

 gases. The first of these phenomena was discovered by Hertz and 

 Hallwachs in 1887. The second was announced first by Lenard in 

 1900. Perhaps on no subject is the literature of the day greater and 

 more contradictory, so we will note only a few of the recent results 

 upon which the bulk of the work has been done, 



(1) With regard to the Hertz effect, the researches from the start 

 showed a great complexity of the phenomenon of photoelectric 

 fatigue — that is, the progressi^^e diminution of the effect observed 

 upon fresh metallic surfaces. According to an important research 

 by Hallwachs,^ ozone plays an important part in the phenomenon. 

 However, other elements enter such as oxidation, the humidity, the 

 mode of polish of the surface, etc. We are not even sure that the 

 fatigue is absent in a vacuum. Eugene Bloch ^ insists that we should 

 work with an exciting radiation of definite wave-length since the 

 fatigue varies from one wave-length to another. He also showed 

 that in certain instances there is an acceleration of the effect which 

 has been refound by various Avorkers. 



A, great many experiments have been made in a vacuum. Some 

 were undertaken to study the Hertz effect at the rear surface of a 

 strip traversed by the light, an effect perhaps greater there than at 

 the front surface (Stohlmann, Kleemann, and others)., Other ex- 

 perimenters have shown a selective effect in the case of certain metals; 

 for instance, with the alkaline metals, according to Pohl and Pring- 

 scheim,^ there are maxima of exciting power at wave length 0.300 ]j. 

 for sodium, at 0.436 \i for potassium, and at 0.390 \i for a liquid alloy 

 of potassium and sodium. The general exciting power increased 

 regularly toward the smaller wave lengths. Several workers have 

 also endeavored to extend the photo-electric sensitiveness of photo- 

 electric cells into the infra-red (Elster and Geitel) or to utilize them 

 for photophony (Bloch). 



However, the greatest effort has been spent in order to find out in 

 vacuum the vari-ation of the initial velocities of the photo-electric 

 electrons with the wave length. This problem has a great theoretical 

 interest, and the simple laws stated by Lenard since 1900 for the 

 ensemble of radiation emitted should be studied separately for each 

 wave length of the exciting radiation. According to Lenard, the 

 total number of electrons emitted is proportional to the intensity of 



1 Hallwachs, Annalen dcr Physik, vol. 23, p. 459, 1907. 



2 Bloch, Radium, vol. 23, p. 125, 1910. 



3 Pohl and Fringscheim, Verh. der Deutsch. Phys. Ges., vol. 12, pp. 215, 349, 1910. 



