December 18, 1896.] 



SCIENCE. 



897 



method by wliicli the negative charge is dis- 

 sipated, they are inclined to adopt Righi's 

 original view that the air particles act as 

 carriers for the electricity. Against the ex- 

 planation of Lenard and Wolf (particles of 

 the body itself carrying charge) they urge 

 the objection that charged dust particles 

 will not be acted on by a magnetic field. 

 (Why not, if in motion ?) Elster and Geitel 

 do not believe in the action of the condensed 

 layer of gas (Hoor) , but think that by a sort 

 of resonance the ultra-violet rays make it 

 easier for the gas particles in the neighbor- 

 hood of the surface to receive a charge. 



LATER INVESTIGATIONS OF ELSTER AND GEI- 

 TEL. ACTINO-ELECTRIC EFFECT OP 

 POLARIZED LIGHT. 



100. Further actino-electric investiga- 

 tions by Elster and Geitel appear in Wiede- 

 mann's Annalen in 1892.* The authors 

 point out that the behavior of an illumi- 

 nated cathode is similar in many respects 

 to that of an incandescent cathode, f If 

 the analogy holds true, the resistance of a 

 Geissler tube should be diminished by illu- 

 mination of the cathode, as it is by heating 

 the latter. The observations of Wiede- 

 mann and Ebert, however, contradicted this 

 conclusion for low pressures. ;|: Elster and 

 Geitel therefore repeat these observations, 

 using potassium as a cathode in a tube filled 

 with hydrogen . Illumination of the potas- 

 sium surface was found very noticeably to 

 diminish the resistance (as shown by using 

 an adjustable spark gap in air, in parallel 

 with the tube). The effect was most marked 

 at low pressures (0.1 mm. to 0.01 mm). 



101. This same tube was then used to 

 study the action of a magnetic field upon 



*Wied. Ann. 46, p. 281. 



t The discharge from incandescent metal surfaces 

 hasheen investigated by Elster and Geitel and others 

 at considerable length. Most of the work on this 

 subject will be found in Wiedemann's Annalen. — 

 E. M. 



X Wied. Ann. 35, p. 217. 



the actino-electric current. It was found 

 that when the lines of force of the field 

 were perpendicular to the direction of the 

 convection current, the action in restrain- 

 ing the discharge produced by light was 

 greatest. If the lines of force were parallel 

 to the current, the action was either nil or 

 else slightly reversed. In these investiga- 

 tions Elster and Geitel found it possible to 

 measure the actino-electric current with a 

 galvanometer, instead of using an electrom- 

 eter. 



102. In 1894 observations were made by 

 Elster and Geitel * upon the actino-electric 

 action of polarized light. Wankaf had 

 already tried to detect a difference in the 

 discharging action of light depending upon 

 the direction of the plane of polarization, 

 but without success. The difficulty seems 

 to have been partly in the character of the 

 surface and partly in the fact that com- 

 pletely polarized ultra-violet rays are not 

 readily obtained. Elster and Geitel avoided 

 the last difficulty by using cells of Na and 

 K, which were sensitive to visible rays. A 

 smooth and approximately plane surface 

 was obtained by using the metals in liquid 

 form. The light used was polarized by 

 means of a nicol. 



103. At oblique incidences the actino- 

 electric effect was greatest when the vibra- 

 tions of the incident light took place in a 

 plane perpendicular to the sensitive surface, 

 (Fresnel's Hypothesis), and least when the 

 vibrations were parallel to the surface. 

 The difference between the maximum and 

 minimum was a function of the angle of 

 incidence, and was found to be greatest for 

 an incidence of about 60°. 



104. In a later article]: Elster and Geitel 

 describe still further experiments on the 

 action of polarized light. Keeping the 



* Berliner Akad. 6, p. 133. Wied. Ann. 52, p. 

 440. Abstracted in Phil. Mag. 38, p. 158. 



t Mitth. d. Math. Gesellschaft in Prag., p. 63, 1892. 

 X Wied. Ann. 55, p. 684, 1895. 



