Intelligence and Miscellaneous Articles. 127 



seems well suited for accurate measurements of the wave-length 

 and of the decrement of the vibrations. — Wiedemann's Annalen, 

 xl. p. 399 (1890). 



LECTURE-EXPERIMENT TO DEMONSTRATE THE EXCITATION OF 

 ELECTRICITY BY LIGHT. BY WILHELM HALLWACHS. 



In the first experiments on the excitation of electricity by light, 

 only very feeble positive charges were obtained of the magnitude of 

 a volt. MM. Bichat and Blondlot then found a method of in- 

 creasing the charges by directing a current of air against the irra- 

 diated plate by which they obtained increases of potential up to 

 30 volts. This method of increasing the excitation afforded at the 

 same time an insight into the connexion between the excitation of 

 electricity by light and the discharge of electricity by light, which 

 latter was increased by the electrostatic power of the negative 

 charges imparted to the plates, just as by a current of air. This 

 connexion was made clear by the experiments of Prof. Bighi, who 

 demonstrated the proportionality between the potential produced 

 by radiation and the electrostatic forces which occur at the surface. 

 In repeating the experiments of Bichat and Blondlot, I have ob- 

 tained charges up to 100 volts by blowing against the bottom of a 

 brass vessel suspended in a sheath. 



In order to show in a lecture the excitation of electricity by 

 light we take advantage of the current of air; for then, if the Held 

 of observation is sufficient and the lamp does not burn too long, we 

 may dispense with the troublesome screens against induction. By 

 means of the following arrangement I have been able to show the 

 phenomenon to a large circle of hearers. In the luminous cone 

 of the electric light, after removing any glass lenses in the lan- 

 tern, a freshly polished zinc plate fixed to a shellac support is 

 introduced laterally. The rays of light which pass by on the side 

 strike a lens at some distance, which concentrates them on the gold 

 leaf of a Hankel's electrometer. After removing the eyepiece, the 

 object-glass of the microscope of the electrometer projects an image 

 of the gold leaves on a screen, on which a scale is affixed to observe 

 the displacement of the gold leaves. When the electrometer was 

 charged with 20 chromic-acid elements and when the plates of 

 the electrometer were tolerably near, the desired sensitiveness was 

 obtained. In order to measure the latter a rigid wire was placed 

 on the electrometer-battery, a few elements removed from the 

 point put to earth, and connected by the conductor with the gold 

 leaf. 



In order to make the experiment, the gold leaf and the zinc plate 

 are connected with each other, a disk of mica is introduced in the 

 path of the rays coming from the lamp, the zinc plate is put to 

 earth for a moment, and after removing the mica disk no percept- 

 ible change is observed in the adjustment of the gold leaves if the 

 sensitiveness of the electrometer is not exceptionally great. If 



