858 



SCIENCE. 



[N. S. Vol. IV. No. 102. 



lluminated the interior surface of a nega- 

 tively charged hollow cylinder without ob- 

 serving any effect, confirms this view. 



21. Since the effect is produced at the 

 surface, it should depend upon the charac- 

 ter of the surface. A well polished surface 

 was, in fact, found to be from 40 to 50 times 

 as sensitive as one that was rough or oxy- 

 dized. With iron the effect was less marked 

 than with zinc. With aluminum it was 

 more marked. 



DEVELOPMENT OF A POSITIVE CHAKGE BY 



ILLUMINATION. WOKK OF HALLWACHS 



AND RIGHI. 



22. At the close of the article just referred 

 to, Hallwachs mentions experiments indica- 

 ting that the discharge of negative electricity 

 may occur, under the influence of ultra- 

 violet rays, even from a neutral body. . In a 

 later article* these experiments are de- 

 scribed somewhat more at length. A pol- 

 ished metal disk was suspended within a 

 metallic box and connected with an electro- 

 meter. When illuminated by rays from an 

 arc lamp 45 cm. distant it was found to ac- 

 quire a positive charge. The maximum 

 potential reached was about \ volt for alu- 

 minium, 1 volt for brass, and a little more 

 than 1 volt for zinc. The metal screen sur- 

 rounding the disk was of rusty iron ; this 

 being chosen so that the contact potential 

 difference between screen and disk should 

 have a tendency to charge the former nega- 

 tively. Otherwise the results obtained 

 might be misleading. In order to prevent 

 direct electrostatic induction from the lamp, 

 the gypsum window through which the rays 

 came was covered with wire gauze. All 

 action ceased upon the interposition of a 

 sheet of mica. 



23. Hallwachs' article, which was quite 

 brief, was followed by a note in the Comptes 

 E,endus, by Bichat,t whose results are, to 



* Wied. Ann. 34, 731, 1888. Phil. Mag. 25, p. 78 

 (Abstract). 



t C. E. 107, p. 557, 1888. Beibl. 13, p. 39. 



say the least, striking. Bichat's experi- 

 ments were made with growing plants 

 placed upon insulating supports and illumi- 

 nated. With the exception of the geranium, 

 all the plants tested were found to acquire 

 a negative charge, with a maximum poten- 

 tial of 7-8 volts. This phase of the subject 

 does not appear to have been carried further 

 either by Bichat or by others. 



24. A more elaborate investigation of 

 the effect of ultra-violet rays was under- 

 taken by Righi* in the same year, the first 

 results being published almost at the same 

 time as the work of Hallwachs. Righi's 

 arrangement of apparatus was as follows : 

 A sheet of wire gauze, B, was set up parallel 

 with a metal disk. A, and a short distance in 

 front of the latter. A was connected to one 

 pair of quadrants of an electrometer, B to 

 the other and also to earth. The needle was 

 charged to about 100 volts. Upon ground- 

 ing A for an instant, insulating and illumi- 

 nating with rays from an arc lamp, the 

 electrometer showed a deflection. The 

 final deflection was the same even when A 

 had an initial charge. When the steady 

 deflection was once reached a change in the 

 relative position of A and B was without 

 influence upon the deflection. Righi con- 

 cluded from this that the metals were 

 brought to the same potential by the action 

 of the light. If this is true the steady de- 

 flection obtained as above described should 

 be a measure of the contact E. M. F. be- 

 tween the metals. 



25. Righi found that the electrometer de- 

 flection increased with increase in surface 

 exposed, and with diminished distance from 

 the arc lamp. An arc between carbon and 

 zinc was found especially effective. Sun- 

 light was without influence. 



26. Upon combining several pairs (disk 

 and gauze) by connecting the wire gauze 



* Eendiconti d. Accademia dei Lincei 6, p. 185, 

 1888. Beibl. 12, 286. Eeprinted in the Journal de 

 Physique 7, p. 153, 1888. 



