860 



SCIENCE. 



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



current is therefore a function of the density 



of the negative charge, or of the electric 



force acting upon the negative plate." A 



E 

 curve plotted with -z- and i as abscissas 



and ordinates respectively resembles a 

 curve of magnetization. If the plate and 

 gauze are of different metals the contact 

 E. M. F. between them must be subtracted 

 from E in computing the current. 



33. If the metal of which the gauze is 

 made is positive with reference to the plate 

 (e. g., gauze of zinc, plate of copper silver- 

 plated), a current may be obtained without 

 any battery.* The current in this case 

 corresponded to the contact E. M. F. be- 

 tween the two metals {i. e., about 1 volt). 

 This was tested by placing a Daniell cell in 

 the circuit and observing the increase in 

 current. The tendency of illumination ap- 

 pears to be to equalize the potentials of the 

 two metals. By connecting the two to a 

 condenser and discharging the latter after 

 equilibrium has been reached, the contact 

 E. M. F. of the metals can be measured.f 



34. Al, Zn, and Pb placed in the arc in- 

 crease the effect. Stoletow calls atten- 

 tion to the fact that these metals are quite 

 strongly electro-positive, and have strong 

 ultra-violet spectra. 



35. The experiment of Stoletow was modi- 

 fied by Borgmann % in such a way as to 

 make the illumination intermittent. If 

 the development of a current followed the 

 illumination instantly it was thought that 

 the intermittent character of the current 

 could be detected by a telephone. Although 

 an E. M. F. of about 120 volts was used, no 



* Stoletow, C. E. 106, p. 1151. Beibl. 12, 605. 



t Eighi has followed up this aspect of the subject 

 at some length. (Accad. del Lincei, 5, 860. Beibl., 

 14, 69.) It was found that the contact P. D. de- 

 pended in|many cases upon the gas, and in some cases 

 the direction of the E. M. F. could be reversed by 

 changing from air to burning gas. 



X C. E. 108, p, 733. Beibl. 13, 565. See also 

 Phil. Mag. 26, p. 272. 



sound could be heard. Borgmann concludes 

 that the action does not occur at once, but 

 requires a finite time. 



36. Stoletow, however, opposed this view* 

 and was of the opinion that the telephone 

 was not sufficiently sensitive to detect the 

 sound. He rotated a disk of cardboard, 

 containing 16 openings, in the path of the 

 rays, while a commutator on the shaft 

 (containing 16 segments) cut out the gal- 

 vanometer and threw in an equivalent re- 

 sistance 16 times each revolution. When 

 the brushes were set for a maximum effect 

 the galvanometer was found to be unaf- 

 fected by changes in speed. Stoletow esti- 

 mates, therefore, that the actinic current 

 must be set up within ^^-^-^ sec. after the 

 beginning of the illumination. In this 

 same paper Stoletow describes experiments 

 with a sectored disk, which was rotated in 

 in the path of the ultra-violet rays in such 

 a manner as to diminish their intensity 

 one-half. Under these circumstances the 

 actino-electric current was also reduced one- 

 half for all speeds of rotation. f 



photo-electric effects probably due to 



convection: work of righi, lenard 



and wolf, hoor, etc. 



Early in his investigations Righi came 



to the conclusion that the effects observed 



were due to a sort of electric convection, 



which for some reason was accelerated by 



ultra-violet rays. This conclusion was 



justified by the following experiments •.% 



37. A rod carrying a mirror was mounted 

 in a horizontal position inside a glass box by 

 a delicate bifilar suspension. Thin sheets of 

 Al were fastened to each end, and one of 

 them was charged negatively by being con- 

 nected with a dry battery. On illumina- 



*C, E. 108, p. 1241. Beibl. 13, 902. For resume 

 of Stoletow's work see also Phil. Mag 30, p. 437. 



tl. c, p. 1242. 



JAccad. dei Lincei 4, p. 6, 1888. Beibl. 12, 721. 

 See also C. E. 107, p. 559, for brief statement of these 

 results. 



