December 11, 1896.] 



SCIENCE. 



859 



of one to the metal disk of the next, a bat- 

 ter}^, with increased E. M. F., was obtained. 

 When plates were used without gauze in 

 front, all metals were found to become 

 charged to a gradually and continuously in- 

 creasing positive potential. 



27. E>ighi was at first of the opinion that 

 those metals which are found least active 

 when judged by the rate of dissipation of a 

 negative charge when illuminated were 

 found most active if judged by the positive 

 charge acquired from a neutral condition.* 

 For example, aluminum and zinc showed 

 the most rapid loss of negative electricity 

 under the action of ultra-violet rays,but gold • 

 and copper (originally neutral) became more 

 strongly charged positively than either. f 

 Still later observations by Righi appear, 

 however, to contradict the results stated 

 above. I A heavily lacquered plate was 

 grounded and punctured with a large num- 

 ber of small holes, through which ultra- 

 violet rays fell upon a parallel polished 

 plate. The final steady potential of the lat- 

 ter (which was insulated) was measured 

 for difierent distances between the plates. 

 The surface density developed by illumina- 

 tion could then be computed. • It was 

 d = 0.000116 C. G. S., for carbon and d = 

 0.000161 for Al.§ Other metals showed a 

 surface density lying between these limits. 

 It appears, therefore, that the charge in- 

 creases until a certain definite density is 

 reached. This density depends upon the 

 temperature, changing in the case of zinc, 

 from .000146 at 24° to .000218 at 10°. In 

 still another article Eighi says|| '^ it is to be 

 observed that the order" in which the 

 metals stand with regard to taking a posi- 

 tive charge " is almost the same as that 



*C. E. 107, p. 559, 1888. Beibl. 13, 40. 



tl.c. 



t R. Ace. dei Lincei 5, p. 331, 1889. Beibl. 13, 566. 



^ Later observations led Righi to correct these values 

 to .000116 and .000241 respectively. See Atti. del. 

 E. Inst. Yen. 7, 1889. Beibl. 13, 976. 



II Ibid. 



which indicates the rapidity with which 

 they lose a negative charge under the influ- 

 ence of illumination." 



28. Attempts were made to find some 

 efiect with positively charged bodies, but 

 without success. 



29. Not only metals, but also sulphur and 

 black rubber, were found to become posi- 

 tively charged when illuminated. 



PHOTO-ELECTRIC CURRENTS. WORK OF STOLE- 

 TOW. 



30. It was shown in 1888 by Stoletow* 

 that under suitable conditions it was pos- 

 sible to obtain a current by the action of 

 ultra-violet rays. A metal plate 22 cm. in 

 diameter and a parallel sheet of wire gauze 

 were illuminated by an arc lamp. The -f 

 pole of a battery was connected to the 

 gauze and the - pole to the metal plate, a 

 sensitive galvanometer being in circuit. 

 Under these circumstances a current was 

 obtained so long as the illumination con- 

 tinued. On reversing the battery connec- 

 tions the galvanometer showed only a 

 small deflection. 



31. The efiect was increased by carefully 

 cleaning the metal plate. The current was 

 found to be proportional to the illuminated 

 surface. On increasing the distance be- 

 tween plate and wire gauze the current di- 

 minished, the law being approximately ex- 

 pressed by the equation i = E -;- (a -f bl) 

 where 1 represents the distance between 

 plate and gauze. The current was propor- 

 tional to E up to 2 volts. Beyond that 

 E. M. F. the current increased less rapidly 

 than the E. M. F. 



32. Stoletow found in later experiments,f 

 however, that the current remained con- 



stant so long as -^ was constant. J ''The 



Beibl. 12, 605. 



*C. E. 106, p. 1149, 

 Phil. Mag. 26, p. 317. (Abst.) 



t C. E. 108, p. 1241. Beibl. 13, 902. 



X This law does not hold exactly at low air pres- 

 sures. See Stoletow, Jour, de Phys. 9, p. 471, 1890. 



