206 Prof. Millikan and Mr. Winchester on Influence of 



effects due to light reflected, hack from the disks to the 

 gauze. But a more prohahle cause is the following : — 



The magnetic control on the back of the wheel W (fig. 2) 

 produced a weak magnetic field which, in our experiments, 

 pulled some of the electrons back to the wheel which would 

 otherwise have escaped to the wall of the tube. This is 

 doubtless the reason that the lead, while showing a slight 

 discharge rate (cf. Table II.), shows no positive potential 

 (cf. Table III.). Attention is called here to this point in 

 order that the impression may not be left that the values 

 given in Table III. necessarily represent either the absolute 

 or the correct relative values of the positive potentials 

 acquired in a vacuum, even under the influence of the 

 particular source of light employed in these experiments. 

 Further experiments are now in progress which, it is hoped, 

 will give the absolute values of the potentials acquired by all 

 of these metals under the influence of certain sources. This 

 particular tube and wheel was designed primarily with 

 reference to the temperature work, and the conclusions 

 already drawn from the results shown in Table III. are 

 entirely independent of whether the corpuscles, in escaping 

 from the metal, moved through a magnetic field or not. As 

 for Table II., not only is the order in which the metals 

 appear, but also the relative values of the discharge shown 

 in this table precisely the same as it would have been if 

 there had been no magnetic field ; for observations upon 

 the discharge rates were taken at a series of potentials 

 between and 300 volts, and it was found that the readings 

 were altogether independent of the potential when the latter 

 exceeded 18 or 19 volts. 



Although it is impossible to determine precisely what was 

 the effect of the weak magnetic field upon the order in which 

 the metals appear in Table III., certain conclusions can, 

 nevertheless, be drawn from a comparison of the orders in 

 which the metals appear in the two tables. Thus, although 

 the iron and gold disks were exactly symmetrically placed 

 with respect to the magnetic field of the control-magnet, yet 

 the metal showing the smaller velocity of projection, namely 

 gold (cf. Table III.), discharges 50 per cent, more electrons 

 than does the iron (cf. Table II.) . Now, if the kinetic energy 

 of emission of the corpuscles were simply an energy imparted 

 to these corpuscles by the absorption of the ultra-violet light, 

 or if it were such an energy added to an energy of agitation 

 which is a function of temperature alone (the assumption 

 made in the Drude-Thomson theory of conduction), then the 

 order in which the metals appear in the potential series 

 would of necessity be the same as the order in which thev 



