290 Prof. J. A. Fleming : Kote on the Photoelectric 



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would the negative electricity move towards it across tlie 

 interspace between the plates. As a matter of fact, the best 

 photoelectric metals are the most electropositive metals. 

 Accordingly, if we imagine a sheet of potassium opposed to a 

 sheet o£ zinc, then to make the photoelectric current agree in 

 direction with the volta-electric current, we should have to 

 illuminate the zinc plate with ultraviolet light, bat keep the 

 potassium plate in the dark. In the present case, the photo- 

 electric current is in the opposite direction to the volta- 

 electric current, assuming the ionized gas replaced by an 

 aqueous electrolyte. 



It seems therefore that in the case considered there are 

 two separate sources of electromotive force, viz. : the volta 

 contact-difference of potential of the metals in the cell and 

 a photoelectromotive force due to the illumination ; and to 

 these may be added an external electromotive force due 

 to any battery inserted in the circuit. In two tubes, made 

 as above, I found that the electromotive force produced by 

 the incident light was equal to 0'45 volt in one, and in the 

 other tube to 0*6 volt. This E.M.F. was measured by the 

 counter E.M.F. (produced by a shunted cell) which had to be 

 introduced into the photoelectric cell circuit to reduce the 

 galvanometer deflexion to zero. These differences are no 

 doubt due to small differences in composition of the alloy in 

 the two cases, to differences in the intensity of the light used 

 at the time of making the measurements, or to differences in 

 the pressure of the residual gas in the tube. The current 

 produced by the tube of E.M.F. equal to 0*6 volt through a 

 galvanometer of resistance 180 ohms, was found to be 5*4 

 microamperes. This indicates that the equivalent resistance 

 of the tube is 74,000 ohms on the assumption that the effective 

 E.M.F. is the same when the circuit is open and closed. The 

 current increases very rapidly at first with the intensity of 

 the incident light, but the author has not yet been able to 

 find time to make measurements of the relation between the 

 illumination per square cm. of the surface and the E.M.F. 

 created in the circuit. 



By the use of two such cells placed in series, I have found 

 that the separate photoelectromotive forces are additive, and 

 that if such photoelectric cells are joined in series like voltaic 

 cells and separately illuminated, the individual E.M.F/s are 

 added together in an external circuit connecting the first and 

 last plate. In the case of the two tubes mentioned above, 

 giving separately 0'45 and 0*6 volt E.M.F., the E.M.F. 

 when in series was found to be 1*0 volt. 



At one time I contemplated constructing a photoelectric 



