292 Prof. J. A. Fleming : Note on the Photoelectric 



eft'oct o£ the plane of polarization of the incident light. If 

 we polarize the incident light by a NicoPs prism, and con- 

 nect the metals of the photoelectric cell with a galvanometer 

 either with or without a battery of 1 or 2 secondary cells 

 inserted in the circuit, then we tind at once that the galva- 

 nometer deflexion is much greater when the plane of penali- 

 zation is at right angles to the plane of incidence, than when 

 these planes coincide. 



On the electromagnetic theory of light, this may be stated 

 by saying that the effect is a maximum when the electric 

 vector of the plane-polarized light is normal to the surface 

 of the photoelectric metal, and a minimum when it is parallel 

 to it. The theory that firs in best with the above facts, is 

 that the light facilitates or causes an escape of negative 

 •corpuscles or electrons from the surface of the photoelectric 

 metal, and to do this it must of course impart to them energy 

 sufficient to give them a velocity enough to carry them 

 beyond the range of attraction of the positive charge which 

 remains behind on the metal. 



Since the electropositive metals are those which most 

 easily lose electrons from their atoms, this is in accordance 

 with the observed fact that the most electropositive metals 

 are the most highly photoelectric substances. Again, if 

 there is an emission under the action of light of electrons 

 from the surface, we have an explanation of another fact 

 easily proved with these tubes, viz. that the formation of a 

 magnetic field parallel to the surface of the alloy greatly 

 reduces the photoelectric current. We can show this easily 

 by the great redaction which occurs in the galvanometer 

 deflexion when even an ordinary horseshoe magnet is placed 

 with its poles across the tube. Again, we can explain on 

 this hypothesis the unilateral conductivity of the vacuous 

 space over the illuminated alloy surface. If we connect the 

 negative pole of a secondary cell to the external terminal of 

 the alloy and the positive to the platinum plate, and insert a 

 galvanometer as usual in the circuit, we greatly increase the 

 galvanometer deflexion, which occurs when no cell is inserted. 

 On the other hand, if the cell is reversed, then, provided the 

 platinum is quite free from splashes of alloy, we cannot 

 reverse the deflexion of the galvanometer. 



It is obvious that if a magnetic field is created parallel to 

 the surface of the alloy, a force will be exerted on the nega- 

 tive ion as it moves normally away from the surface, deflecting 

 it from its path, and this will reduce the number of ions 

 which in any time reach the platinum plate, and therefore 

 will diminish the current. Again, the negative electrification 



