The apparatus used by Kazda and Dunn for the determination of the critical photo electric potential of mercury has been completely reconstructed. The stop cocks, the grease of which was the source of contaminating vapors, have been replaced by mercury cut offs and the high temperature cement used in the construction of the photo cell has been replaced by a graduated quartz-pyrex seal. Fresh clean mercury was returned to the still.
With this new apparatus and under these new working conditions the long wave length limit for running mercury has been found to be 2735Å. The conditions within the apparatus have been found to be practically free from contaminating vapors so that the critical frequency has been found for stationary mercury, namely 2375±10Å, which is in entire agreement with Kazda for flowing mercury.
This impurity in the old apparatus caused the photo current to increase four fold its original value in thirteen to twenty minutes after the still was turned off. It then slowly receded, falling below its initial value in three or four days. This increase in the photo current was accompanied by a raise in the threshold to 2850[angstroms], hence falling in time to a constant value 2680Å. In the apparatus as it now stands this same four fold increase is not reached until some eighty hours after the still is turned off and remains perfectly constant indefinitely thereafter. The long wave length limit for this maximum sensitivity was found to be 2910Å.
The effect of hydrogen, helium and argon in contact with the surface and dissolved in the body of the mercury was found to have no influence whatever upon the photo electric behavior of the mercury or upon the rate of rise of sensitivity upon turning off the still except that each had a marked cleansing effect in reducing the concentration of the impurity which slowly contaminates the surface.
Nitrogen in contact with the surface was found to have no effect except that the maximum threshold reached when nitrogen was present was 28OOÅ. Water vapor at very low pressure inhibits the action of the sensitizing impurity and gives a threshold of 2800Å after being in contact with the surface for seventy hours. Oxygen has a decided reducing effect upon the threshold value, bringing it down to 2555Å in eighteen hours. Another fifty-two hours exposure to oxygen does not materially change this value. Air, at atmospheric pressure, in contact with the surface gave this same threshold value, namely, 2555Å.
