﻿588 Mr. Karl T. Compton on Contact Difference of 



the numbers given that, in general, the electropositive 

 elements show electrons with smaller velocities than the 

 electronegative elements. But Millikan and Winchester 

 used little disks of these different metals fastened near the 

 circumference of a large aluminium disk. This disk was so 

 rotated as to bring one metal after another into the path of 

 the ultra-violet light. Thus the contact difference of potential 

 acting on an electron escaping toward the receiving-plate 

 would be due to the somewhat uncertain combination of the 

 metal disk and the aluminium wheel on the one side and 

 the receiving-plate on the other. Furthermore, these metals 

 were polished with emery and then heated to 400° 0. in 

 contact with the air. The writer has found that the softer 

 metals, such as aluminium, lead, and copper, give abnormal 

 values of the contact difference of potential when polished 

 with emery because the particles of emery stick in the metal 

 and modify its surface. Also, while the treatment to which 

 these metals were subjected was useful for the particular 

 object these experimenters had in view, yet it was such as to 

 make the metals especiall} 7 subject to the time-change in the 

 contact difference of potential ; and so it seems possible that 

 metals may have been changed out of the usual order of the 

 electropositive series. Thus it seems possible that if the 

 correct values of the contact difference of potential were 

 known and corrected for in these experiments, the electrons 

 from the different metals might be found to possess equal 

 initial velocities. The results of E. Ladenburg show differ- 

 ences which may easily be accounted for by differences in the 

 contact difference of potential. The results obtained by 

 0. v. Baeyer and A. Grehrts seem to contradict the results of 

 other physicists, although there may be another explanation 

 of their values. 



With a view to testing the suggestion made in the pre- 

 ceding paragraph, mirror plates exactly similar to D, fig, 1, 

 were prepared from brass, zinc, copper, and platinum. These, 

 together with the aluminium plate, were then tested, using 

 exactly the same procedure as had been used previously in 

 section 5, except that now the plates D were interchanged, 

 while the perforated box, C, was kept the same throughout. 

 For C the nickel wire gauze box, which had become so 

 tarnished as to have a constant electropositive character, was 

 employed. The potential between C and B was 200 volts. 

 The first and last curves were taken with the aluminium 

 plate to make sure that no change had occurred in the setting 

 of the apparatus during the series of experiments. The 

 values, in volts, for the contact differences of potential 



