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L. The Influence of the Contact Difference of Potential 

 between the Plates emitting and receiving Electrons liberated 

 by Ultra-violet Light on the Measurement of the Velocities 

 of these Electrons. By Karl T. Compton, M.S., Porter 

 Ogden Jacobus Fellow, Princeton University *. 

 [Plate IX.] 



1. JNTRODUCTION—Ib is well known that when two 

 metallic plates are connected to the terminals of any 

 instrument measuring difference of potential, the actual field 

 between the plates is measured by the reading of the instru- 

 ment plus or minus a certain contact difference of potential 

 which depends on the materials of the plates and the con- 

 ditions of their surfaces. Thus one would expect that an 

 electron, moving between the plates, would be acted on by 

 a force due to the contact difference of potential as well as 

 by the force due to the difference of potential indicated 

 by the voltmeter or electrometer. The contact difference of 

 potential between metals may be as large as one volt. The 

 measurements of the velocities of the electrons liberated by 

 ultra-violet light have shown that they possess sufficient 

 initial kinetic energy to carry them against fields of from 

 zero to six volts, approximately. Thus the contact difference 

 of potential between the plates may be a very large per- 

 centage of the total potential difference, and yet its influence 

 seems never to have been taken into account. It is the 

 purpose of this paper to present evidence proving that the 

 contact difference of potential must be taken into account 

 when making velocity measurements in the photo-electric 

 effect, and then to show how this fact modifies some of the 

 results which have been obtained in making measurements 

 of the electron velocities. 



2. Apparatus (fig. 1). — Light from an iron arc passes 

 through a diaphragm d and a quartz window A ; then 

 through suitably lined holes in the mica shield F, the brass 

 cylindrical box B, the perforated metallic box C, and 

 strikes the polished aluminium plate D. It is thence re- 

 fleeted out again through holes in 0, B, and F and through 

 the glass window E. The parts B, 0, and D are insulated 

 from each other, and the electrical connexions are shown in 



fi g- 2 : 



B is 5 cm. in diameter and 1 cm. high. The hole through 

 which the light enters B is 1 mm. in diameter, and the hole 

 in the mica screen above it is just enough smaller to prevent 



* Communicated by Prof. 0. W. Richardson, 

 2Q2 



