Contact Difference of Potential, 305 



fact that the contact difference decreases from other causes 

 with the lapse of time. But it is quite easy to show that, if 

 a current of negative electricity be sent from a battery of 

 cells from a copper to a zinc electrode in ionized air, the 

 copper becomes more, and the zinc less, electropositive. 



The difficulty in the case of the uranium-oxide cell was 

 that, in applying Helmholtz's theory, it was found that an 

 amount of work, presumably at the surfaces of the metals, 

 must be accounted for. In the light of the above remarks, 

 it seems clear that this work is the total work done in ren- 

 dering the copper more, and the zinc less, electropositive. In 

 the case of the cell described in the paper it was found that 

 the amount of this work per unit quantity of electricity was 

 such as to produce a back electromotive force of 0'276 volt. 

 Let us assume that the only electrons taking part in the cell 

 action are bound electrons set free at the surface of the 

 copper, which again become bound electrons at the surface 

 of the zinc. The work done per unit of electricity is, 

 therefore, '216 x 10 8 erg. But one unit of negative elec- 

 tricity contains 6*4 x 10 19 electrons. The work done per 

 electron is, therefore, 



♦276 xlO 8 lo 1A 12 

 — — -t^t =-43 x 10 ~ 12 erg. 

 b'4 x 10 ly 6 



On the above assumption, this is the difference between the 

 work necessary to take an electron out of an atom of copper 

 and the work necessary to take an electron out of an atom of 

 zinc. Various estimates have been made of the amount 

 of energy necessary to ionize a molecule of a gas, and 

 H. A. Wilson and Townsend obtained by experiment a value 

 for this energy of about 3 x 10 ~ 12 erg in the case of a 

 molecule of air. Campbell ( k Modern Electrical Theory,' 

 p. 222) states that the energy required to drag an electron 

 out of an atom probably lies between 130xl0~ 12 and 

 l'3xl0- 12 erg. 



To return to the experiment with ultra-violet light, the 

 method used will be clear from fig. 1 (p. 306). C is a cylindrical 

 copper vessel which can either be earthed or connected to a 

 potentiometer, according as the deflexion or nul method is used 

 in measuring the contact difference. Z is a zinc plate con- 

 nected to one pair of quadrants of the electrometer by means 

 of a zinc wire which passes through an amber plug P. The 

 vessel is closed by a quartz plate Q in contact with which 

 and in connexion with the cylinder is a network of copper 



Phil. Mag. S. 6. Vol. 24. No. 140. Aug. 1912. X 



