282 Prof. Franz Exncr on Contact Electricity. 



opposite, and measured, with a Delmann's electrometer, the 

 electric charge in both cases. If the difference of potential 

 oi' the two condenser-plates be P, and that of the poles of the 

 cell D, then the condenser-charge for one connexion will be 



p 

 P + D, for the other P — D, from which the required ratio -^ 



can be found. A direct connexion of the plates by a wire 

 gives a check experiment. 



In this way Kohlrausch finds the difference Zn|Pt='6 



- . „ ,,, ,. Zn|Pt . ,, 106 , 111-2 



Darnell, and tne ratio „ — ~- varying between T7 r 7 r and -. 



Zn|Ou J to 100 100 



We shall have an opportunity further on of returning to 

 the probable cause of this variability. 



Quite recently a number of other experiments bearing on 

 this subject have been made, as, for example, by Clifton *, who 

 likewise employed the condenser method. He finds : — 



Zn 

 Fe 



and from these Zn 



Fe = -694 Daniell, 

 Cu=-095 „ 

 Cu=-789 „ 



by application of Volta's law of tensions, the truth of which 

 will not be questioned. 



The results of Kohlrausch appear to me, however, to possess 

 greater accuracy. He finds, according to the above figures, 

 Zn | Cu about *5 Daniell. Ayr ton and Perry f have obtained 

 results generally analogous to those of Clifton. 



Furthermore must be mentioned an old experiment by Sir 

 W. Thomson J, giving an account of the absolute value of 

 Zn|Cu. In his well-known ring electrometer the two half- 

 rings were made of Zn and Cu. Over the slit swung a needle 

 charged as might be desired. When the two half-rings were 

 metallically connected the needle deflected to one side or the 

 other ; but if they were joined each to one pole of a Daniell 

 cell in the proper manner, the force of the latter overpowered 

 the contact force of the two metals, and it was possible, by a 

 proper division of the electromotive force of the Daniell, to 

 arrange so that the needle remained undeflected. Zn|Cuwas 

 consequently less than one Daniell, which agrees with all other 

 experiments. 



Thomson observed further, that when the copper half-ring 

 was oxidized in the lamp-flame, its electromotive force with 

 zinc rose considerably, and appeared to be even greater than 

 that of 1 Daniell. 



The foregoing data form, as it were, the basis of the two 

 * Proc. Roy. Soc. xxvi. 

 t Proc. Roy. Soc. xxvii. 

 X Proc of Manchester Soc. ii. 



