ON THE CONTACT THEORY OF VOLTAIC ACTION. 
29 
of which dipped into the two porcelain dishes, there were twisted round it a few clean 
cotton threads, which had been previously soaked in distilled water or in the non-acid 
liquid employed, alum, copper sulphate, &c. The threads were of such a length that 
one of their ends dipped into the dish containing the non-acid liquid, while the other 
descended nearly, but not quite, to the acid. 
Date. 
The following— 
Has to the following— 
At— 
A contact 
difference 
of 
potential, 
in volts. 
Remarks. 
21/5/78 . 
Sulphuric acid . . 
Saturated zinc sulphate 
°C. 
-1-699 
After long delay goes up slightly. 
22/5/78 . 
Distilled water 
Sulphuric acid . . . 
22 
+ 1-278 
Saturated solution 
alum. 
Saturated solution 
iy >» ... 
}> )> ... 
22 
22 
rising 
slightly 
+ 1-458 
+ 1-269 
Increased a very little in two hours. 
copper sulphate 
y. 
As a test of the accuracy of the measurements we may see how nearly for any 
three metals A, B, C, the sum 
AB+BC+CA 
equals nought, as it ought to, since it is well known that in any compound metallic 
circuit at uniform temperature there is no electro-motive force. In order that the 
test may be the more rigid we will employ only the measured contact differences of 
potential, and not the calculated ones or those marked with an asterisk in the table 
of contact differences of potential of solids with solids. 
Volts. 
Sum. 
1. 
(Copper, Lead) + (Lead, Platinum) 
= 
+ 0-229 
(Platinum, Copper) 
= 
-0-238 
— 0-009 
2. 
(Copper, Zinc) + (Zinc, Lead) 
= 
— 0-540 
(Lead, Copper) 
— 
+ 0-542 
+ 0-002 
3. 
(Iron, Brass) + (Brass, Copper) 
= 
+ 0-151 
(Copper, Iron) 
— 
— 0-146 
+ 0-0O5 
4. 
(Zinc, Platinum) + (Platinum, Copper) 
- 
+ 0-743 
(Copper, Zinc) 
= 
-0-750 
-0-007 
5. 
(Zinc, Platinum) + (Platinum, Lead) 
= 
+ 0-210 
(Lead, Zinc) 
= 
— 0-210 
o-ooo 
6. 
(Iron, Copper) + (Copper, Brass) 
= 
+ 0-059 
(Brass, Iron) 
-0-064 
-0-005 
