230 
Proceedings of the Royal Society 
2. On an Iodine Battery. By Mr A. P. Laurie. 
Communicated by Professor Tait. 
This is a one fluid battery. The two plates are zinc and carbon. 
The solution is a solution of iodide of potassium in water, in which 
iodine is dissolved. The usual strength I use is 1 oz. of iodide 
of potassium in 2 oz. of water. In this dissolve J oz. of iodine, 
but the strength of the iodine and of the iodide of potassium can 
be considerably varied without affecting the electromotive force. 
The iodine is meant to prevent polarization by uniting with the 
nascent hydrogen to form hydriodic acid, which will then unite 
with the caustic potash, to form iodide of potassium and water. 
In this way the iodide of potassium acts as a carrier of iodine, from 
the iodine in solution, to the zinc plate. 
It is quite possible, however, that the iodine acts directly on the 
zinc plate. 
I made the following experiments : — 
On connecting a single cell with a quadrant electrometer, I got 
a deflection of 60 divisions. The zinc and carbon were then con- 
nected by a short wire for ten minutes, and again rapidly connected 
with the electrometer. 
The deflection was now 58. It was joined up for another ten 
minutes, the deflection was then 57. 
The first reading, namely 60, was unusually high, as in no future 
case did I get more than 58 or 59 to start with. 
The following experiment is more trustworthy 
Initial deflection 58. Current allowed to run for thirty minutes, 
deflection 57. I tried a whole series of metals, but found none so 
good as zinc. 
The zinc cannot be amalgamated, as the solution acts on mercury. 
In all these experiments I used commercial zinc. If it is left in 
the solution it is slowly dissolved, and must therefore be removed 
from the solution, when the battery is not working. 
I made up a Daniell cell with a saturated solution of sulphate of 
copper, and 1 of commercial sulphuric acid in 12 of water. The 
Daniell gave a deflection of 54, while my cell gave a deflection of 
56. Its electromotive force is therefore about one Yolt. 
