332 
Proceedings of the Royal Society 
To check this cell, I used two Latimer Clerk cells, one made np 
two years ago, the other made up quite recently. Taking Lord 
Raleigh’s recent determination of the electromotive force of this cell 
(1*454 volts), the three cells agreed with each other within the 
errors of experiment. 
The electrometer was connected to a vocker so constructed that the 
zero of the instrument, the deflection due to the cell to he measured, 
and the deflection due to the Daniell cell, could be noted one after 
the other. This was always done, and the numbers given at the 
end are always the mean of several readings. The Daniell did not 
alter at all from week to week. 
When very small variations in electromotive force had to be 
measured, two or three cells were placed in series, and in some cases 
cells were connected to the electrometer through similarly constructed 
cells, especially to note the effects of temperature. No correction 
has been made for the new determination of the B.A. unit, as this 
affects both the electromotive force and the zinc dissolved by unit 
current. Consequently the error introduced in calculating from 
electromotive force to heat of combination is very slight.* 
With the above apparatus the following experiments were 
made :■ — 
1. Experiments to find the effect on the electromotive force of 
the cell of varying the nature of the zinc. 
2. Experiments to find the effect on the electromotive force of 
the cell of varying the temperature of the cell. 
3. Experiments to find the effect of varying the strength of the 
iodide of zinc solution. Eor these the cuprous iodide cell was used, 
as simplifying the changes taking place in the cell. 
4. Experiments on varying the strength of the free iodine pre- 
sent. 
5. Experiments on varying the strength of the iodide of zinc 
solution in the iodine cell. 
(1) In making measurements with the electrometer, the surface 
condition of the metals used is evidently of great importance, as so 
little current is drawn from the cell. If wet zinc is exposed to the 
air it becomes coated with a film of hydrate, which perceptibly 
lowers the electromotive force of the cell. 
* See Alder Wright’s recent papers in the Phil. Mag. for 1885. 
