ME. LATIMER CLARK ON A STANDARD VOLTAIC BATTERY. 
5 
Several experiments were made to determine the variation of the electromotive force 
at different temperatures ; from the mean of these it appears that the force decreases 
with increase of temperature in the ratio of about '06 per cent, for each degree Cent. ; 
for example, an element gave relative values of *9993 at 0° Cent, and of *9412 at 100° 
Cent. The element varies much more at temperatures near 0° Cent, than at temperatures 
near 100° Cent. The variation for about 10 degrees above or below 15'5 is '06 per 
degree Cent. When the temperature is lowered from 15°*5 to 0° the force increases at 
the rate of -08 per cent, per degree ; when raised from about 15°*5 to 100° Cent, it 
diminishes at the rate of '055 per cent, per degree. 
The element maintains a sensibly constant electromotive force for one or two years, 
and possibly longer if the salts be prevented from drying by an air-tight covering. 
It is not intended that this element should supersede any of the existing combinations 
in practical use for the production of a current ; for it, like the Marie Davy and many 
other batteries, falls rapidly in electromotive force when allowed to work through a 
circuit of small resistance, though it recovers its original electromotive force if allowed 
to remain inactive for a short time. It is intended to be used chiefly as a standard of 
electromotive force with which other elements or sources of potential can be compared 
by means of an electrometer or of instruments (similar to the one described below) 
which do not require any current. 
It will, however, continue to supply a permanent current through a circuit of large 
resistance, say 10,000 ohms, without any sensible diminution of its force, and has been 
advantageously applied to the testing of submarine cables. 
The instrument used in comparing the elements was one devised by the author in 
1859 (see fig. 6, p. 14); the following diagram will explain its construction: — 
Fig. 1. 
WORKING BATTERY. RHEOSTAT. 
& 
a a represents a length of ten metres of platinum-iridium wire about *5 millimetre 
