566 ON A NEW METHOD FOR ASCERTAINING 
if not impossible to obtain exactly similar portions, 
even of the same length of wire, it will be always 
proper to examine the conducting powers of the 
wires. For this purpose they must be immersed 
in similar vessels, containing the same quantity 
(say a tb) of water, and then placed in different 
parts of the same voltaic circuit. If the voltaic 
battery employed consists of six large Daniell’s 
cells, the copper element of each of which ex- 
poses an active surface of two square feet, the 
heat evolved in the two jars will be at the rate 
of about one degree per minute. Connexion with 
the battery must be broken after the current has 
passed for five or ten minutes, when the increase 
of temperature must be ascertained to the hun- 
dredth part of a degree, by the help of accurate 
thermometers. The influence of the surrounding 
atmosphere must be obviated as far as possible by 
arranging matters, so that the temperature of the 
water shall be as much above that of the air at the 
conclusion, as it was below it at the commencement 
of an experiment. A series of ten experiments 
will be sufficient to determine the relative heating 
powers and resistances of the two wires to one 
thousandth at least; and still greater accuracy 
may be attained by taking the mean of this and 
another series, in which the wires shall have 
