Measure of Resistance to Galvanic Currents. 37 
It appears from this Table that the specific conducting power 
of annealed silver wire is ]0 per cent. greater than that of the 
same Wire unannealed; that of annealed copper wire on the 
average 6 per cent. greater. The difference of conducting power 
is especially great in the case of brass. As the density of wire 
depends on the amount it has been stretched since the last heat- 
ing, that, as well as the conducting power, must vary much even 
when the metal is perfectly uniform. The temperature to which 
the wire was raised, the duration of the operation, and the 
rapidity of the cooling, all affect its specific conducting power. 
Column 5 of the above Table was calculated by means of the 
formula 
Peal 
or Fg tt a+ Te 
ti eas ane 
Bat Sr I 
at/a+ ee : 
The eee the correction for the conicality of the 
conductor, may, in the case of metal wires, be almost always 
neglected, since it never differs sensibly from unity. This method 
is obviously much more exact than that hitherto employed, in 
which the mean diameter of the wire had to be determined by 
direct measurement; and the square of the magnitude so obtained, 
which was never exact, entered into the calculation. By my 
method all the data may be determined with great accuracy, 
especially the length of the wire, which alone enters in the 
square. 
_ If the above Table be compared with that of Arndtsen, it 
appears that the mean conducting power of unannealed platinum 
wire, viz. 8-257, and the smallest value found for unannealed 
silver wire, viz. 56°252, are exactly m the ratio given by Arndatsen ; 
while the resistance of copper wire according to Arndtsen, agrees 
pretty well with that of annealed copper in the above Table. As 
the silver and platinum wires employed both by myself and 
Arndtsen were chemically pure, I have in the following Table 
taken the resistance of platinum and hard silver as a standard. 
The values taken from Arndtsen’s Table are indicated by an A, 
those observed by myself by an 8S. 
