11^ Mr Barlow on the Laws qf Electro-Magnetic Action. 
the approximation is altogether accidental. Indeed, there can 
be no doubt, that the exhibited effect is governed by some law 
very nearly the same as that proposed ; and it will remain for 
those who insist upon electric phenomena being due to the ac- 
tual transmission of material or imponderable fluids, to prove that 
their hypothesis is consistent with this law of action, or with one 
but slightly different from it. 
Experiments to ascertain the Conducting Power of different 
Wires, as depending upon their Thiclmesses or Diameters. 
I here procured a number of copper and brass wires, each 
two feet long, and weighing from 40 grains to near 4000 grains. 
An inch at each end was bent at right angles, which inch, after 
being well cleaned, and rubbed with nitrate of silver, was im- 
mersed in the cups filled with mercury, at N and P, as explain- 
ed in the preceding article, and the deflection produced on the 
compass was carefully registered ; but in this case, as in the last, 
it was necessary to guard against the variable power of the bat- 
tery. This was done by employing one fixed wire to ascertain 
the relative power before and after each experiment. The weight 
of this standard wire was 470 grains for the two feet in length. 
Being thus provided, the experiments commenced by immersing 
this standard in the cups above mentioned. The result being 
registered, the smallest specimen was immersed, and the stand- 
ard removed ; then the specimen was removed, and the stand- 
ard again immersed, — the mean of the first and third experi- 
ment being assumed as the mean standard power of the battery 
due to the period when the specinien was in the cups ; and, in 
this way, we proceeded from the smallest to the largest copper- 
wire, and then from the smallest to the largest brass-wire. The 
following are the results, in which the first and the second ob- 
servations on the standard wire are placed in adjacent columns, 
for the convenience of having their mean value also in an adja- 
cent column to the results of the different specimens ; but it 
will be understood, that the specimen was always submitted to 
trial between the observations on the standard, which latter was 
brass, and weighed 470 grains to the two feet in length. 
