24 REPORT—1840. 
however, this remarkable fact, that the number of coils disappears from 
the formula, simply because the force of the machine is ina direct ratio, 
and the velocity is in an inverse ratio, to the square of this number. 
It is thus that the number of coils, the dimensions of the rods, and the 
other constituent parts of an electro-magnetic machine, should be con- 
sidered simply as occupying the range of the ordinary mechanisms 
which serve for the transmission or transformation of the velocity, 
without increasing the available power. So it would be possible to use, 
instead of the ordinary wheelwork, rods of greater or less length, or a 
greater or less quantity of wire, in order to establish between the force 
and the velocity, the relation which the applications to manufacturing 
processes may require. 
3rd, The mean attraction of the magnetic rods, or the pressure which 
the machine can exert, is proportional to the square of the current. 
This pressure is indicated by the galvanometer, which in this manner 
performs the function of the manometer of steam-engines. 
4th, The economic effect, i. e. the duty or the available power, 
divided by the consumption of zine, is a constant quantity, which is ex- 
pressed most simply by the relation between the electromotive force 
and the factor k, which has been previously noticed. I may here repeat, 
what I stated elsewhere, that by employing platinum instead of copper, 
the theoretical expenses may be reduced in the proportion of nearly 23 
to 14. 
5th, The consumption of zinc, which takes place while the machine 
is at rest, and does not work at all, is double that which takes place 
while it is producing the maximum of power. 
I consider that there will not be much difficulty in determining 
with sufficient precision the duty of one pound of zine, by its transfor- - 
mation into the sulphate, in the same manner that in the steam-engine, 
the duty of one bushel of coal serves as a measure to estimate the effect 
of different combinations. The future use and application of electro- 
magnetic machines appears to me quite certain, especially as the mere 
trials and vague ideas which have hitherto prevailed in the construction 
of these machines, have now at length yielded to the precise and definite 
laws which are conformable to the general laws which nature is accus- 
tomed to observe with strictness, whenever the question of effects and 
their causes arises. In viewing on the one hand a chemical effect, and 
on the other a mechanical effect, the intermediate term scarcely presents 
itself at first. In the present case, it is magneto-electricity, the admi- 
rable discovery of Faraday, which we should consider as the regulating 
power, or, as it may be styled, the logic of electro-magnetic machines. 
On the Theory of Electricity. By C. J. Kennepy. 
The author shows in the commencement of his paper, that on the 
theory of a single fluid, the electrified aérial current proceeding from 
the positive wire must be superior in force to the electrified aérial 
eurrent proceeding from the negative wire. He describes various ex- 
