8.3 27.2 226 37.4
11.4 37.4 182 67.4
14.5 47.6 254 53.4
17.9 58.7 157 86.2
20.7 67.9 166 89.6
20.7 67.9 225 74.6
24.1 79.0 238 76.7
24.1 79.0 389 64.0
27.6 90.5 207 83.9
It will be observed that these experiments relate to low pressures; it
would be desirable to extend them to higher pressures.
IV. _Transmission by Electricity._--However high the efficiency of an
electric motor may be, in relation to the chemical work of the electric
battery which feeds it, force generated by an electric battery is too
expensive, on account of the nature of the materials consumed, for a
machine of this kind ever to be employed for industrial purposes. If,
however, the electric current, instead of being developed by chemical
work in a battery, is produced by ordinary mechanical power in a
magneto-electric or dynamo-electric machine, the case is different; and
the double transformation, first of the mechanical force into an electric
current, and then of that current into mechanical force, furnishes a means
for effecting the conveyance of the power to a distance.
It is this last method of transmission which remains to be discussed. The
author, however, feels himself obliged to restrict himself in this matter
to a mere summary; and, indeed, it is English physicists and engineers who
have taken the technology of electricity out of the region of empiricism
and have placed it on a scientific and rational basis. Moreover, they are
also taking the lead in the progress which is being accomplished in this
branch of knowledge, and are best qualified to determine its true bearings.
When an electric current, with an intensity, i, is produced, either by
chemical or mechanical work, in a circuit having a total resistance, R, a
quantity of heat is developed in the circuit, and this heat is the exact
equivalent of the force expended, so long as the current is not made use of
for doing any external work. The expression for this quantity of heat, per
unit of time, is Ai²R; A being the thermal equivalent of the unit of power
corresponding to the units of current and resistance, in which i and R are
respectively expressed. The product, i²R, is a certain quantity of power,
which the author proposes to call _power transformed into electricity_.
When mechanical power is employed for producing a current by means of
a magneto-electric or dynamo-electric machine--or, to use a better
expression, by means of a _mechanical generator of electricity_--it is
necessary in reality to expend a greater quantity of power than i²R in
order to make up for losses which result either from ordinary friction
or from certain electro magnetic reactions which occur. The ratio of the
quantity, i²R, to the power, W, actually expended per unit of time is
called the efficiency of the generator. Designating it by K, we obtain, W
= i²R/K. It is very important to ascertain the value of this efficiency,
considering that it necessarily enters as a factor into the evaluation of
all the effects to be produced by help of the generator in question. The
following table gives the results of certain experiments made early in
1879, with a Gramme machine, by an able physicist, M Hagenbach, Professor
at the University at Basle, and kindly furnished by him to the author: