ater years, to a great extent, been replaced by registering meters that
can be read by the customer."

The principle employed in the Edison electrolytic meter is that which
exemplifies the power of electricity to decompose a chemical substance.
In other words it is a deposition bath, consisting of a glass cell in
which two plates of chemically pure zinc are dipped in a solution of
zinc sulphate. When the lights or motors in the circuit are turned on,
and a certain definite small portion of the current is diverted to flow
through the meter, from the positive plate to the negative plate, the
latter increases in weight by receiving a deposit of metallic zinc; the
positive plate meantime losing in weight by the metal thus carried
away from it. This difference in weight is a very exact measure of the
quantity of electricity, or number of ampere-hours, that have, so to
speak, passed through the cell, and hence of the whole consumption in
the circuit. The amount thus due from the customer is ascertained by
removing the cell, washing and drying the plates, and weighing them in
a chemical balance. Associated with this simple form of apparatus
were various ingenious details and refinements to secure regularity of
operation, freedom from inaccuracy, and immunity from such tampering
as would permit theft of current or damage. As the freezing of the zinc
sulphate solution in cold weather would check its operation, Edison
introduced, for example, into the meter an incandescent lamp and a
thermostat so arranged that when the temperature fell to a certain
point, or rose above another point, it was cut in or out; and in this
manner the meter could be kept from freezing. The standard Edison meter
practice was to remove the cells once a month to the meter-room of the
central-station company for examination, another set being substituted.
The meter was cheap to manufacture and install, and not at all liable to
get out of order.

In December, 1888, Mr. W. J. Jenks read an interesting paper before the
American Institute of Electrical Engineers on the six years of practical
experience had up to that time with the meter, then more generally in
use than any other. It appears from the paper that twenty-three Edison
stations were then equipped with 5187 meters, which were relied upon for
billing the monthly current consumption of 87,856 lamps and 350 motors
of 1000 horse-power total. This represented about 75 per cent. of the
entire lamp capacit