the wires and set the coil in action.
Upon turning the lights off in the room you see the wires strongly
illuminated by the streams issuing abundantly from their whole surface
in spite of the cotton covering, which may even be very thick. When
the experiment is performed under good conditions, the light from the
wires is sufficiently intense to allow distinguishing the objects in a
room. To produce the best result it is, of course, necessary to adjust
carefully the capacity of the jars, the arc between the knobs and the
length of the wires. My experience is that calculation of the length
of the wires leads, in such case, to no result whatever. The
experimenter will do best to take the wires at the start very long,
and then adjust by cutting off first long pieces, and then smaller and
smaller ones as he approaches the right length.

A convenient way is to use an oil condenser of very small capacity,
consisting of two small adjustable metal plates, in connection with
this and similar experiments. In such case I take wires rather short
and set at the beginning the condenser plates at maximum distance. If
the streams for the wires increase by approach of the plates, the
length of the wires is about right; if they diminish the wires are too
long for that frequency and potential. When a condenser is used in
connection with experiments with such a coil, it should be an oil
condenser by all means, as in using an air condenser considerable
energy might be wasted. The wires leading to the plates in the oil
should be very thin, heavily coated with some insulating compound, and
provided with a conducting covering--this preferably extending under
the surface of the oil. The conducting cover should not be too near
the terminals, or ends, of the wire, as a spark would be apt to jump
from the wire to it. The conducting coating is used to diminish the
air losses, in virtue of its action as an electrostatic screen. As to
the size of the vessel containing the oil, and the size of the plates,
the experimenter gains at once an idea from a rough trial. The size of
the plates _in oil_ is, however, calculable, as the dielectric losses
are very small.

In the preceding experiment it is of considerable interest to know
what relation the quantity of the light emitted bears to the frequency
and potential of the electric impulses. My opinion is that the heat as
well as light effects produced should be proportionate, under
otherwise equal conditio