large and the keeping cool of the surfaces was necessary, and in cases
when, the discharge not being oscillating of itself, the arc as soon
as established was broken by the air current, thus starting the
vibration at intervals in rapid succession. I have also used
mechanical interrupters in many ways. To avoid the difficulties with
frictional contacts, the preferred plan adopted was to establish the
arc and rotate through it at great speed a rim of mica provided with
many holes and fastened to a steel plate. It is understood, of course,
that the employment of a magnet, air current, or other interrupter,
produces no effect worth noticing, unless the self-induction, capacity
and resistance are so related that there are oscillations set up upon
each interruption.
I will now endeavor to show you some of the most noteworthy of these
discharge phenomena.
I have stretched across the room two ordinary cotton covered wires,
each about 7 metres in length. They are supported on insulating cords
at a distance of about 30 centimetres. I attach now to each of the
terminals of the coil one of 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
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