arcs, the polished surfaces are made to last much longer; and,
thirdly, the apparatus affords some gauge in the experiments. I
usually set the pieces by putting between them sheets of uniform
thickness at a certain very small distance which is known from the
experiments of Sir William Thomson to require a certain electromotive
force to be bridged by the spark.

It should, of course, be remembered that the sparking distance is much
diminished as the frequency is increased. By taking any number of
spaces the experimenter has a rough idea of the electromotive force,
and he finds it easier to repeat an experiment, as he has not the
trouble of setting the knobs again and again. With this kind of
discharger I have been able to maintain an oscillating motion without
any spark being visible with the naked eye between the knobs, and they
would not show a very appreciable rise in temperature. This form of
discharge also lends itself to many arrangements of condensers and
circuits which are often very convenient and time-saving. I have used
it preferably in a disposition similar to that indicated in Fig. 2,
when the currents forming the arc are small.

I may here mention that I have also used dischargers with single or
multiple air gaps, in which the discharge surfaces were rotated with
great speed. No particular advantage was, however, gained by this
method, except in cases where the currents from the condenser were
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