wly fed in. Proceeding in this manner, it is impossible
that any big bubbles, which are the principal danger, should remain
between the turns. The air is most completely excluded, probably
better than by boiling out, which, however, when gutta-percha coated
wires are used, is not practicable.

For the primaries I use ordinary line wire with a thick cotton
coating. Strands of very thin insulated wires properly interlaced
would, of course, be the best to employ for the primaries, but they
are not to be had.

In an experimental coil the size of the wires is not of great
importance. In the coil here used the primary is No. 12 and the
secondary No. 24 Brown & Sharpe gauge wire; but the sections may be
varied considerably. It would only imply different adjustments; the
results aimed at would not be materially affected.

I have dwelt at some length upon the various forms of brush discharge
because, in studying them, we not only observe phenomena which please
our eye, but also afford us food for thought, and lead us to
conclusions of practical importance. In the use of alternating
currents of very high tension, too much precaution cannot be taken to
prevent the brush discharge. In a main conveying such currents, in an
induction coil or transformer, or in a condenser, the brush discharge
is a source of great danger to the insulation. In a condenser
especially the gaseous matter must be most carefully expelled, for in
it the charged surfaces are near each other, and if the potentials are
high, just as sure as a weight will fall if let go, so the insulation
will give way if a single gaseous bubble of some size be present,
whereas, if all gaseous matter were carefully excluded, the condenser
would safely withstand a much higher difference of potential. A main
conveying alternating currents of very high tension may be injured
merely by a blow hole or small crack in the insulation, the more so as
a blowhole is apt to contain gas at low pressure; and as it appears
almost impossible to completely obviate such little imperfections, I
am led to believe that in our future distribution of electrical energy
by currents of very high tension liquid insulation will be used. The
cost is a great drawback, but if we employ an oil as an insulator the
distribution of electrical energy with something like 100,000 volts,
and even more, become, at least with higher frequencies, so easy that
they could be hardly called engineering feats. With oil in