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 insulation
and alternate current motors transmissions of power can be effected
with safety and upon an industrial basis at distances of as much as a
thousand miles.
A peculiar property of oils, and liquid insulation in general, when
subjected to rapidly changing electric stresses, is to disperse any
gaseous bubbles which may be present, and diffuse them through its
mass, generally long before any injurious break can occur. This
feature may be easily observed with an ordinary induction coil by
taking the primary out, plugging up the end of the tube upon which the
secondary is wound, and filling it with some fairly transparent
insulator, such as paraffine oil. A primary of a diameter something
like six millimetres smaller than the inside of the tube may be
inserted in the oil. When the coil is set to work one may see, looking
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