Electrical Engineers at the beginning of the past year, the interest
excited in it has been considerable. It affords us a means for
producing any potentials by the aid of inexpensive coils operated from
ordinary systems of distribution, and--what is perhaps more
appreciated--it enables us to convert currents of any frequency into
currents of any other lower or higher frequency. But its chief value
will perhaps be found in the help which it will afford us in the
investigations of the phenomena of phosphorescence, which a disruptive
discharge coil is capable of exciting in innumerable cases where
ordinary coils, even the largest, would utterly fail.
Considering its probable uses for many practical purposes, and its
possible introduction into laboratories for scientific research, a few
additional remarks as to the construction of such a coil will perhaps
not be found superfluous.
It is, of course, absolutely necessary to employ in such a coil wires
provided with the best insulation.
Good coils may be produced by employing wires covered with several
layers of cotton, boiling the coil a long time in pure wax, and
cooling under moderate pressure. The advantage of such a coil is that
it can be easily handled, but it cannot probably give as satisfactory
results as a coil immersed in pure oil. Besides, it seems that the
presence of a large body of wax affects the coil disadvantageously,
whereas this does not seem to be the case with oil. Perhaps it is
because the dielectric losses in the liquid are smaller.
I have tried at first silk and cotton covered wires with oil
immersion, but I have been gradually led to use gutta-percha covered
wires, which proved most satisfactory. Gutta-percha insulation adds,
of course, to the capacity of the coil, and this, especially if the
coil be large, is a great disadvantage when extreme frequencies are
desired; but on the other hand, gutta-percha will withstand much more
than an equal thickness of oil, and this advantage should be secured
at any price. Once the coil has been immersed, it should never be
taken out of the oil for more than a few hours, else the gutta-percha
will crack up and the coil will not be worth half as much as before.
Gutta-percha is probably slowly attacked by the oil, but after an
immersion of eight to nine months I have found no ill effects.
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