To exclude more perfectly the air, an excellent way to proceed, and easily practicable with small coils, is the following:  Construct a box of hard wood of very thick boards which have been for a long time boiled in oil.  The boards should be so joined as to safely withstand the external air pressure.  The coil being placed and fastened in position within the box, the latter is closed with a strong lid, and covered with closely fitting metal sheets, the joints of which are soldered very carefully.  On the top two small holes are drilled, passing through the metal sheet and the wood, and in these holes two small glass tubes are inserted and the joints made air-tight.  One of the tubes is connected to a vacuum pump, and the other with a vessel containing a sufficient quantity of boiled-out oil.  The latter tube has a very small hole at the bottom, and is provided with a stopcock.  When a fairly good vacuum has been obtained, the stopcock is opened and the oil slowly 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