Tesla Coil - A Glass Tube Secondary

The subjoined figure will, I hope, do away with the necessity for a very verbose description:— Procure a glass tube, 2in diameter and 6in. in length. This can be bough for 5d. or 6d. — the short glass chimneys used Argand burners exactly answer the purpose. Fit a bung to each end, insert about the half of a 3in. wire nail, and mount between a couple of small brackets. It will then be comparatively easy to wind upon it the length of No. 36 wire required.

The wire I use is double silk-covered. The reel of wire is tested for continuity, then soaked in melted paraffin wax, and mounted on a temporary spindle ready for winding from. Coil about 12in. of the wire into a spiral, and fasten it securely with a bit of waxed thread in. from one end of tube and start winding, fairly tight, close and even turns, until within 1/4in, of other end of tube; tie here same side as started from and leave 12in. free wire. Now wind primary much in the same way. The rod is a length of hardwood ruler, lin. diameter and lin longer than the glass tube. The extra lengths shown (as at B) are merely to support the primary exactly in centre of tube to keep it fixed there.

With these extensions, therefore, the primary rod will be the exact length of box. Put on primary 50 turns of No. 22 wire, evenly separated; tie the ends tightly, then go over the whole surface of wire turna three or four times with shellac varnish. It is hardly necessary to say that the G.P. supports for tube (as C, Fig. 1) are made to adhere to the glas simply by warming the edges.

The containing-box is of 1/4in. walnut (fretwork wood), and measures Inside 4 hy 4 by 8, put together with glue and screws, thoroughly well varnished inside and out, and the joins inside covered with pitch. Boiled oil provided sufficient to reach within 1/2in. of top. All the free ends of wires must be passed through rubber tubes: the two from secondary taken out on one side of box, and the primary on the other. Below (Fig. 2) is ground plan of connections.

I mount these on one end of a board, 10 by 12, or so; thus leaving room for Tesla tank to stand at the other end. The terminals a and care mounted on raised blocks of wood; the stand itself has been practically soaked in shellac varnish, and is mounted on four glass sup- ports. The condenser is a quart Leyden jar. The utmost care must be taken with the "second- ary" ends to secure the best insulation. In default of "hard rubber tubing of great thickness," I take these rubber-covered ends as soon as they come out of the oil and are "on the straight," so to speak, through glass tubes of various thicknesses fitting one within the other, building them up in two upright cases, one at each end of the tank, using plenty of pitch and rubber wherever possible, then terminal binding screws at top of each.

A lid should be fitted to tank, and after pouring in the oil some hours should elapse before attempting to use the coil. I think above will be clear enough; if not, I shall be glad to assist anyone, in spite of a feeling that I am quite a tiro myself. I repeat, I get with abore coil no longer spark than with the ordinary tail used by itself, but increased vibrations as I anderstand it, different effects altogether-e.g., attach to the terminals two L-shaped pieces of brass wire, so as to get two parallel lengths of wire 3in. to 4in. apart. From this little instrument, which from its cost, so cheap is it, that it might almost be reckoned a toy, to quote Dr. Tesla's article, "the discharge appears in the form of powerful brushes and luminous streams issuing from all points of the two straight wires," &c.

J. P. N.