Tesla Coils Resurrected

(2nd installment)

The importance of properly “tuning” an oscillation circuit is strikingly shown in the operation of the Tesla or the Oudin resonator, which is practically the same with the exception that one terminal of secondary is grounded to the end of the primary, thus making it a form of auto-transformer. Unless resonance is obtained, the spark length and the thickness is greatly reduced. The difference of but two turns on the primary of the Oudin resonator, from which this description is taken, will cause a difference of nearly 3 in. in the spark length. For this reason in particular, the air-insulated coil, in which the primary is wound on a drum or cage the same as a wireless telegraph helix, permits a greater flexibility in use; that is, it may be used with various coils and condensers of different capacity, with high efficiency as the circuit may readily be tuned by moving the primary clips from one turn to another until the proper point is found.

For the reason that many readers may already own a coil or transformer which would operate a Tesla, the construction of the high-frequency coil itself will first be taken up. Later articles will describe both open and closed core transformers suitable for operating the Teslas of various sizes, and another article will be devoted to a description of the experiments, both practical and interesting, which may be performed with the apparatus. Photographic illustrations of the effects produced will be shown and the subject of tuning will receive due consideration.

At the start, it may be well to state that an induction coil giving a long thin spark is quite useless for practical work with this apparatus. The sole purpose of the induction coil or transformer in this case is to charge a condenser of large capacity and it is the oscillatory discharge from the condenser across the gap in series with the Tesla primary which produces the high-frequency-sparks. As most readers will understand, the long, thin spark possesses but little current or amperage, although the voltage is enormous. When such a coil is connected to a big condenser there is not sufficient current to charge it, and the discharge is very weak. A wireless telegraph transformer is splendid for the purpose as are also the induction coils which give a very thick and hot spark. A very high voltage is not essential and the writer has obtained some of his best results with a closed core transformer which delivers a secondary voltage of but 5,000. From this figure to 20,000 is a good margin and no advantage accrues from the use of a higher voltage unless the transformer is of large capacity so that the current output may be correspondingly great.

The first requisite in building this apparatus is to have the materials perfectly dry. The cardboard tubing and woodwork should be thoroughly dried in the oven and then varnished while warm. This will seal up the pores and prevent moisture from entering. Complete sets of parts for making these coils may be purchased from advertisers and the materials in these sets are thoroughly dried and treated.

The Oudin resonator will first be described, as its construction is somewhat simpler than that of the Tesla coil. The Oudin is more useful in public demonstrations than the Tesla, as it has a single terminal at the top from which sparks may be drawn, etc. Its appearance in the dark is still more striking than that of the Tesla, and the brush discharge is in the form of long, flame-like streamers.

Fig. 1 gives a side elevation of the completed instrument. Fig. 2 shows a cross-section through the center. The secondary is wound on a heavy cardboard tube, 18 in. long and 5 in. in outside diameter. This tube is fitted with turned wooden heads, the upper one of which is made slightly larger, so as to provide a neat finish to the top of the cylinder. The construction is entirely with wooden pegs and glue. No nails should be used. A brass ball is mounted on the top of the head of cylinder and its shaft is passed through into the cylinder. The starting end of the secondary winding is soldered to this shaft or rod. If the ball has a ring around it as shown in the drawings, the effect will be still more weird.

The cylinder may be mounted in a lathe or an improvised winding machine and the wire laid on. The winding consists of 800 to 900 turns of No. 30 s.c.c. magnet wire. The turns should be spaced about the width of a piece of the wire apart. There are several methods of doing this, and the easiest and best is to use the screw-cutting lathe with the gears set to cut a No. 56 thread. If no lathe is available, the winding may be put on with a piece of heavy cotton thread between turns, the thread being taken off when winding is completed. No. 30 s.c.c. wire winds 64 turns to the inch with close winding, therefore the winding of 56 to the inch will put on rather more than 900 turns if a space of 1/2 in. is left at top and bottom of cylinder. The spacing is not absolutely necessary if the coil is not to be worked to its greatest capacity, but there will be sparks between turns unless this precaution is taken if a large transformer is used. After completing the winding, the wire should be carefully covered with thin shellac varnish, several coats being applied and each coat permitted to dry before the next is put on. The final drying should be in a warm oven to harden the varnish.