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High Frequency Currents and Apparata

Ever since 1891, on which memorable date Nikola Tesla, the wizard of high frequency electrical phenomena, delivered his famous lecture before the American Institute of Electrical Engineers covering his discoveries and experiments in this wonderful field of science, the greatest scientists of the whole world have been busily occupied in developing and perfecting apparatus of this nature for various purposes, including invaluable electro-therapeutical applications, wireless telegraphy, etc. Possibly the electro-medical profession has benefited more by the discoveries in this little known field of electrical science than any other branch of workers.

In this article will be outlined a few of the more interesting arrangements and types of apparatus which can be employed fur the production of ultra-high frequency, high potential electric currents, and which, as is generally known, can be taken through the body without feeling any appreciable pain. It is these high frequency currents, which oscillate at anywhere from 100,000 up to 1,000,000 or more times a second, which are utilized in the many electrical stage acts touring the country and which probably most readers have seen at some time or other. The apparatus described in this article produces high frequency currents of this character, and many pleasing, startling experiments can be made therewith, similar to those shown by the stage lecturers and college professors.

At Fig. 1 there are outlined at A. B and C the principal hook-ups and layouts of apparatus necessary for the production of these ultra-high frequency currents. At Fig. 1A is depicted the regular Tesla coil circuits with a step-up transformer, T, spark gap S, glass or other high tension condensers C. preferably of the adjustable type, and TC the air-core Tesla transformer. At P the primary of this Tesla coil is indicated, while at S is shown the secondary winding. The high frequency current discharges take place between the electrodes at HF. To properly tune the closed oscillating circuit SCP the condenser C should be made adjustable as aforementioned, and also the primary inductance P should be variable by means of a clip X. This permits the capacity and the inductance of this circuit to be altered until maximum results are obtained in the Tesla coil secondary.

In some cases resort is had to a compound Tesla circuit as depicted in Fig. 1B. Here the usual step-up transformer excites the circuit SCPX. The secondary S of Tesla coil TC1 then charges or excites the second closed oscillatory circuit S1 C1 P1, the final high frequency discharge taking place from the secondary S1 of the No. 2 Tesla coil.

Where a discharge of great intensity is desired the usual hook-up for a high frequency outfit is indicated at Fig. 1C. Here the same symbols refer to similar parts as just explained, and for this Oudin coil arrangement one end of the secondary S is joined to the primary coil P; the other end of the secondary coil is connected to a brass or copper ball B. This scheme is different from those shown at A and B, Fig. 1. in that the secondary is placed at the end of the primary and not inside of same, as is the case in the regular Tesla coil arrangement.

While on the subject of high frequency schemes and circuits for same, it will undoubtedly be of interest to cover one of the latest plans for this kind of work, or the Collins-Sanchez high frequency generating circuit, as outlined in Fig. 2. This constitutes the idea now utilized in a large majority of the extra compact style high frequency sets supplied for physicians' requirements, etc. To begin with, a small size and very well-insulated Oudin coil consisting of primary and secondary P and S is made use of. The secondary, as understood from diagram at Fig. 1C. connects to a metal ball or other electrode as observed, and its other free terminal is joined to the primary P. High frequency currents are caused to be generated and to oscillate around the circuit CXP. which comprises a mica or other fairly high voltage condenser C, a vibrating spark gap X and the primary coil of the Oudin transformer P. The vibrator X carries extra heavy silver contacts and the iron armature of the vibrator spring is attracted by the magnet coil as perceived. Thus this vibrator performs two functions, viz., it serves to interrupt its own (magnetic coil) circuit as supplied with current from 110- volt direct current or alternating current, and also it serves as a spark gap for the high frequency oscillating circuit CXP.

These high frequency currents thus produced are transformed by induction into the secondary circuit of the Oudin coil S. This type of apparatus gives a very powerful and steady uni-polar discharge, and the current thus generated is much in use nowadays for ultra-violet ray treatment, excitation of X-ray tubes, etc.

A great many experimenters possess a high voltage wireless transformer rated at 1/2 kw. or more and giving anywhere from 12,000 to 15,000 volts at the secondary terminals. For those possessing such a transformer, and also for those who may be interested in building a fair-size high frequency outfit and who can easily purchase a suitable transformer of the size aforementioned, the following data is suggested.

The Tesla coil here described will produce 12 to 15-inch high frequency sparks from the secondary when excited by a transformer of the type just mentioned.

Besides the transformer in question, there will also be required a suitable glass plate condenser or the equivalent made up of Leyden jars, and also a spark gap, which shall preferably be of the rotary type so as to be well cooled. A quenched spark gap proves very efficient for this class of work.

Fig. 3 shows the appearance of the Tesla coil design here proposed. The base of same may be made of some well-dried wood, and this had best be boiled in paraffine wax so as to exclude all dampness. Upon this base are placed glass supporting rods for the secondary and primary coils, and this will be found to give the very best efficiency where such high frequency, high potential currents are to be produced. The primary coil is composed of four to six turns of about No. 4 B. & S. copper, brass or aluminum wire (stranded best), and these convolutions of heavy wire are wound to a diameter of about 12 1/2 inches, spacing the turns about 1 1/2 inches apart.

The primary turns are held in shape by four wax impregnated wooden clamps, as perceived. Also a clip is provided for one lead of the primary so that the number of turns in circuit, and consequently the inductance, may be varied in tuning up the set.

The secondary coil may be composed of a cardboard cylinder 24 inches long by 5 inches in diameter. This is provided with two binding' posts at the end of same, as shown, and the winding comprises one even layer of No. 26 enameled or silk-covered magnet wire, each turn being spaced from its neighbor the thickness of the wire itself to improve the insulation. This can be done easily in a screw-cutting lathe.

This coil should be hooked up as per diagram. Fig. 1A. Regarding the glass plate condenser suitable for this 1/2-kw. size high frequency coil, and considering that the exciting transformer is rated at 12,000 volts (secondary), with a frequency of 60 cycles, then .06 microfarad condenser capacity is required. If 1/16-inch common glass is used in making this condenser about 5,333 square inches of such glass is required. This is coated on both sides with tin-foil; 1 1/2 inches margin should be allowed around every tin-foil leaf on the glass plates. If the foil can be made 24x20 inches on each plate, then 11 such plates will be necessary for this outfit.

At Fig. 4 are shown a number of details which can be followed more or less exactly in making up a suitable condenser rack to hold these glass plates. This rack may be made up of wax impregnated wooden pieces, and inside of same there are placed two hard rubber strips in either side of the frame, which strips are slotted as perceived, so that the glass plates may rest edgewise in these slots, thereby reducing current leakage to a minimum. By looking at the top view of the condenser frame it is seen that two metal strips run along the top hard rubber strips and a series of binding posts are mounted on these metal pieces. From each binding post there runs down between the glass plates a brass wire carrying a spring contact shoe, detail of which is shown in sketch at Fig. 4. It is thus perceived how each alternate foil-leaf on the glass plates receives a positive and negative charge. The terminal lead wires Tl and T2 should be hooked up to the condenser frame as in diagram, i. e., to opposite end of the metal strips, so that the high frequency inductance will be balanced, no matter how many condenser plates are in use. The number of plates in use can be varied by simply sliding them out of the rack, or also by removing one or more of the spring contact shoes.

Regarding the details of rotary spark gap suitable for this outfit, they are illustrated at Fig. 5. Any small motor operating on battery or 110-volt current is to be utilized in driving the rotary spark disc B. This disc is made up in the regular fashion as employed for wireless sets, but the plan here advocated is a very good one, especially from the air-cooling point of view.

This design calls for a 1/8-inch zinc or other metal disc about 6 inches in diameter, and 12 spark plugs are cut into the disc as detail sketch B portrays. These sparking electrodes are cut on three sides and then the lug is bent up. All of these lugs when finished should be filed off or turned off in a lathe, so as to be perfectly true.

This operation, however, had best be postponed until the disc is firmly screwed or riveted to a central hard rubber insulating hub, as drawing shows. The spark disc is held securely to the motor shaft either by means of a regular hub or by means of hexagon nuts threaded onto the shaft. In front of the revolving spark wheel are mounted two stationary electrodes, and detail sketch A shows how the ends of these are filed down so as to correspond with the thickness of the rotating disc electrodes. It is well to force a few cooling vanes (washers) tightly on the ends of the stationary electrodes as indicated. A marble base is best employed for mounting the motor and stationary electrodes with their upright standards. There are thus two spark gaps in series in this design. It is well to have a rheostat in series with the spark gap motor, so that the speed of same may be adjusted, and also the spark frequency, in tuning up the complete high frequency sets.

When all of these parts have been properly made up or assembled and the diagram followed as per Fig. 1, there should be very little trouble experienced in producing a heavy high frequency spark 1 foot long or more, depending upon the adjustments of the circuit. The spark gap, condenser and primary coil clip X should be adjusted one after the other or alternately until the maximum resonance is obtained in the circuit as manifested by the production of the largest spark in the secondary circuit.

There is appended to this article a short bibliography of the more important articles which have appeared in The Electrical Experimenter on high frequency currents, and also a number of the best books available on the subject.

For those vitally interested in this subject it will be well to purchase one or more of the books mentioned. A few experiments of general interest are cited below. An experiment not very well known, although dating from the time of Tesla's first lecture on high frequency currents, is that which demonstrates how a motor may be operated on one wire, and in some cases without any wires connected to it. Upon this and other experiments employing very powerful currents Tesla has taken out a number of patents on the wireless transmission of energy through space.

At Fig. 6 is depicted Tesla's scheme for a one-wire motor. One terminal of a small-size high frequency Tesla coil is hooked up to a coil of wire wound on an iron core and in front of which is placed a delicately mounted metal disc, which can rotate upon its axis as perceived. The other end of the magnet coil is connected to a metal plate suspended in the air and which picks up energy out oi the ether, presumably. When the Tesla coil is excited in the usual way the high frequency current passes through the magnet coil, magnetizing the iron core, and the rotating disc starts to move. Thus we have a single wire motor, and in further tests made by Tesla with very powerful apparatus it was found possible to make a device of this character operate with simply a ground connection and the other electro-magnet terminal joined to an insulated capacity or plate suspended in the air. He has also, by this and other arrangements, produced a wireless light which can be lighted at a considerable distance from the generating station. He claims in his patents that it is easily and simply possible to thus generate vast quantities of high frequency electrical energy and to transmit it for hundreds, nay, even thousands of miles, where it will be picked by an elevated capacity or aerial joined to a suitable translating mechanism, such as a transformer and motor, etc., having its second terminal connected to earth.

A fact not usually considered and which seems to possess considerable promise in this direction, as well as in many other lines of electricity's application, is that of freezing the high frequency or other circuits so as to reduce the resistance to an inappreciable value. Tesla mentioned this in one of his early patents over 20 years ago, and lately very commendable work has been done in this direction by Prof. Kammerlingh Onnes of Holland. By suitable refrigerating apparatus of special type, which can produce a cold approaching that of absolute zero, or nearly so, it has been ascertained that if induced currents are set up in such a refrigerating circuit, then it is possible for that initial flow of current to pass on around that circuit for a very considerable period of time before it dies down to zero. In some of the later experiments it was found possible for such a current to oscillate around a circuit for many hours before exact measurements with a galvanometer, properly joined to the circuit, indicated that the current had depreciated in value to any great extent. This is an important point which as yet has remained undeveloped, and it seems very possible that it could be worked out with up-to-date and perfected refrigerating apparatus so as to be applied to wireless telegraph sets, particularly those employing high power, where there is a great amount of heating and considerable losses occasioned thereby. As is well known, in such high frequency circuits the resistance plays a very important part, as it acts directly with respect to the damping of such a circuit.

(To be continued)

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