STUNTS WITH High-Frequency Current (Nov, 1935)
STUNTS WITH High-Frequency Current
By Kendall Ford
READERS who have followed the constructional articles on high-frequency apparatus that have appeared in past issues will be interested in learning how some of the amazing experiments are performed. The 36-in. high-frequency coil and its associated apparatus (P. S. M., May ’35, p. 82, and July, p. 82) will be used for the purpose of illustration in this article.
The 110-volt line current is stepped up to approximately 12,000 volts by means of the transformer. The high-voltage current flows from the secondary of the transformer into the condensers, which become charged. If the circuit comprising the condensers, primary of the high-frequency coil, and spark gap has been properly adjusted, the condensers will discharge across the spark gap with a series of sparks, the frequency of which is many times the original 60-cycle charging current.
The high-frequency discharge of the condensers is somewhat analogous to the swing of a pendulum. The 60-cycle current that charges the condensers may be likened to the single motion required to start the pendulum from rest, and as the swing of the pendulum will gradually diminish until it comes to a standstill, so will the strength of the discharges from the condensers across the spark gap gradually diminish until the condensers are discharged. These rapid discharges from the condensers occur between the peaks of the 60-cycle charging current, and in a properly adjusted circuit they in no way interfere with the charging current. Where a rotary spark gap is used, it has the effect of still further increasing the frequency of the spark several hundred times a second. Since high-frequency current travels over the surface of a conductor, or along the skin of a human being, it may be readily understood why a person may take a high-frequency discharge of several hundred thousand volts and suffer no ill effects.
As the 12,000-volt high-frequency current flows through the primary winding of the high-frequency coil, a current is induced in the secondary coil, the terminal voltage of which depends upon the ratio of the secondary to the primary turns. The voltage induced in the secondary coil is proportional to the number of secondary turns, and at a point along the. secondary even with the top of the primary coil the voltage is high enough* to produce a spark several inches long. To avoid sparking between the coils at that point, the primary coil is tapered away from the secondary.
The importance of properly adjusting the high-frequency circuit cannot be too strongly emphasized. The author has seen high-frequency apparatus where merely adding a portion of a turn to the high-frequency coil primary changed the discharge from a weak, stringy spark to a mass of beautiful long streamers. For greater flexibility and as a means of adjusting the coil to its maximum output, it is suggested that an additional loading coil be inserted between the condenser lead and the primary coil. Six to ten turns of bare copper or brass wire wound on a cardboard or wood form 6 to 8 in. in diameter will be quite satisfactory. Any size wire larger than No. 10 will do, and it should be spaced so the connecting lead may be clipped to any turn.
If the stationary electrodes of the rotary spark gap are separated too far from the revolving studs, a spark will occur only in unison with the 60-cycle charging current, and this may be decidedly unpleasant to take through the body. The proper separation will depend upon the speed of the rotary part of the gap. It may usually be determined by the sound of the spark. When a pure musical note is given off by the discharge across the gap, the adjustment may be considered correct. The maximum separation between the gap elements should not in any case exceed 1/16 in. When operating high-frequency apparatus for the first time, especially in broad daylight, the experimenter is apt to be somewhat disappointed with the results. Only the strongest sparks will be visible in a bright light, and the beautiful brush discharge will be completely lost. Some views of spectacular night displays are shown. Of particular interest is the 6-ft. halo produced by the revolving wire described in the May issue.
If a small branch of a shrub or tree is fastened to the discharge rod of the coil, it will wilt and collapse almost immediately after the current is turned on. If the discharge is continued, the branch will burst into flames within a few seconds. If a dry, well-seasoned board is placed against the
discharge rod, the spark will branch out and slowly creep to the top and sides of the board. At night the spectacle appears like a luminous tree slowly taking shape before the spectator’s eyes.
A stunt that never fails to mystify the layman is the lighting of a lamp with the current flowing through the lamp into the body. This may be performed by holding a lamp with attached wires and socket between the experimenter and the discharge spark, or by placing the lamp between two persons, one of whom is taking the discharge through a metal rod.
A neon-gas tube will light up weirdly if held within several feet of the spark. Outlines of figures and letters, if formed from a continuous piece of wire and suspended in the air with string, will glow with a strange blue light when the end of the wire is connected to the discharge rod.
Another interesting experiment may be performed by lighting a torch with the spark. The torch may be shaped from a piece of wood, one end of which is hollowed out to hold the burning material. A bare wire should extend from the hollowed end to the part that is held in the hand. If the hollowed end is filled with cotton waste or bits of cloth and brought near the spark, it will immediately burst into flame. The effect may be enhanced by previously saturating the waste or cloth with kerosene.
Ordinary insulators become excellent conductors for the high-frequency discharge. If a glass bottle is placed over the discharge rod, the sparks appear to meet little or no resistance in passing through the glass.
Some interesting stunts may be performed with only part of the apparatus. If the high-frequency coil is removed from the circuit and a coil of insulated wire connected in place of the high-frequency coil primary, an interesting demonstration of electromagnetic induction may be given. A second coil is made, to which is connected a socket or receptacle. The diameter of the coils may be any size that will hold its shape, and each coil may consist of from four to ten turns. If a lamp is placed in the socket and the lamp coil brought near the stationary coil, the lamp will light brilliantly when the current is turned on. If the lamp coil is moved back and forth before the stationary coil, the extent of the magnetic field may be readily observed. Since the action of the coils is similar to a transformer, the experimenter may vary the number of turns in each coil and note the various distances at which it is possible to light the lamp. In this experiment all connections remain the same as when using the apparatus for a high-frequency demonstration except that the two leads that formerly went to the primary of the high-frequency coil are connected to the ends of the stationary coil.
By attaching two bare wires to the terminal posts of the high-voltage transformer, the novel effect of a Jacob’s ladder may be produced. The wires are arranged so that they come within 1/4 in. of each other at one point, then slant upward and away from each other at an angle of about 45 deg. to a height of about 10 in. from the point of closest separation. When current is applied to the primary of the transformer, an arc will form across the points of least separation, and will then climb upward until it reaches a gap of several inches, where it is extinguished, only to be followed by a series of arcs as long as the current is on.
Although the discharge from a high-frequency coil may be taken with no ill effects, the spark should not be allowed to play on the bare skin, otherwise a painful burn may result. If the spark is taken through a metal rod held in the hand, the possibility of burns is eliminated. Caution should be exercised when working around the high-voltage transformer circuit, or when performing experiments requiring the transformer alone.