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Electro-Staic Experiments

By Frederick Von Lichtenow

The attractive and repulsive forces peculiar to static electricity enable the person experimenting in this field to do things which would be impossible with any other kind of electricity. However insignificant these may appear to the uninitiated, for the true experimenter they carry a deep meaning.

Experimenting in static electricity is playing with the electricity of the earth in microform. This fact alone throws a vast amount of fascination into this odd and yet so natural a branch of electrical science.

Static electricity shows itself In probably more ways and certainly requires less apparatus for its production and experimental conduction than any other form of experimental electricity. A rod of glass and a piece of silk or a sheet of hard rubber and a piece of fur, together with some bits of tissue paper, are sufficient apparatus for the practical study of its elementary principles.

A small static machine, however, such as the Wimshurst influence machine, is required for t lie successful reproduction of the following experiments, which will aid the novice in grasping the principles underlying them.

The opposing or repelling chains give a simple and yet quite impressive way of showing the repulsive effect of like-charged bodies. Two very light and equally long brass chains, such as are usually found around static Instruments for connection purposes, are suspended horizontally side by side by their respective ends as shown in Fig. 1. They must hang well off the table and under just enough tension to form only a slight down-ward curve. The electrodes of the machine are then set beyond sparking distance, when with a few turns of the crank handle the chains will he caused to press sideways, each strongly repelling the other, in which position they will remain for some time after the machine has been stopped, gradually and slowly falling back into normal position with the leaking away of the static charge.

If the discharge hails are brought within sparking distance, so that sparks may pass at certain intervals, the chains will set up a rhythmic motion - separating upon being discharged, meeting again upon neutralization - as long as tile plates are rotated.

Fig. 2 shows the apparatus required for the static ball pendulum, which clearly illustrates tile principle of alternate attraction and neutralization, helped along by the weight of the ball in got gathering momentum, which in the end effects the pendulum motion. This latter will continue as long as the machine is working. Both balls consist of solid brass and should be kept in a well polished condition. The smaller, swinging ball, one-half inch in diameter, is suspended by a piece of very thin copper wire, 3 1/2 or 4 inches long, having a loop on its upper end to insure the necessary free movement. The larger one, measuring one inch in diameter, is stationary, while the whole is sup- ported upon insulating stands. No sparks must pass across the static machine gap.

Working under the same principle as that involved in experiment No. 2, the static vibrator, as I will call it, forms another highly interesting piece of apparatus. The smaller ball is here replaced by a short piece of very fine, perfectly straight copper wire, about 2 1/2 inches long, held rigid In a clip as indicated in Fig. 3. The free end of this horizontally placed wire must reach nearly across the entire width of the brass ball, without however touching it, and must center upon it. Both should be separated by a gap of from one-quarter to one-half inch, this depending entirely on the size and condition of the static machine employed. With the discharge rods set far apart and machine put to work, the wire will immediately be attracted to the ball, since both are oppositely charged, as quickly released under the neutralizing spark, attracted again under the new charge and so on, which, assisted by the spring element existing in it, will cause it to vibrate at an incredible speed.

The "cushioning" effect of the shark can be shown in our experiment, which at once affords a spectacular way of lighting Geissler tubes causing them to swing at the same time, and gives an opportunity for the study of the "cushioning" effect of the static spark.

Two medium sized Geissler tubes of equal length (of the rarefied gas and not the heavy liquid type) are suspended a couple of inches or so apart from insulating stands connected to the respective poles of tile Static machine (Fig. 4).

With the passage of the electric charge they will at once approach each other, being attracted as a consequence of their opposite polarity, when upon meeting by their lower bulbs the spark discharge will take place through their entire lengths, strongly lighting them up for the moment. Being released under the effect of the neutralizing spark, they fall back into their former positions, only to be attracted to one mother again with the approach of the new charge (Fig. 5). This in repetition causes a sort of swinging motion on the part of the tubes, which in the end - one would think at least - must lead to their striking hard together; but they never do. Each time they meet, the resulting spark acts as if it were a cushion placed between them; in fact they sometimes seem to cling together for the instant, while the discharge is taking place, which on the other hand forces them always to a fresh start, in this way limiting the momentum gained by the tubes on each trip. They will perform in this manner as long as the machine is in action, the terminals of which tire to be separated beyond their spark limit.

In working out these static "stunts" I had in mind not only the beginner, but the less capitalized experimenter who, unable to buy the more expensive auxiliary apparatus, may not be satisfied with the average run of experiments belonging in the tissue paper - tinfoil - pithball class.

The next experiments are described by Percival G. Bull. The "bronze" or "metal" paper needed in the experiment seems, as I faintly remember, to be an uncertain article on the local market. There is something entirely wrong with it. Either the demand for it is so brisk that stocks are early exhausted, or there is no call for it at all, and, consequently, nobody bothers with it. I was for a time inclined to think the latter was the case, until finally, after a prolonged and fruitless search among the various stationery stores, I was shown at some small place what looked to be the remnant at of a once glorious pile. Whether I purchased the real, honest-to-goodness "metal" paper or not has been an open question with me to this day, since it was not sold to me under that somewhat mysterious sounding name. At any rate, It works.

The following illustrations and short descriptions give the results of my tests with three kinds of the metal-coated paper (Fig. 6).

One characteristic of this paper is that the sparks always show a strong tendency toward branching out over its surface, whether the distance between the jars be a few inches or a foot, or even more. Their color is a beautiful bluish-white. With the jars separated a few Inches, up to six inches or so, the discharge manifests itself in thousands of bright little stars hanging together by shiny threads.

These very striking effects are due to the relatively high conducting quality of the metal particles covering this paper.

The paper illustrated in Fig. 7, offering a somewhat higher resistance to the condenser discharge titan the former, limits the distance between the jars to nine inches. At or near that distance the sparks are very pronounced and appear concentrated in the form of miniature lightning bolts of a clear white color. They hit around in curves and are accompanied by a loud report. If the jars are approached to within four inches or less, as indicated, the sparks will dart in spray fashion across the intervening space, lighting up in a vivid emerald green. This paper is the worst conductor of the three, and, consequently, permits only a separation of a few inches between the Leyden jars. Set at that distance, the spark effect is very similar to the one last noticed on the "copper-bronze" paper, however, it is not quite so distinct. The color shade of the sparks runs more into a dull yellowish green, not unlike that of oxidized brass.

The above spark-and-color effects are those as observed in an artificially (moderately) lighted room. The papers may be placed in triple or quadruple layers. thus insuring a better insulation for the Leyden jars, in addition to which an oil-cloth covering on the table may be advisable. Care must be taken that the discharge balls of the machine are first to be separated beyond sparking distance while charging the jars and not set "a few inches apart" as prescribed by the text book, which may be misunderstood, since the small Wimshurst machine I used in these tests delivers a three-inch spark when in a healthy condition, not to speak of the many larger static machines with their correspondingly much greater spark lengths. After thus charging the jars for a short while the electrodes are gradually and slowly brought toward each other, when upon reaching the stress limit the resulting spark wil1 be accompanied by the condenser discharge across the paper. Following the above tests I was led to another experiment, terminating in the following discovery - if I may call it such - which I will give here for what it is worth:

In order to ascertain the conducting value of these metal papers as a circuit link, I had introduced a separate gap (spark gap) into the former set-up. With the conductors of the machine placed wide apart I was testing the spark across this new gap under various adjustments, when, happening to glance around while turning the crank, I noticed my gold leaf electroscope standing some distance away near the further end of the table, under the influence of a strong discharge. I discharged it and tried again with the same result, then looked at the gap, where only a silent discharge was taking place, caused by being set at the spark limit.

Without disturbing anything, I studied their respective positions and found the knob of the electroscope to be at exactly the same elevation as the busy end of the gap, with the latter squarely facing the former. Therein rested the secret, evidently. The oscillatory waves set up by the spark were in this way forced upon and recorded by the very sensitive instrument, which latter fact proves that a strong, unipolar factor predominated in the charge (Fig. 10).

The discharge rods of the spark gap, being in a vertical position, are curved in order to be capable of a wide range of adjustments. The rods consist of heavy, polished brass wire and carry at their terminals one-inch solid brass balls well polished, as all the terminals on static instruments should be. All in all, the spark gap is a "concoction" of my own, brought about by the dire need for just such a gap (Fig. 9). No sparks will occur between the Leyden jars in this connection.

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