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Electro-Static Experiments

By Frederick Von Lichtenow

Part II(Conclusion)

Several years ago I happened upon the instructive and interesting little work entitled, "Simple Experiments in Static Electricity," by Percival G. Bull. Experiment No. 9, Chapter IV, of this book, dealing with spectacular condenser discharges, imprest me very much; in fact, to the extent that I couldn't help but give it the due tryout. The "bronze" or "metal" paper referred to and 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 way,' 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 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.

I mention this little incident only as an example of the handicaps, which at times confront the experimenter even in big cities, and which are only too frequently responsible for the fact that many otherwise highly instructive and fascinating experiments are left untried. Needless to say, I have had several real disappointments of this nature since, not only "close calls" like the above.

My aim in this article is to put down the results obtained with, and various spark effects noted on, three different grades of "bronze" or "metal" paper - the only number I was able to secure - which are not given in the aforementioned book. Following in the order of their relative resistance capacity they are "silver" paper, the "copper- bronze" paper and the "brass-bronze" paper, the latter being the poorest conductor of the three. They measure each, as cut by me, 19 1/2 inches in length (the original width of the paper) by 8 1/4 inches wide, an ample size for the accommodation of even the largest Leyden jars ordinarily used.

I employed in these experiments two very finely made static Leyden jars of one pint size each in connection with the always dependable little "Electro" Wimshurst static machine. According to the book in reference the jars are to be placed one at each end of the paper and connected with their inner coatings to the respective poles of the machine. I have placed them in various positions, these latter depending on the spark effect desired, as well as necessitated by the nature of the paper itself. The following illustrations and short descriptions give the results of my tests ( Fig. 1).

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 by only a few inches, and 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. 2, offering a somewhat higher resistance to the condenser discharge than the former, limits the distance between the jars to 3/4 of a foot. 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 4 inches or less, as indicated in "b," the sparks will dart in spray fashion across the intervening space, lighting up in a vivid emerald green.

(Fig. 3.) As I have stated before, this paper is a poor conductor, 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 noticed on the "copper-bronze" paper, Fig. 2-b ; however, it is not quite so distinct. The color shade of the sparks runs more into a dull yellowish green, not unlike that or 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 oilcloth 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 alone, when in a healthy condition, not to speak of the many larger static machines with their correspondingly greater output. After thus charging the jars for a short while the electrodes are gradually and slowly approached toward each other, when upon reaching the stress limit the resulting spark will 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 included a separate gap (spark gap) into the former set-up. With the conductors of the machine set 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 charge. 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 element predominated in the charge (Fig. 4).

While the metal paper could by no means be clast as a conductor, this experiment demonstrates that it possesses sufficient conducting elements, however small, as to sustain a certain form of circuit; but it is, on the other hand, its feeble conducting value that makes the experiment at all possible. 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. They consist of heavy, polished brass wire and terminate in 1-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. 5). No sparks will occur between the Leyden jars in this connection.

Success in static experiments depends a great deal on the nature of the connections employed. Chains or wires used for this purpose must terminate in balls or rings, respectively, with the links of the chains preferably soldered. Open ends and sharp edges are certainly to be avoided, while earth-connected or other objects not needed in the experiments are to be kept at a respectable distance from the instruments in operation.

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