By Frederick Von Lichtenow
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
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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