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$ 72. Electrical figures.—By means of electricity, figures can be produced on the surface of different bodies, which are either directly visible or are rendered visible by strewing dust, or by breathing upon hem. Riess has made an extensive series of experiments (Pog. Ann., LXIX, 11 on these phenomena, the best known of which are the Lichtenberg figures, and he has determined very accurately the circumstances under which these figures and images appear.
Riess divides them into primary electrical delineations, or such as are caused by different parts on the surface of poorly conducting substances being placed in unlike electrical condition, and becoming visible on being sprinkled with powders; and
Secondary electrical delineations, which are produced when the film of foreign matter which covers nearly all bodies is affected by the electrical discharge; in this case the figures are made to appear by breathing upon the plate, or else visible marks may appear immediately, if the surface of the body itself has been in any way attacked.
We shall first consider the figures made visible by sprinkling powder upon them.
$ 73. Dust figures.—To produce the Lichtenberg figures Riess used square copper plates, covered on one or both sides with a coat of pitch about } line thick.
The formation of dust figures (Lichtenberg figures) is a consequence of the electroscopic action of electrified spots on the resinous surface upon the powder itself, electrified by shaking in the bag through which it is sifted. A mixture of flour of sulphur and minium is best for this purpose. Positively electrified places on the plate are covered with the sulphur, and therefore appear yellow; the minium, on the contrary, is collected on the negative spots, which thus appear red.
The spark having passed over the pitch surface, so that a dust figure would have appeared if it had been immediately dusted, no figure will be formed if the pitch surface is first exposed for a second to the flame of a spirit lamp, by which the electricity is removed from the plate.
The simplest mode of producing dust figures is the following, used also by Riess : A copper plate, covered on one side only with pitch, is touched by a conductor, and an insulated metallic point is placed on the pitch surface. The upper end of the point being touched by the knob of a positively charged jar, remove the insulated point, and on powdering with the above described mixture a round yellow sun, with dense rays, will appear.
The experiment being conducted in the same manner with a negatively charged jar, a perfectly red circular disk will appear.
This diversity in the appearance of the figures is well known; but Riess has directed attention to another remarkable distinction, namely, that the positive figure is much larger than the negative, though equally strong charges have been used.
With a given positive charge of the jar the yellow sun had (as a mean of 3 experiments) a diameter of 16.1 millimetres.
With an equally strong negative charge the red disk had a diameter (also a mean of 3 experiments) of 5.8 millimetres.
The diameters of the negative and positive figures, produced by
equally strong charges of the jar, are, consequently, in the ratio of 1 to 2.77, or the surfaces covered by them are as 1 to 7.67.
A plate, coated on both sides with pitch, on being brought between the insulated point and the conducting wire, and subjected to the above process, the positive figure appears on one side and the negative on the other.
When the jar was charged with negative electricity, the disk appeared above and the sun below, but the yellow sun in this case was only 2.2 times as large as the red disk.
The cause of the negative figures being relatively greater than in the previous experiment was owing to the excess of negative electricity, which was transmitted to the upper surface; in fact, a sun appeared on the upper side, which was 3.3 times as great as the red disk on the under side, when a positively charged jar was used in a similar experiment.
Riess has shown that the dust figures appear only when the passage of electricity on the insulating plate is accompanied by a discontinuous discharge, which may be recognized generally by a peculiar hissing. By holding the pitched plate to the knob of a charged jar a spark passes with a crashing noise ; a discontinuous discharge thus takes place, and a figure appears on dusting; but the plate being placed at such a distance from the knob of the jar that a spark cannot pass, some electricity still gradually goes over, producing a continuous discharge. If the plate is dusted atter standing from 30 to 70 minutes opposite the knob of the jar, a number of round spots appear irregularly distributed-yellow, if the jar had a positive, red, it a negative charge. These spots exbibit no trace of rays; they are perfectly alike in size and form for both electricities.
Pence, electrical dust figures appear when electricity is transmitted by a discontinuous discharge to an insulating plate.
Upon this fact Riess founds a very ingenious explanation of the difference between positive and negative dust figures. In a discontinuous discharge passing over the surface of an insulator the condensed atmosphere, which covers the surface of all bodies, is forcibly penetrated, and a part of the stratum, containing vapor of water, is projected with violence against the surface of the body.
But Faraday has shown that, if moist air impinges forcibly against any body, the latter is negatively electrified; thus, then, in this case* the surface of the plate becomes negatively electrified in consequence of the discharge which takes place over the surface; the remaining electricity of this discharge then has only to spread over a negatively electrified insulating surface.
The surface being charged with negative electricity, it spreads from the point over an insulating surface already negative; the circumstances, therefore, not being favorable for the distribution of the negative electricity the figure cannot become enlarged, and a rounded form is assumed.
The jar being positively charged, the remainder of the positive charge spreads from the point over an insulating surface negatively
* The experiments of Faraday referred to, scarcely allow of such a conclusion.-(Sea Report for 1856, p. 364.)
G. C. S.
electrified by the discontinuous discharge; the fact that electricity is already present on the surface, acting attractingly on that issuing from the point, occasions a greater diffusion of the positive electricity; but the circumstance that the positive electricity spreading forth is partially neutralized by the presence of the negative, causes the radiating form of the positive dust figure.
To sustain this view, Riess produced a modification of the phenomenon in rarified air. On a plate covered with pitch, placed under a glass receiver, was placed the blunt end of a wire, which received a spark from a jar charged with positive electricity. With the whole pressure of the air the sun appeared on dusting the plate ; but when the air was exhausted to 27} lines pressure, only an irregular yellow speck appeared ; negative electricity behaved in like manner. The difference between the positive and negative figures was no longer observed at this degree of rarifaction.
When the air was exhausted to 2 or 3 lines the end of the wire left only a point, which, with positive electricity was red, with negative, yellow; and consequently caused, not by the transmission of electricity to the plate, but by induction.
The penetration of the stratum of air surrounding the plate is, therefore, the origin of dust figures.
§ 74. Dust images.-If a stamp (as simple as possible, having a few raised letters, and for this reason printing types will answer) be placed on a single pitch plate, (so Riess calls a copper disk coated on one side only with pitch,) and electricity be communicated to the stamp, it acts inductively on the pitch surface, the latter becoming electrified at the spot where touched ; and this electricity is opposite to that of the stamp, for on removing it and powdering the plate with the mixture mentioned already, a red image of the letter is obtained, if the stamp is positive; a yellow one, if negative; for the Acur of sulphur attaches itself to the positive, the minium powder to the negative spots of the resin plate.
The above described phenomenon underwent numerous modifications, according to the manner in which the stamp was electrified.
The stamp being touched by the knob of a charged Leyden jar, and then removed in an insulated condition, leaves an image as above indicated; it is, however, very little covered with dust; while the ground, by the formation of dust figures becomes yellow, if the letter is red, or red, if the letter is yellow.
The stamp being removed uninsulated, the dust figure changes, whereby the clearness of the image also suffers.
By electrifying too strongly, an actual passage of electricity in part occurs at the place where the stamp touches the plate, so that a dust image appears, partly red and partly yellow.
Then we have at the same time a dust image and dust figures. To obtain the dust image clearly, the formation of the dust figures must be avoided, which Riess accomplished in various ways.
The knob of a Leyden jar was exchanged for a four-inch ball, and the jar fastened horizontally, so that the pitch plate and the stamp could be placed under the ball; the stem of the stamp was half an inch from the ball. By the inductive action of the ball the end of the stamp
touching the pitch was electrified like the ball ; too strong an accumulation of electricity was prevented by providing the stamp with a point. After the stamp had been exposed from 20 to 30 minutes to the inductive action of the ball, a clear dust image appeared without any dust figure, but irregular spots appeared in the ground, which were not of the color of the image.
Similar results were obtained when the stamp was placed for several hours in connexion with one pole of a powerful dry pile, while the electricity of the other pole was conducted off as completely as possible.
In these cases, in which generally no dust figures appeared, it was indifferent whether the stamp was insulated or not, on its removal.
The color of the irregular spots showed that they originated in the electricity actually passing from the stamp to the pitch plate at the places which admitted of a slight current. To avoid these, more ready passage to a conducting medium must be furnished for this electricity, as in the case when the dust images were produced in rarified air. Riess obtained in this manner the most perfect dust images.
The dust figures and images, just considered, are, according to Riess, primary electrical delineations, the figures and images now to be considered are secondary electrical delineations.
$ 75. Electrical breath figures.—The surface of glass, mica, &c., over which an electrical discharge stroke has passed, gives, by breathing upon it, peculiar ramified figures, which stand out from the surface obscured by the breath with a mirror-like lustre.
The breath figure indicates the path taken by the electrical discharge over the surface; and its form differs therefore, according to the nature of this surface. On metal, it appears as a round disk; on resin, as serpentine stripes ; on njica, as fine, many times ramified lines.
The breath figure is independent upon the kind of electricity employed.
That these figures do not originate in the electricity which continues to adhere to the surface is established by the fact that they are seen on metallic surfaces, on which they appear after the breathing, as distinct circles, surrounded by more or less obscure rings; the breath figures also appear a long time after the discharge stroke has passed over the surface, or after the surface has been passed over the flame of a spirit lamp. Hence, the breath figures cannot be owing to adhering electricity ; they are to be ascribed to a change of surface which the substance used has been subjected to, by the electrical discharge.
On a fresh surface of mica, that is on such as is obtained by a fresh cleavage, breath figures do not appear. This depends upon a peculiar property of fresh mica surface, which Riess has described in the 67th volume of Poggendorf's Annalen, page 354.
A clean plate of mica being breathed on, or held over evaporating water, the result is, as with all bodies, that it will be covered with a rapidly disappearing stratum of water, consisting of very small drops, which are not in contact with each other.
But when the mica has received a fresh surface by cleavage, it remains perfectly clear, shining and transparent after being breathed on.
This phenomenon is by no means owing to the fresh surface not
condensing vapor of water, for the breath causas it to show the colors of thin plates; it is consequently covered with a coherent stratum of water.
A drop of water which stands at rest on an old surface of mica at once spreads on a fresh surface, and completely covers it. Hence, a mica surface made by cleavage possesses, in consequence of its great purity, so great an attraction for the vapor of water that it condenses the water into a coherent stratum, while, had the mica been exposed a long time to the air, it would have condensed the water in separate drops.
While an old surface of mica is an excellent insulator of electricity a fresh surface discharges an electroscope in a few seconds; it acts hygroscopically by condensing the vapor of water of the atmosphere into a coherent stratum, which conducts electricity.
This remarkable peculiarity of fresh mica is preserved but a short time in the air ; in a few days it may be clouded by breathing upon it.
Very powerful electrical discharges produce not only a change in the film of foreign matter covering the body, but they alter the surface of the body itself. This is the cause of the traces noticed in § 41, occasioned by the discharge spark on glass and mica (electrical colored stripes) and of the rings of Priestley, which occur when numerous discharges of a battery take place between a point and a polished metallic surface, whereby oxidation of the metal forms many colored concentric circles.
$ 76. Karsten's Electrical Figures. The analogy which Riess describes in the VI volume of Dove's Repertorium der Physik, between electrical breath figures and the images of Moser, occasioned Karsten to examine whether such images could not be obtained in the electrical way.
For this purpose he placed (Pog. Ann., LVII, 492) a coin on a mirror, resting on a discharging metal plate, and caused sparks to strike from the conductor of the machine upon the coin, thence passing to the metal plate, (around the edge of the glass.) After 100 revolutions of the machine the coin was removed ; the glass plate seemed wholly unchanged, but when breathed upon the image of the coin appeared distinctly.
Besides the memoir cited, Karsten has published two others, in Poggendorf's Annalen, (LVIII, 115, and LX, 1,) on electrical images, but as he has not succeeded in discovering their true nature, it is unnecessary to go further into the details of these memoirs; and the more, since Riess, as we shall see, has correctly ascertained the condition for producing electrical images. The report upon Riess' researches will therefore suffice to bring the facts at least, to the knowledge of the reader.
We must, however, briefly notice, by the way, Karsten's last treatise in one particular. In the beginning he adduces many experiments which have been made to explain the cause of Moser's images ; besides Moser's own theory, he presents the opinion of Hunt, Know, Fizeau, Daguerre, Masson and Moore. Why is the excellent work of Waideles on this subject ignored ? it appears in the first half of the 59th volume of Poggendorf's Annalen, and after these images had been the