Electricity.

Date:
[1829]
Catalogue details

Licence: Public Domain Mark

Credit: Electricity. Source: Wellcome Collection.

    43/150 (page 53)
    quickly write the letter P on the resinous plate : take the bottle containing the mixed powders, and by gently striking the sides of it between the palms of the hands, the mouth of it being directed towards the plate, which must be placed in a vertical position, project on the electrified part a little of the powder, and the letter will instantly appear, well defined, and covered with the red lead, while the resin will be dispersed. If now the whole be wiped clean off, and, with a jar charged at the negative conductor, the letter Nbe written on the plate, and the powder pro¬ jected as before, it will be found that the nega¬ tive electricity has selected the resinous particles for the formation of the letter, while those of the red lead have been rejected. Magnetic Effects of Electricity. 259. The connexion between magnetism ana electricity will be fully treated of in a subse¬ quent part of this article; at present we shall merely offer the following general remarks in the way of showing the amount of what was known on the subject previously to the interest¬ ing discoveries recently made. 2G0. Dr. Franklin appears to have been the first who paid any serious attention to this sub¬ ject. He sent the charge of some large electrical jars through fine sewing-needles ; the ends of the needles were rendered blue, and on being carefully laid on water they traversed, evincing evident proofs of polarity. The most remark¬ able circumstance attending these experiments was, that if the needle lay east and west when the charge was passed through it, the end which was entered by the fluid pointed to the north: but if it lay south and north, the end which lay pointing to the north would continue to do so, whether the charge entered by that end or the other; although the Dr. imagined that a still stronger charge would have reversed the poles, even in that situation, since this effect had been actually produced by lightning. The polarity he also found to be strongest when the needle received the charge while lying north and south, and weakest when it lav so as to point east and west. 261. Rut the experiments most to be relied on were made by Van Marum with the large machine and battery in the Tylerian museum at Haarlem. He and his assistants tried to give polarity to needles made of watch-springs, of from three to six inches in length; and also to steel bars nine inches long, from a quarter of an inch to half an inch broad, and about a line in thickness. The result was, that when the bar was placed horizontally in the magnetic meridian, whichever way the shock entered, the end of it that stood toward the north acquired the north polarity, and the opposite end acquired the south. If the bar, before it received the shock, had some polarity- and was placed with its poles contrary to the usual direction, its natural polar¬ ity was uniformly diminished, and often reversed; so that the extremity of it, which in receiving the shock pointed to the north, became the north pole, &c. 262. When the bar was struck standing per¬ pendicularly, its lowest end became the north pole in any case, even when the bar had previ¬ ously some magnetism, and was placed with the south pole downwards. Things remaining the same, the bars seemed to acquire an equal degree of magnetic power, whether they were struck whilst standing horizontally in the magnetic meridian, or perpendicular to the horizon. When the needle was placed in the magnetic equator, whichever way the charge entered, it never produced any magnetism; but if it was passed through its width, then the needle acquired a considerable degree of magnetism, and the end. which lay towards the west became the north pole, and the other end the south pole. If a needle or bar, already magnetic, or a real magnet, was struck in any direction, its power was always diminished.. For this experiment they used considerably large bars, one of which was 7-08 inches long, 026 broad, and 0-05 thick. When the shock was so strong in proportion to the size of the needle, as to render it hot, then the needle generally acquired no magnetism at all, or very little. These experiments were made with the power of a battery composed of 135 phials, containing among them about 130 square feet of coated surface. Medical Electricity. 263. The very remarkable properties of the electric fluid seem to have occasioned the appli¬ cation of its powers to organised bodies at an early period of its history; and the results, whether real or imaginary, gave rise to various opinions, which are now only viewed as monu¬ ments of credulity and imposture. It is not our intention to enter here minutely into an investi¬ gation of this department of the science of elec¬ tricity ; yet, although this would be improper on several grounds, we must not pass it over in silence. The application of electricity as a medical agent has recently been much revived; but that application is, we believe, most success¬ fully made in the form of Voltaic electricity; and will therefore fall, with more propriety, under consideration in another place. From the numerous respectable testimonies extant, as to the real utility of electricity in the healing art, we shall make a choice selection, which it is hoped will satisfy the reader, that in the hands of a skilful operator, it may be applied in nume¬ rous instances with certainty of success. Let it however be observed that no one ought to attempt its application who is not thoroughly conversant with the use of the apparatus, and capable of forming a correct judgment, as to the strength and duration of the application. 264. Speaking of the power of the apparatus which ought to be used in the application of electricity for medical purposes, Mr. Singer justly remarks, that the machine employed ought to be such as will furnish a constant stream of sparks; if a plate machine, it ought to be two feet in diameter; if a cylinder machine, not less than ten inches, and fourteen if possible. Ma¬ chines called medical electrical machines, are sometimes made on a very small scale, and hence the application of them only produces trouble, waste of time, and final disappointment. 265. In. connexion with a powerful electrical