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Felix Savary's Experiments

An overview of Savary's experiments were written in Treatises on Electricity, Galvanism, Magnetism, and Electro-Magnetism published in 183 1 . It explains Savary's several major experiments, all of which involved the discharging of a Leyden jar.

One experiment describes a straight conducting wire connected to the jar with steel needles placed at different distances. Savary observed that at different distances, the needles would have different polarities. We now know that this is due to the wire's self induction, and this essentially creates a LC circuit.

This is an excerpt from Treatises, and I have bolded the most important parts of the paper :

"There appears to be a very essential different in the effects of the shock of an electrical battery discharged through a wire, and that of a voltaic battery, in communicating permanent magnetism to steel bars or needles. Mr. Savary has brought to light several very curious particulars relating to this subject, which have hitherto received no explanation. When the discharge from a Leyden battery is made through a straight wire, different needles, though equal in size, and parallel to each other, and placed transversely on the same side of the wire, but at different distances, have their polarities not disposed in all of them in the same manner. In some the poles have the same relative situation as those of a needle previously magnetized, and free to move, which has taken the position it would have when under the influence of a continued voltaic current passing in the same direction along the wire.

But in others the position of the induced poles is the reverse of this. For the sake of conciseness of expression, we shall call the action which produces an arrangement of poles similar to that resulting from a voltaic current, positive magnetization; the contrary effect being that of negative magnetization. Thus, in a series of experiments in which the needles were placed at distances from the wire which increased by equal intervals, at the point of contact with the wire the needle was magnetized positively, at a small distance negatively; a little further off it had acquired no magnetism whatever; at a distance somewhat greater than this, it exhibited positive magnetism; and this effect continued for a certain interval, beyond which the magnetization was again negative. When still more remote, it was positive, and continued so to all greater distances that were tried. Hence the action appears to be periodical with relation to the distance at which it is exerted."

An article from Le Globe (no. 96) published on August 2, 1826, highlights some of the most important aspects of the Leyden jar discharge observed by Felix Savary. It includes the second part of his experiment, which is a Leyden jar connected to a helix.

This is a picture of one of the needles that Savary probably used for his experiments with the helix. A very fine needle is wrapped around by a conducting wire, and its polarity is checked using a compass. In this picture, the needle is enclosed in a glass tube for insulation.

This is an excerpt from Le Globe:

"1. The direction of the magnetic polarity of small needles, exposed to an electric current, directed along a wire stretched longitudinally, varies with the distance of the wire.

2. This action is periodical;-that is, when the small steel needle, which is in relation with the wire, is magnetised in a certain direction, at a certain distance, the magnetism diminishes as the needle is removed, till at a certain distance it becomes nothing. At a greater distance it recovers its magnetism, but in a contrary direction, and it goes on increasing till it reaches its maximum at a particular distance. It then diminishes as the removal of the needle is continued, and again becomes nothing. The magnetism then resumes its first direction, which constitutes a new period. M. Savary has observed three periods.

3. The distances at which the zero and the maximum of magnetism take place vary with the length and diameter of the wire, and with the intensity of the discharge.

4. When a helix is used for magnetising, the distance at which the needle placed within it is from the conducting wire is indifferent, but the direction and the degree of the magnetism depends on the intensity of the discharge, and on the ratio between the length and size of the wire.

5. The maximum of intensity, which can be produced with a given wire, depends on the ratio between its size and length, so that it is only for a certain value of this ratio that we can obtain the degree of magnetism called the state of saturation. For all other ratios the maximum is less.

6. Any metal placed in the vicinity of the needle has a very powerful influence on the direction and degree of the magnetism.

7. These effects vary with the relative positions of the wire, the needle, and the metal.

8. The direction of the action of the metal depends on the intensity of the discharge, so that discharges different in intensity, develops in the metal a series of opposite states, analogous to the polarities of opposite signs, which small needles acquire at different distances from the conducting wire, or for different intensities of electricity."

A complete translation of Felix Savary's paper, Mémoire sur l'Aimantation, can be found below. In addition to the information above, Savary's original paper includes charts and data that he collected during his experiments.

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