Abbildungen der Seite


Magnetical experiments.

1. By Mr. Sellers*.

On magnetic needles. MR. Sellers states, that he had often made trial with many needles, touching them on each hemisphere of the stone, in all variety of ways he could imagine, to find if it were possible, by that means, to cause any of those needles to vary in its direction: but that he always found the contrary, all of them conforming to the magnetical meridian, and standing north and south, as other needles that were touched on the very pole of the stone.

He adds, that some of these experiments he tried in London, when there was no variation known.

That on frequent trials of touching needles with different load. stones of several magnitudes, as also of different virtue, the needles touched gave all of them the same directions. This he thinks is confirmed by all the needles, and sea compasses, made in several parts of the world, and consequently touched on several stones of different countries, yet all agreeing in this magnetical harmony, that they all give the same directions. That having sometimes drawn a needle only over the pole of the stone, within the sphere of its virtue, without touching the stone, it has received the same directive quality from the stone as if it had been really touched on the stone itself, though not altogether so strong as if it had touched the stone. Again, that having touched needles on the stone with faint strokes, and other needles with stronger, all these needles received the same effect from the stone, both for strength and direction; he conceiving that it is not the fainter or stronger touches on the stone, nor the multiplicity of strokes, that varies the needle's strength or direction; but that the nature of the steel whereof the needle is made, and the temper that is given thereunto, cause different effects as to the strength it receives from the stone; himself having tried all sorts of steel that he could pos. sibly procure, and all the different tempers he could imagine, for

This seems to be the first notice of making artificial magnets. Mr. Sellers is probably the person of the same name who was the author of Practical Navigation, in 1669.

the most powerful receiving and retaining the virtue from the load. stone ; he also affirms that he has fully satisfied himself that he can infase such virtue into a piece of steel, that it shall take up a piece of iron of two ounces weight or more ; and give also to a needle the virtue of conforming to the magnetical meridian, without the help of a loadstone or any thing else that has received virtue therefrom.

[Phil. Trans. Abr. 1667.

2. By Mr. (afterwards Dr.) Gowan Knight ; shewn before the

Royal Society, November 15, 1774*. Mr. Knight, of Magdalen College, Oxford, being introduced to a meeting of the Royal Society on Thursday the 15th of Novem. ber, 1744, produced, before the gentlemen there present, several curious artificial magnets contrived by bimself; some of which consisted of plain bars of steel naked, and others of bars or blocks of the same substance, armed with iron after the common manner of natural loadstones : but as he was apprehensive the triathe had before made of the weights these magnets were respectively capable of lifting could hardly be repeated with sufficient exactness, and advantage before so large a company, he desired to refer himself, for those particulars, to what the president of the Society bad seen at his lodgings on Wednesday the 7th, and on Tuesday the 13th of the same month of November.

On which the president acquainted the company, that he had lately been several times at Mr. Knight's lodgings, where he had seen many experiments made with his artificial magnets; and that, particularly on the days abovementioned, he had been present, and had taken minutes of the following trials then made by that gentleman, by which it appeared, that

A small eight.cornered bar of steel, of the length of 7-10th inches, and about half an ounce Troy weight, lifted by one of its ends about eleven of the same ounces.

'That another plain bar of steel, of a parallelopiped form, of the length of 5 9-10th inches, the breadth 4.16ths, and the thickness

* There is another paper containing an additional series of experiment on the same subject, made by the same experimentists, contained in vol. xliv. year 1747, of the Transactions; but it is not much more than an expansion of those presented above.


2.10ths of an inch, weighing 2 oz. 8! dwt. lifted by one of its ends twenty Troy ounces,

That a steel bar, almost of the same form as the last, but only four inches in length, capped or armed with iron at each end, cramped with silver, and weighing all together 1 oz, 14 dwt. lifted by the feet of the armour full 4 lb. Troy.

That a single block of steel of a parallelopiped form, almost four inches long, 1 2. 10th inches in height, and 4-10ths of an inch in thickness, armed with iron, cramped with brass, suspended by a ring of the same, and weighing all together 14 oz. 1 dwt. lifted by the feet of the armour 14 lb. 2{ oz. Troy weight.

That a compound artificial magnet was also tried, consisting of twelve bars of steel armed, and that it was found to lift by the feet of the armour, as the last, 23 lb. Troy, 2į oz.

The twelve bars, composing this last magnet, were each a little more than four inches long, 3-10ths of an inch in breadth, and 16.100ths of the same in depth, weighing one with another about 25 dwt. each. They were all placed one on another, so as to make together one paralellopiped body, of the common length and breadth of the several bars, but of the height of near two inches, being the sum of the respective thicknesses of all the bars taken together : and this paralellopiped body, being cramped with brass, and fitted with a handle of the same metal, was armed at the two ends that were made up of the common extremities of all the bars, with two substantial pieces of iron, after the common manner of arming natural loadstones, the whole frame weighing together about twenty ounces Troy.

Besides these, the president made also the following report of some trials he had seen made at the same time of the effects of an art Mr. Knight is master of, by which he can improve or increase the lifting powers of natural loadstones.

He carried with him, on Wednesday the 7th of November, a small armed loadstone belonging to an acquaintance, which weighed, with its armour, 7 dwt. 14 gr.; but which, being reputed but of an ungenerous nature, took up, and with some difficulty, barely two ounces. Mr. Knight took it into his study, and returning it in about a minute, it then took up more than four ounces with ease : but, on his saying it would still gain some more strength, by remaining with him some time, it was left till the 13th, when it took op distinctly, with the same apparatus as before, 6 oz. 18 dut. 3 gr. ; since which time it has also several times been found to lift nearly the same quantity.

Mr. Knight further, at the same time, showed the president the following instances of his ability to invert or change the direction of the poles in natural loadstones.

Such a stone belonging to Mr. Francis Hauksbee, weighing about 5 oz. 14 dwts, of an irregular cylindrical form, with two of the sides somewhat fatted, on which armour had formerly been applied, had the direction of its polarity from one of these flatted sides to the other, notwithstanding the stone had a distinct grain running at right angles to that direction. It was tried, and observed that one of these flatted sides strongly attracted the north end, and repelled the south ; and that the other attracted the south, and repelled the north end of the magnetic needle. The end of the stone, attracting the south end of the needle, was then marked, by the rubbing of a piece of silver upon it, as on a touchstone: after which, Mr. Knight carried the stone into his stady; and reproducing it in about a mioute, showed that the poles were then directly inverted ; and that the same end, which before attracted the south end of the needle, now attracted the north, and repelled the south, and vice versa.

After this, Mr. Knight, again taking the stone, brought it back in as short a time as before, with the direction of its polarity turn. ed at right angles to its former direction, and into the direction of the natural grain of the stone, the poles now lying in the flat ends of the cylinder; one of which, being the smoother end, attracted the south end of the needle, while the other, which was of a roagher texture, attracted the north end, and repelled the south end of the same : when it was also observed, that the polarity appeared stronger in this case, than either of the former.

Lastly, Mr. Knight, in about the same time, inverted this last direction of the poles, keeping it still parallel to the axis of the cylinder, but causing the smooth end of the stone to attract the north end of the magnetic needle, and the rough end to attract the sonth, and repel the north end of the same needle.

After this report, Mr. Knight proceeded to show, at the meeting, some of the same artificial magnets there mentioned ; and it was found, that the compound magnet, consisting of twelve steel bars, and which had, in the experiment made before the president,

Sifted 23 lb. 2} oz. Troy weight, did here, under all the inconveniencies and disadvantages of a crouded room, still lift a weight amounting to 21 lb. 11 oz.

It was also found, that the single armed block of steel, which had before lifted 14 Ib. 2 oz. did here, under the same disadvan. tages as the former, lift 131b. 7 oz.

And lastly, Mr. Knight produced to the company the above. mentioned natural loadstone belonging to Mr. Hauksbee, but with the direction of its polarity again altered from what it was, when it was last seen by the president.

P.S. Since the artificial magnets mentioned in the foregoing paper, Mr. Knight has caused some others to be made of a less size, but of a very great lifting power: and one of these, weighing, without its armour, just an ounce, and with the armour, cramps, and rings, 1 oz. 17 dwt. lifted 6 lb. 10 oz.

This magnet consisted of three plates of steel, each two inches long, 7-10ths of an inch in breadth, and not above 6.100ths of an inch in thickness: they were laid flat on each other, and screwed together by two small brass screws going through the three plates. After which, the little parallelopiped block so made up, was armed with iron at the two ends, cramped together with silver, and fitted with a double ring of the same metal, for the convenient holding of it.

[Phil. Trans 1741.

3. By Mr. Benjamin Wilson. Containing an account of Dr. Knight's method of making artificial Loadstones.

The method was this: having provided himself with a large quan. tity of clean filings of iron, Dr. K. put them into a large tub that was more than one-third filled with clean water ; he then, with great labour, worked the tub to and fro for many hours together, that the friction between the grains of iron by this treatment might break off such smaller parts as would remain suspended in the water for some time : the obtaining of which very small particles in sufficient quantity seemed to him to be one of the principal desi. derata in the experiment. The water being by this treatment ren. dered very muddy, he poured it into a clean earthen vessel, leaving the filings behind; and when the water had stood long enough to

« ZurückWeiter »