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On the Magnetism of Drills. By Mr. BallARD, 1698. I caused six or seven several drills to be made before my face, and the bit or point of every one became a north pole, only by hardening, before they ever came to be worked, either in iron, or any other matter; so that I cannot suppose those found in a shop to have acquired their polarity so much from their after use, as from their first make.

That pieces of plain iron, in shape like drills, that is, something long and small, always change their poles as they are inverted, the end downwards being always the north pole, I find not always true : for though it hold generally in such small pieces, and always in pieces of any bulk, as large hammers, anvils, andirons, bars of windows, &c., yet I found several small pieces of steel, such as the drills are made of, to have fixed poles, one end north, the other south, in whatever positions I held them; some of these very vigorous in their polarity, others showing plainly a tendency to such a pole, rather than the other, yet so faintly, that it applied contrary to their inclination; that is, at the upper 'end, if it affected to draw the south; or the lower end, if the north. They caused the needle to stand in equilibrio east and west; the particular inclination of either one end seeming in some pieces quite to conquer, in others, quite to hinder, that more general polarity they both acquire, by being either upward or dowoward. Yet this seems only to be found in small stems of iron; the being either upward or downward always prevailing in pieces of greater bulk.

As to the opinion of the magnetic philosophers, that nothing gives or receives a magnetism, but what is in itself truly magnetic, as is only iron; as to the last part, that is, only iron receiving a magnetism, I have nothing certain to say; but for giving the same, I suppose it very questionable, whether only iron or loadstone can bestow or impart such virtue, since not only the quenching in water will do it, but the heating also of an iron by violent motion, will do the same; as by quick and hard filing, which is the very same thing as brisk drilling in the iron; and therefore may be said to proceed from the file, which is either steel or iron. But to show that it comes from the mere motion of heat, which is nothing else but the motion continued, this experiment may suffice, if it succeed to others as it seemed to do to me. I took my knife, which had been formerly touched a quarter of a year or more before, and proffering it to the needle, it drew the north pole, which happened right for my purpose. I wetted it briskly on a dry dirty threshold, and being very thin, it soon became hot towards the point, the edge being whetted away to a wire, as it were, I struck the very top, and back towards the top, against the ground, as I had done the sides, to destroy and rub off, if I could, all its former polarity which was southward; then offering it again to the needle, it drew the south end, and was quite changed. To confirm the thing, I touched the same knife again with the north pole of my loadstone, and it drew

vigourously the north end of the needle. I whetted it again strongly in the same manner, and it changed again. This I repeated five or six times, and it still changed by whetting, especially on the sides towards the top of the knife, the very top and back, which could not be whetted to so great a heat, retaining still some affection for that pole, the loadstone had inclined them to. This I tried with a knife of a thicker blade; but I could not with my hand whet it to that heat as to have the same effect, as on my own; though I used such force as at last to break it in two. I intend therefore to try it at a cutler's wheel, laid with emery and oil; and likewise on a grindstone, both wet and dry; and I rather choose the grindstone, because the other wheel may be supposed to have much iron worn into it from the many knives that have been grourd on it, and so the effect if produced, will prore no more than that of filiog with, or drilling in iron. And the wet grindstone, though it want heat to give a new polarity, yet probably it may wear off those parts of the iron in which the old did inhere, and so render it simple again.

As to the 4th, whether brass or copper will, as well as iron, give a north polarity to a drill, this cannot well be tried; since the very making, if it be well hardened, will certainly give it. Wherefore the drill Mr. Hunt made could not, if well hardened, according to what I can find, be indifferent to either pole.

As to the conclusions, first, that a drill is naturally a north pole, I suppose may be true, but it is contrary directly to what is just affirmed, viz., that the drill made by Mr. Hunt was indifferent to either pole, &c. And I suppose that bare drilling might be able to give a polarity to a drill, if it could be made indifferent, as well as filing does, if the drill be used so briskly as to be made as hot as the file makes the iron. Secondly, that though a south pole given by the magnet cannot be taken off by the heat of a brisk motion, as that of drilling; which yet, by the experiment of my knife seems to be contradicted; yet perhaps the heat may be great enough to produce a polarity in an indifferent piece of iron, as was before said to be done, in little indifferent drill-like pieces of steel by filing.

On Magnetism, particularly on the Polarity of a Piece of Iron.

By Mr. J. C., 1694. It is known that a rod of iron held perpendicular to the horizon, or inclining, the lower end is its north pole, or attracts the south end of a magnetic needle ; and that the same end held upwards becomes a south pole, or attracts the north end of a needle, and repels the south end. The south I call a mutable pole, which may be north or south, according as you hold it. I call a fixed pole, that which does not change, however you hold it; particularly that is a fixed north pole, which, though held upwards, attracts the needle's south end, and repels the north end : and that is a fixed south pole, which held downwards, attracts the needle's north end, and repels the south end. It is known that the magnet makes such fixed poles. But to do it without the magnet, is what I here chiefly consider.

1. The species of the pole, whether north or south, may be found by passing the iron rod through cork or wood, and then leaving it to swim on water, it will turn to its proper pole. But this way is not nice, being in some cases so slow, that you would think it to be at rest when in motion towards its pole. A better way to try, for instance, a north pole, is to hold the iron perpendicular to the horizon, and to try whether, being held under the north end of the needle, it attracts it. But a yet better way is to try whether the upper end of the rod attracts the south end of the needle, for attraction is more sensible than expulsion.

2. A fixed north pole may be made with all the ways and rods that you can make a fixed south pole, but not on the contrary ; for there are many cases wherein you can make a fixed north pole but not a fixed south pole ; and whatever way you get a fixed south pole, it is weaker than a fixed north pole made the same way. Applying a needle to an erect bar, beginning at the top and so down, the needle turns not at the middle, but nearer. Of some rods you cannot make a fixed south primarily, yet you may consequentially; so you may make one end a north pole, and then the other end of those rods may, without more-a-do, become a fixed south pole : but this does not always hold, for the one may be a fixed north pole, the other may be a mutable pole.

3. Fire destroys all fixed poles, whether made by the magnet, or otherwise ; but it increases, or rather less obstructs that magnetism which proceeds from the earth: a wire or rod of iron heated at one end, that end is a mutable pole, but more vigourous while hot than if cold; or the ignited end held downwards, will attract the said end of the needle more vigourous than if cold; and so if held upwards, it more attracts the north end. The vigour of mutable poles is more in great than small rods, but it is otherwise in fixed poles.

4. Heat the end of a rod of iron red hot, or heat all the rod, and cool that ignited end northward, it will be a fixed north pole ; if cooled south, it becomes a fixed south pole. This Gilbert and others assert from experience. But this holds only in some cases : viz., if the rod is short you cannot make a fixed pole that way. Take a round wire, its diameter 4 inch, and length ten inches, you cannot produce a fixed pole by ignition ; but if this wire is longer, as suppose thirty inches long, or ever so much longer, it is capable of a fixed pole by ignition. Again, take a round rod thirty inches long, and one inch diameter; this rod is not capable of a fixed pole at that length, though the lesser was capable at that length. And thus my experiments give me reason to think that there is no rod or bar of iron ever so thick, but which, if had length enough, would be capable of a fixed pole, by bare ignition, for of that only I speak in this paragraph ; and there is no rod ever so short, but which if you make it sufficiently thin, is capable of a fixed pole. So, when in a rod I could not obtain a fixed pole at twenty-one inches length in that thickness, I could by making the rod thinner produce a fixed pole, even

in the length of one inch and less, and the pole should be of which kind I pleased. The terminus, or necessary length for every thickness, increases more than one would think.

5. Heat a rod, or its end, red-hot, and thoroughly cool this end downwards, it will have somewhat more magnetism than if cooled horizontally towards the north. But the better way is to cool it a little inclining towards the north. I cannot find that multiplicity of ignitions produces more magnetism than one good ignition ; but it must be thoroughly ignited. Nor can I find by many experiments that quenching in water signifies to the producing or hindering magnetism ; but many ignitions may accidentally promote it by purifying the iron.

6. Dr. Power says, that if we hold a rod northward, and hammer in that position the north end, that will become a north pole, i.e., a fixed north pole; contrarily, if you hammer the south end. But this is true only in some cases, viz., it holds in rods only of a certain length; for I say here again, as before of ignitions, that of round bars of the same diameter, there is required a certain length, under which a fixed pole cannot be produced by hammering; but of any length more than that certain length you may make it; and then if you take a bar shorter than that length of which you cannot make a fixed pole, while you keep that diameter ; if you take a rod of the same length but less diameter, you may by blows produce a fixed pole; or if you only beat that thicker bar thinner, you may produce a fixed pole, though the rod be never so short, provided you beat it thin enough.

7. What is said of hammering, is to be understood of filing, grinding, drilling, sawing, or a hard rubbing, or even a soft rubbing, provided it is long, will produce fixed poles ; the heavier the blows are, cæteris paribus, the magnetism is the stronger; I say cæteris paribus, as when the blows are not so heavy in either case, as to flat the iron, for flatting it produces more magnetism, though other things do not vary. A few hard blows will produce as much magnetism as many, as to sense, as if you give ever so many blows, yét a soft blow may produce but little magnetism. The utmost magnetism that I could produce in ordinary rods this way, did not exceed that which an ordinary loadstone would have infused.

8. Beating many rods northward, whose lengths I knew sufficient, I never failed of producing a fixed north pole ; but hammering the same or like rods southward, I found that I could not produce a fixed south pole, only a mutable pole; nay, hammering one full south, I produced a fixed north pole; the reason I thought might be, that the hammered south end on the anvil was a little lower than the end which I held in my hand. I then held the end higher, and go hammering it south upwards, I never failed producing fixed south poles in proper rods.

9. Old drills and punches are fixed north poles, because almost constantly used downwards; but new drills are either mutable poles,

or weak north poles ; when I say a new drill, I do not mean one made on the spot, for that is probably a north pole, because quenched downwards in water, but then such polarity made by bare ignition is a weak pole, and soon decays, and turns to a mutable pole ; but I mean a drill, which though never or little used, yet has been made some days or weeks, drill with this southward horizontally, and it is a chance if you produce a fixed south pole, but much less if you drill south downwards, but if you drill south upwards you may make it a fixed south pole.

10. The stronger the polarity is, the longer it will last; a weak fixed pole may degenerate into a mutable pole in a day's time, yea, I have known it in a few minutes, while exposed to the air, and held in a position contrary to its pole ; on the contrary, we find needles touched with good loadstones hold that virtue a great while, if kept from air, and in a meridional position.

11. The loadstone itself will not make a fixed pole of any iron, only it must have a proper length if it is thick, or if it is short it must have a sufficient thinness ; so, ordinary or weak loadstones cannot fix a pole in a thick short key, which yet they will do in a little key. So in a short thick iron tapering, a loadstone may fix a pole in the little end, when it cannot in the great end.

12. When ignition, hammering, or a loadstone cannot make fixed poles, it must not be thought that it can do absolutely nothing on such rods, for even then it may be found that there is an effect of magnetism in them discernible enough otherwise, though not enough to make fixed poles.

13. When you have the due length for making a fixed pole, you will find the making one a fixed north will consequently render the other a fixed south pole; but, if keeping the same diameter of this rod, you increase its length sufficiently, the making one end a fixed north pole, will not necessarily make the other a fixed south pole, but leave it a mutable pole. So if you by a like primary operation make the second end a fixed pole, the first end will lose its fixity and become mutable. There is a certain length suited to every thickness of iron, to leave one end mutable while the other is fixed, and the thicker the iron is, the greater is this length.

14. If you farther increase the length of the same rod, you will attain such length, that the oftener you have a fixed pole on one end, and then go to fix the other end, the fixity of the first will not be destroyed, and that end become mutable as before, but the fixity of the first end will remain, and so you make both ends two fixed north poles, or two fixed south poles. The shortest length (for there is no terminus of the greatest length) for this, is more in thiek than in thin

iron.

15. The aforesaid lengths are less, according to the strength of magnetism, viz., ignition requires a greater length than when a rod is actuated by a loadstone ; and a rod touched with a strong loadstone requires less length than one touched with a weak one.

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