Abbildungen der Seite
PDF
EPUB

The Circulating Lamps.

After keeping the company thus long in the dark, it will be proper to illuminate the room before you dismiss them. In order to which, introduce the circulating wheel described in page 186. To the upper axis of which let there be fixed a number of radii, made of baked wood, at the end of each of which must hang a small globular lamp, filled with spirits; and let that of each lamp be ringed with a different colour. The wheel, having previously acquired its greatest velocity, is to be placed on the table, and a chain, depending from the branch, is to dip into each lamp as it passes by; so that all of them will become illuminated in a very short time. These lamps will not only enlighten the room, but by their variegated colours, and continual revolution, afford a very pleasing phenomenon.

MAGNETISM.

DEFINITIONS.

1. MAGNETISM is the science that explains the several properties of the attractive and repellent powers in the magnet or loadstone.

2. The magnet is a rich, heavy, iron ore of a hard substance, a dusky grey colour, with some mixtures of a reddish brown, and sparkling when broke.

3. The magnetic virtue is called the third species of attraction; gravity being the first, and electricity the second.

4. The two ends of a magnet, when it is properly formed, are called its poles; and when it is placed on a pivot, in just equilibrium, one end will turn toward the north, and is called its north pole, and the other end the south pole.

5. When the two poles of a magnet are surrounded with plates of steel, it is said to be armed.

6. If the end of a small iron bar be rubbed against one of the poles of a magnet, it is said to be touched, and is then called an artificial magnet.

7. If such a magnet be supported on a pivot, it is called a magnetic needle; one end of it turning toward the north, and the other toward the south.

8. The difference between the position of the needle, and the exact points of north and south, is called its declination.

9. A needle which is touched will incline toward the earth, and that is called its inclination or dipping.

APHORISMS.

1. The magnetic attraction is produced by effluvia emitted by the magnet, and passing from one pole to the other.

2. One pole of a magnet will attract iron, and the other repel it, but no other body.

3. The magnet attracts iron as well in vacuo as in the air.

4. The magnetic attraction will be continued through several pieces of iron placed contiguous to each other. 5. The magnetic effluvia pervades all bodies.

6. The magnetic attraction extends to a considerable distance.

7. The north pole of one magnet will attract the south pole of another: and the similar poles will repel each other.

8. The end of a needle touched by the north pole of a magnet will turn south, and that touched by the south pole will turn north.

9. The declination of the magnetic needle is different in different parts of the earth, and in the same part at different times.

10. The inclination of the needle is not always the same in different places, nor in the same place at different times.

11. The strength of natural magnets differs in those of different magnitudes, but not in proportion to their magnitudes.

12. The strength of a natural magnet is considerably increased by its being armed.

13. Iron acquires a magnetic power by being continually rubbed in the same direction.

14. Iron bars become magnetic by standing a long time nearly upright.

15. The magnetic virtue may be communicated by electricity.

16. A strong blow at one end of a magnetic bar will give it a magnetic power.

17, Fire totally destroys the power of magnets, as well natural as artificial.

The Magnetic Wand.

Bore a hole, three tenths of an inch diameter, through a round stick of wood; or get a hollow cane about eight inches long, and half an inch thick. Provide a small steel rod, and let it be very strongly impregnated with a good magnet; this rod is to be put in the hole you have bored through the wand, and closed at each end by two small ends of ivory that screw on, different in their shapes, that you may better distinguish the poles of the magnetic bar.

When you present the north pole of this wand to the south pole of a magnetic needle, suspended on a pivot, or to a light body swimming on the surface of the water (in which you have placed a magnetic bar,) that body will approach the wand, and present that end which contains the south end of the wand, to the north or south end of the needle, it will recede from it.

The Mysterious Watch.

You desire any person to lend you his watch, and ask him if it will go when laid on the table. He will, no doubt, say it will; in which case, you place it over the end of the magnet, and it will presently stop. You then mark the precise spot where you placed the watch, and moving the point of the magnet, you give the watch to another person, and desire him to make the experiment; in which he not succeeding, you give it to a third (at the same time replacing the magnet) and he will immediately perform it.

This experiment cannot be effected, unless you use a very strongly impregnated magnetic bar, (which may be purchased at the opticians',) and the balance of the watch must be of steel, which may be easily ascertained by previously opening it, and looking at the works.

The Magnetic Dial.

Procure a circle of wood or ivory, of about five or six inches diameter, which must turn quite free on a

stand with a circular border; on the ivory or wood circle fix a pasteboard, on which you place, in proper divisions, the hours, as on a dial. There must be a small groove in the circular frame to receive the pasteboard circle, and observe, that the dial must be made to turn so free, that it may go round without moving the circular border in which it is placed.

Between the pasteboard circle and the bottom of the frame, place a small artificial magnet, that has a hole in its middle. On the outside of the frame, place a small pin, which serves to shew when the magnetic needle is to stop. This needle must turn quite free on its pivot, and its two sides should be in exact equilibrio.

Then provide a small bag, with five or six divisions, like a lady's work bag, but smaller. In one of these divisions put small square pieces of pasteboard, on which are written the numbers from 1 to 12. In each of the other divisions put twelve or more similar pieces, observing that all the pieces in each division must be marked with the same number. The needle being placed upon its pivot, and turned quickly about, it will necessarily stop at that point where the north end of the magnetic bar is placed, and which you previously know, by the situation of the small pin in the circular border.

You then present to any person that division of the bag which contains the several pieces, on which is written the number opposite to the north end of the bar, and tell him to draw any one he pleases. Then placing the needle on the pivot, you turn it quickly about, and it must necessarily stop at that particular number.

The Magnetic Cards.

Draw a pasteboard circle; you then provide yourself with two needles, similar to those used in the foregoing experiment, (which you must distinguish by some private mark) with their opposite points touched with the magnet.

When you place that needle, whose

« ZurückWeiter »