223. An iron bar may be rendered magnetic more readily by various processes, technically termed touches, all depending upon inductive action (213). The simplest mode, is to

pass a pole of a magnet over the whole length of a bar of iron or steel, of course always in the same direction; the end of the bar last touched by the boreal pole of the magnet becoming an austral pole. process of the single touch.

A

C

B

This is usually termed the Another and convenient mode, is to join the opposite poles of two magnets, AB, to place them over the centre of the bar of iron c, and to separate AB from each other, draw

ing them in contrary directions over c. They are then removed, again placed together, and reapplied to c, once more separated, and so on, the bar c ultimately acquiring a very energetic degree of magnetic intensity. The process of the separate touch is somewhat similar to the last, except that the ends of the bar c, rest upon the opposite poles of two sets of magnetic bars made by fastening four or five together, with their poles in the same direction. A and B are, instead of simple bars, similar compound magnets, not lying on the bar c, but elevated at an angle of about twenty-five or thirty degrees; they are united and separated, by drawing them to the opposite ends of the bar c, as in the last-described process. In the process of Opinus, or the double touch, the bars are similarly placed, as in the separate touch last described, but the magnetising bars are inclined at an angle of fifteen or twenty degrees, and not separated; but moved from the middle to the ends of the bar of iron backwards and forwards, commencing and ending the friction in the middle. In the following figure, AB is the bar to be magnetised, ns and n`s',

the fixed magnets on which it rests, and NS, N's the moveable magnets, kept asunder at sN' by a small piece of wood: by this process very thick bars may be readily magnetised. The magnets employed in these processes do not give up any portion of their fluids to the bars, they are used merely to excite in the manner already explained (212); as each particle of magnetic fluid is firmly tied to the atom of iron to which it belongs, and consequently they do not suffer by the process. 224. Magnetic bars are sometimes bent into the shape of the

letter u, and are then termed horse-shoe magnets, and several are not unfrequently fastened together, with their similar poles, in the same direction, constituting a battery of magnets. In this case they are peculiarly fitted for lifting heavy weights, as, by applying a bar of soft iron, A, to their poles, it becomes by inductive action (211), a magnet, and will adhere to the poles

with a very considerable force. In constructing magnets, it is usual to draw, with a file, a line on that end of the bar which it is intended to convert into an austral pole, or that which, if freely suspended, would point towards the north pole of the earth.

225. Magnets, if left to themselves, gradually and in a space of time varying with the hardness of the metal composing them, lose their magnetic properties, from the recombination of the separated fluids. This is prevented by keeping their poles united, by means of a piece of soft iron, which, becoming magnetic by induction, reacts on the magnetism free in the poles of the magnetic bar, and tends to increase instead of diminishing their intensity. The power by which a bar of iron, or steel, retains its magnetic state is termed its coercing force.

226. The coercing force (225) of the other magnetic metals, especially nickel, is not so energetic as that of iron, according to the experiments of Biot. The bars used for these researches were prepared by Baron Thenard, and

were as free from iron as the chemical skill of that philo sopher could render them: M. Biot found that the magnetic intensity of bars of steel and nickel, of the same size, were to each other as 0.002215 to 0.000684, the intensity of the steel magnet being more than three times as great as that of nickel. The magnetic intensity of cobalt has not been examined so carefully as that of nickel; indeed, it is doubtful whether the supposed magnetic properties of that metal, as well of chrome, titanium, and manganese, may not depend upon the presence of small traces of iron.

227. The magnetic intensity of the earth itself, is by no means constant, undergoing frequent variations in different parts of the world, as well as at different times in the same place. The comparative intensities of its magnetism may be observed by noticing the time required to complete a given number of oscillations of a magnetic needle, when a pole of the needle is thrust out of the magnetic meridian, as the intensity of the forces acting on a needle are to each other, as the squares of the number of oscillations performed in a given time.

On this subject I would refer the student to the late valuable report of Major Sabine, (Seventh Report, Brit. Assoc. 1838.)

228. A beautiful illustration of the mode of determining the intensity of forces acting on a needle, by the number of oscillations it performs in a given time, is found in the demonstration of the law of intensity of magnetic action, for which among a host of other invaluable investigations, science is indebted to M. Coulomb. A small needle suspended by a single thread, and protected from the influence of aerial currents, performed fifteen oscillations in one minute; let the directive force (220) of the earth producing these be called m. A long steel magnet placed in the magnetic meridian, had one pole approached to the distance of four inches from the needle, the latter made forty-one oscillations in one minute; the force thus exerted may be called m'. On

FORMULA FOR MAGNETIC INTENSITY.

159

removing the pole to eight inches from the needle, the latter made twenty-four oscillations in the same time; this force may be represented by m". The action of the magnet on theneedle, in the first experiment, is m'―m, and in the second m"-m, because its effects resulted from its own force plus the attraction of the earth, thus,

here, in the second experiment, when the distance of the needle from the pole was twice that of the first experiment, the magnetic intensity was found to be diminished, as nearly in accordance with the law, viz. of its being inversely as the squares of the distances, as experimental investigation could be expected to approach.

229. Artificial magnets have been constructed by reducing to powder the native magnetic oxide, and forming it into bars with wax and oil. They may also be constructed by forming the artificially prepared black oxide of iron, into bars with wax, and magnetising them by one of the processes already described (223).

A great number of mineral and even organic matters appear to exert a certain amount of action of low intensity, on the magnetic needle, so that, from the researches of Coulomb, it appears probable that almost every substance in nature is capable of assuming a faint and transient degree of polarity.

CHAPTER XI.

ELECTRICITY.

(PRIMARY PHENOMENA.)

Excitation, 230. Attraction and Repulsion, 231-3. Conductors, Insulation, 234-5. Natural State of the two Electricities, 236-7. Electroscopes, or Electrometers, 238-241. Excitation of different Substances, 212-4. Spark, 245-6. Superficial diffusion of Free Electricity, 246-8. Induction, 248-253. Electrophorus, 254. Electrolasmus, 255, Electric tension, 256-7. Electrostatic Laws, 258.

230. If a thick glass tube, previously made dry and warm, be briskly rubbed, for a few seconds, with a piece of silk or woollen cloth, also dry and warm, and then held near small pieces of paper or cork, placed on the table, these light substances will leap towards the excited tube, being attracted by it; and, after adhering to its surface for a short time, will be repelled towards the table, after touching which they will be again attracted by the tube; and these phenomena will be repeated, until the electricity excited on the surface of the glass vanishes. A piece of amber, sulphur, or sealing-wax, after excitation by a woollen cloth, will exhibit the phenomenon of attracting light bodies, like the glass tube.

231. Suspend a light ball of pith of elder, or cork, by a long silken thread from the ceiling, or any convenient support, and approach towards it an excited glass tube, the ball will be attracted, and, after adhering for a short time to the tube, will be repelled to a considerable distance, nor will it be again attracted until it has touched some substance connected with the earth, and thus given up the electricity it had acquired from the tube.