The Boy's Playbook of Science Part 31

[Ill.u.s.tration: Fig. 239. The break is removed, and the double blades, B, fixed in its place. The bra.s.s cup, A, containing mercury is so adjusted that the points will leave the surface of the mercury when the armature is vertical. Ether or alcohol poured on the surface is quickly inflamed by the electric spark.]

With the a.s.sistance of the magneto-electric machine, telegraphic communication may be conducted without the a.s.sistance of a battery. It has also been applied to the art of electro-plating by Mr. J. P.

Woolrich, of Birmingham; and whilst visiting that place, the author had the opportunity of witnessing the arrangement employed.

It consists of a very powerful magneto-electric machine turned by a steam-engine, and connected with the large troughs containing the silvering solution. If it is required to deposit a thin coating of silver on the article, a short period suffices for the action of the machine, whilst a thick deposit of the precious metal is only obtained by the constant operation of the magnets for several hours. At Mr.

Woolrich's factory, the goods which were being coated with silver were all kept in motion, moving slowly backwards and forwards in the trough by means of an eccentric connected [Page 246] with the same steam-engine that worked the electro-magnetic machine. (Fig. 240.)

[Ill.u.s.tration: Fig. 240. Silvering and plating by the magneto machine, turned by a steam-engine.]

The magneto-electric telegraph patented by Mr. Henley in 1848, offers another example of the application of the electric current induced in electro-magnetic coils, when they rotate in close proximity to the poles of a powerful steel magnet. This telegraph is now in constant use by the English and Irish Magnetic Telegraph Company, through a distance of more than 2100 miles. The whole length of wires in use amounts to the astonishing quant.i.ty of 13,900 miles, of which 6350 miles are hidden underground, and 7500 conducted above.

This telegraph is considered to be one of the simplest and most economical yet brought into practical working.

[Page 247]

CHAPTER XX.

DIA-MAGNETISM.

At the end of the chapter devoted to the subject of light, will be found an experiment devised and carried out by Dr. Faraday, in which it is shown that if a bar of a peculiar gla.s.s (called after the inventor, _Faraday's heavy gla.s.s_, or silicated borate of lead) is subjected to the inductive action of a very powerful electro-magnet, that it has the power of changing the direction of a ray of polarized light transmitted through it. This effect is not confined to the poles of an electro-magnet, but is also perceptible (though in a diminished degree) with ordinary magnets.

The result of this important experiment was communicated to the Royal Society by Dr. Faraday on the 27th November, 1845, the enunciation of the fact by this learned philosopher being, "that when '_the line of magnetic force_' is made to pa.s.s through certain transparent bodies parallel to a ray of polarized light traversing the same body, the ray of polarized light experiences a rotation." Now, "_the line of magnetic force_" means that continual flow of the magnetic current which pa.s.ses from pole to pole, and is indicated by iron filings sprinkled on paper placed above the poles of a magnet, and usually termed _magnetic curves_, or the curved lines of magnetic force. (Fig. 241.)

[Ill.u.s.tration: Fig. 241. The curved lines of magnetic force.]

The heavy gla.s.s already alluded to, upon which the magnet exerts a certain influence, is called

THE DIA-MAGNETIC;

and by this term is meant a body through which the lines of magnetic force are pa.s.sing without affecting it like iron or steel. At page 212 is a picture representing (at Figs. 201 and 202) the direction of the electricity and that of the magnetic current or whirl at right angles to it. If, then, Fig. 202 be considered as a piece of gla.s.s, the arrow A B [Page 248] will show "the line of magnetic force," the point B being the north pole, and the shaft A the south pole of the magnet, and the arrows traced round will represent direction. This simple drawing expresses the whole of the law of the action of the magnet on the gla.s.s, and if kept in view, will give every position and consequence of direction resulting from it.

The phenomenon of the affection of the beam of polarized light is immediately connected with the magnetic force, and this is supposed to be proved by the _brightness_ of the polarized ray being developed _gradually_, as the iron coiled with wire requires about two seconds to acquire its greatest power after being connected with the battery.

In another experiment of Faraday's, where a beam of polarized light was sent through a long gla.s.s tube containing water, and introduced as a core _inside_ a powerful electro-magnetic coil, the image of a candle viewed with a proper eye-piece, appeared or disappeared as the battery connexion was made or broken with the coil; but this result is not considered by many philosophers to be conclusive of the action of magnetism on light, but rather as an alteration of the _refracting_ power of the medium through which the light pa.s.ses. These experiments were the precursors of the other effects of magnetism upon different kinds of matter which Faraday discovered, and he commenced his examination with a small bar of heavy gla.s.s suspended by a filament of silk between the poles of an electro-magnet, and when the twisting or effects of torsion had ceased, the battery was connected. Directly the current pa.s.sed, Faraday's keen eye detected a movement of the gla.s.s, and on repeating the experiment, he discovered that the movement was not accidental, but always took place in a certain fixed direction--viz., a direction at right angles to a line drawn across and touching the two poles of a horse-shoe-shaped magnet--_i.e._, supposing the feeder or bit of soft iron usually placed in contact with the poles of the horse-shoe-magnet to represent the "_axial line_," any line drawn across it at right angles would be called the _equatorial line_, whilst the general s.p.a.ce included between the poles of the magnet is called "the _magnetic field_." The movement of the heavy gla.s.s was therefore _equatorial_, and it pointed east and west instead of north and south, like iron and steel.

[Ill.u.s.tration: Fig. 242. A cube of copper suspended between the poles of a powerful electro-magnet.]

By the use of the apparatus (Fig. 242) Faraday proved that every [Page 249] substance, whether solid, fluid, or gaseous, was subject to magnetic influences, a.s.suming either the axial or equatorial position.

The apparatus consists of a prolongation of the poles of a powerful electro-magnet, between which _the_ cube of copper, weighing from a quarter to half a pound, suspended by a thread, may be set spinning or rotating. If the electro-magnet is connected with the battery, the cube stops immediately, and whilst still in the same position or in the _magnetic field_, with the magnet in full action, it is impossible to set it spinning or twisting round again. (Fig. 242.)

A large number of other substances, solid, liquid, and gaseous, were submitted to the action of the magnet, the liquids and gases being hermetically sealed in gla.s.s tubes, and some of the results are detailed in the following list:

_Bodies that point axially, or are paramagnetic, like a suspended needle._

Iron.

Nickel.

Cobalt.

Manganese.

Chromium.

Cerium.

t.i.tanium.

Palladium.

Platinum.

Osmium.

Paper.

Sealing-wax.

Fluor spar.

_Peroxide of lead._ Plumbago.

China ink.

Berlin Porcelain.

_Red-lead._ Sulphate of zinc.

Sh.e.l.l-lac.

Silkworm-gut.

Asbestos.

Vermilion.

Tourmaline.

Charcoal.

All salts of iron, when the latter is basic.

Oxide of t.i.tanium.

Oxide of chromium.

Chromic acid.

Salts of manganese.

Salts of chromium.

Oxygen, which stands alone as a paramagnetic gas.

_Bodies that point equatorially, or are diamagnetic, like Faraday's heavy gla.s.s._

Bis.m.u.th.

Antimony.

Zinc.

Tin.

Cadmium.

Sodium.

Mercury.

_Lead._ Silver.

Copper.

Gold.

a.r.s.enic.

Uranium.

Rhodium.

Iridium.

Tungsten.

Rock crystal.

The mineral acids.