Bundles of Magnets.-Armatures.

357. A BUNDLE OF MAGNETS consists of a group of magnetized bars united, so that their poles of the same name may be coincident.

Sometimes these bundles are composed of straight bars, like that shown in Fig. 245, and sometimes they are curved in the shape of a horse-shoe, as shown in Fig. 247.

Magnets, if abandoned to themselves, would lose in a short time much of their power; hence it is, that armatures are employed.

An ARMATURE is a piece of soft iron, placed in contact with the poles of a magnet. Thus, ab, in Fig. 247, is an

armature.

(357.) What is a Bundle of Magnets? What is an Armature?

The poles, acting by induction upon the armature, convert

a into an austral, and b into a

boreal pole. These two poles reacting upon the poles of the magnet, AB, prevent the recomposition of the two fluids, and thus preserve its magnetism. The armature is sometimes called a keeper.

If weights be attached to the keeper till it separates from the magnet, we can, from the number of pounds applied, judge of the power of the mag

net.

For many kinds of magnetic experiment the horse-shoe form is. preferable. It is also the form best adapted to the application of an armature or keeper.

The most powerful horse-shoe magnets are formed by means of electrical currents. Magnets of this kind have been constructed by Prof. HENRY, of the Smithsonian Institution, capable of sustaining a weight of more than a ton and a quarter.

Fig. 247.

A keeper? How can we judge of the power of a magnet? What are the advantages of the horse-shoe magnet?

358. ELECTRICITY, as a science, is that branch of Physics which treats of the laws of attraction and repulsion exhibited by bodies under certain circumstances. Such phenomena are called electrical phenomena. The name electricity is derived from the Greek elektron, which means amber.

Discovery of Electrical Properties.

359. Six hundred years before the commencement of the Christian era, THALES, of Miletus, knew that when yellow amber was vigorously rubbed with wool, it acquired the property of attracting light bodies, such as small pieces of paper, barbs of quills, straws and the like. Comparing this action to suction, the ancients said that amber had a power of suction, and sucked light bodies towards it. In consequence of the rarity of amber, whose origin is even in our day unknown, they went so far as to say, that it was formed from the tears of an Indian bird, grieved at the death of King MELEAGER.

Six centuries later, PLINY, an eminent Roman naturalist, writes: "When the friction of the fingers imparts heat and life to yellow amber, it attracts straws, just as the magnet attracts iron." This

(358.) Define Electricity as a science. What are electrical phenomena? Whence the name? (359.) Give an outline of the history of electrical discoveries.

was all of the knowledge had on the subject until the end of the sixteenth century, when WILLIAM GILBERT, an Englishman, called anew the attention of scientific men to the properties of amber, and showed that a great number of other substances, such as glass, resin, silk, sulphur, and the like, acquired the power of attracting light bodies, on being rubbed with woolen cloth or cat's skin.

To repeat these experiments, rub a tube of glass or a stick of sealing-wax with a piece of woolen cloth, then present them to light bodies, as shreds of gold leaf, barbs of quills, or fragments of paper, and the latter will be seen to approach and adhere to the excited glass or sealing-wax. The manner of making these experiments is indicated in Fig. 248.

of the seventeenth century the progress of discovery in electricity has been rapid, and a multitude of new facts have been developed, which have been so well studied as to form a very extensive branch of natural science.

Sources of Electricity.

360. The sources of electricity may be divided into three classes: Mechanical, Physical, and Chemical. The mechanical sources are: friction, pressure, and separation of the molecules of bodies. When a piece of sugar is broken suddenly in a dark room, a feeble light is observable,

Explain GILBERT's experiments, and the manner of making them. (360.) What are the principal sources of electricity? What are the mechanical sources ?

which is due to the development of electricity at the moment of separating the molecules.

The physical sources are variations of temperature, and the like. Some minerals, particularly tourmaline and topaz, manifest electrical phenomena on being heated or cooled.

The chemical sources are chemical compositions and decompositions of bodies. Metals, like zinc, iron, and copper, when plunged into acids, are attacked by them, forming compounds known as salts. During these combinations considerable quantities of electricity are developed.

The most powerful of the causes of electricity are friction and chemical action. These will be studied in their order.

Electroscope.-Electrical Pendulum.

361. An ELECTROSCOPE is an apparatus for showing when a body is electrified.

The most simple electroscope is the ELECTRICAL PENDULUM, which consists of a small ball of elder pith, suspended by a fine silk thread, as shown in Fig. 249. The thread is fastened to the upper end of a stem of copper, which stem has a support of glass.

To ascertain whether a body is electrified or not, the pendulum is presented to it; if it is electrified, the pith ball will be attracted, otherwise not. When the quantity of electricity is too small to produce sensible attraction upon the pith ball, more delicate instruments are sometimes employed, called electrometers.

Two kinds of Electricity.

362. That there are two kinds of electricity, may be shown by the action of glass and resinous bodies, after being rubbed, upon pith balls.

What is the chief physical source? The chemical sources ? What is the most powerful cause of electricity? (361.) What is an Electroscope? Describe the Electrical Pendulum. How used? (362.) How many kinds of electricity are there?