Part 42 (1/2)

Comparison between Magnetism and Static Electricity.

Substances are: { magnetic, { conductors, { non-magnetic. { insulators.

Produced by: induction. friction, or induction.

Theory: molecular. electron. (fluid)

{ attraction, { attraction, Fields of Force { repulsion, { repulsion, Explain: { induction, { induction.

{ action of compa.s.s.

{ magnetoscope, dip, { electroscope, electron, { declination, pole, { positive, negative, { retentivity, { potential, capacity, Terms: { permeability, { condenser, electrophorus, { lodestone, { oscillatory discharge, { magnetic meridian. { lightning.

Likeness: { _a_--produced by induction, _b_--attract both are: { and repel, _c_--have fields of force.

{ _a_--electricity can be _conducted_, { magnetism cannot.

{ Differences: { _b_--electricity in _all substances_, { magnetism in few.

{ { _c_--magnetism with the compa.s.s indicates { direction.

CHAPTER XI

CURRENT ELECTRICITY

(1) ELECTRICAL CURRENTS AND CIRCUITS

=237. Sources of Electric Currents.=--In studying the production and distribution of static electricity it was seen that if two bodies at _different potentials_ are connected by a copper wire a _movement of electricity to the body_ having the _lower potential_ occurred along the conducting wire. This movement of electricity is called an _electric current_ (Art. 227). _A difference of potential_ is therefore often called an _electromotive force_ (E.M.F.), since it produces the movement of electricity in a conductor. The current between two _oppositely charged_ bodies lasts for so short a time as to be of little or no practical value unless some means are found for continually recharging the bodies. That is, some device must be used to restore the difference in potential as fast as the conducting wire equalizes it. The continual charging of the bodies takes work. In other words, it requires a continual expenditure of some form of energy (which is converted into electrical energy) to produce the electric current. Two forms of energy are commonly used for this purpose.

(A) _Chemical energy_ is employed in _voltaic cells_ for producing electric currents. (B) _Mechanical energy_ is used for the same purpose in the _dynamo_ and similar devices.

=238. The voltaic cell= is named after Volta, an Italian physicist, who in 1800 invented it. In its simplest form it consists of a strip of copper and a strip of zinc placed in dilute sulphuric acid (one part acid to fifteen or twenty of water) (Fig. 215). By the use of sensitive apparatus, it can be shown that the copper plate of the voltaic cell has a positive charge and the zinc plate a negative charge. For example, let a flat plate 10 cm. in diameter be placed upon the k.n.o.b of an electroscope and a similar plate, coated with sh.e.l.lac and provided with an insulating handle, be set upon it to form a condenser. (See Fig.

216.) If now wires from the two plates of a simple voltaic cell be respectively connected to the plates of the condenser, charges from the copper and zinc plates will acc.u.mulate upon the two condenser plates.

Now remove the wires and lift the upper plate. The ”bound” charge upon the lower plate will spread over the leaves and cause them to separate.

Upon testing, the charge from the zinc plate will be found to be _negative_ and that from the copper plate, _positive_. Since a positive charge is found upon the copper plate it is called the _positive electrode_; the zinc plate is called the _negative electrode_.

[Ill.u.s.tration: FIG. 215.--Cross-section of a simple voltaic cell.]

[Ill.u.s.tration: FIG. 216.--Testing the charges upon the plates of a simple voltaic cell.]

=239. Test for an Electric Current.=--If the copper and zinc plates of a voltaic cell are connected by a wire, a current of electricity is set up in the conductor. Evidence of the current may be obtained by holding the conducting wire over and parallel to the needle of a magnetoscope.

The needle is deflected by the action of the current parallel to it (Fig. 217). This _magnetic effect_ of a current is the means usually employed for the _detection_ and _measurement_ of an electric current.

Such a device which detects an electric current by its _magnetic effect is called a galvanoscope_, in honor of Galvani, who in 1786 was the first to discover how to produce an electric current.

[Ill.u.s.tration: FIG. 217.--The magnetic needle is deflected by the current.]

[Ill.u.s.tration: FIG. 218.--Diagram of an electric bell circuit.]