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 knob of an electroscope and a similar plate, coated with shellac 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 accumulate 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.
Fig. 215.—Cross-section of a simple voltaic cell.
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.
Fig. 217.—The magnetic needle is deflected by the current.