The discharge of electricity that takes place when a spark passes between the coatings of a Leyden jar is an electric current. The time during which the current passes, however, is very short. Continuous currents are produced whenever a conductor connects two points at which a difference of potential is constantly maintained.

Demonstrations. - Make a solution of sulphuric acid (H₂SO₄) by pouring slowly 5 c.c. of the acid into 100 c.c. of water. Place a strip of copper and a strip of zinc in the jar containing the dilute acid. As long as the plates are separated, the only action that takes place is the formation of a few hydrogen bubbles on the surface of the zinc plate. But as soon as the two plates are brought in contact - which may be done by bending the plate as in Fig. 1- there is a rapid giving off of hydrogen from the surface of the copper plate Solder a copper wire to a thin sheet of copper, and attach a binding post to a zinc battery plate. Repeat the preceding demonstration with these plates and observe that the change in the formation of gas takes place just when the copper wire is attached to the binding post on the zinc plate.

Disconnect, bring the wire over and parallel to a magnetic needle, and the needle will be deflected as soon as the wire is again touched to the binding post. The needle swings back to its original position as soon as the wire is disconnected, or when either plate is lifted out of the solution.

In the last demonstration the deflection of the needle is caused by the passage of an electric current. As soon as the plates are joined by a wire, as in Fig. 2, the current passes in the external circuit from the copper to the zinc, while the difference of potential is maintained by continued chemical action in the cell.

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مواضيع ذات صلة

​Arc Lighting

Electric Welding

Electric Railways

The Incandescent Circuit

The Transmitter

The Telephone

Thermoelectric Currents

The Transmission of Electrical Energy

True Metal Filament Lamps

Telephone Line Circuit

The Inclosed Arc Lamp has

The Resistance of a Cell