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ELECTRICITY

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ELECTRICITY

5. Stephen  Gray   found   that   he   could charge a conductor by simply bringing it into

The phenomenon of electrostatic induction

the field of another charged body. If now the conductor be connected to earth and immediately afterward disconnected, it will be charged with electrification opposite in sign to that of the original charge. This is known as the phenomenon of electrostatic induction, and will be clear from Fig, 2.

6. A sixth important fact was thoroughly established by Faraday: When electrification is produced by friction, by induction or by any other means, the

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Maxwell's method  of   protection   against   lightning

positive and negative charges so produced are always equal.

7. Faraday also established the fact that the charge on the outside of any closed conductor is distributed in such a way that it produces no electric field—no electric force—inside the closed conductor. It was with this principle in mind that Maxwell suggested, as the best method for protecting houses from lightning, to place them in a network of conductors (see fig- 3) which shall as nearly as possible be equivalent to a closed conductor. For more extended description of the phenomena of electrostatics see S. P. Thompson's Elementary Lessons in Electricity and Magnetism. For a simple and interesting mathematical discussion of the subject see J. J. Thomson's Elements of Electricity and Magnetism.

II. ELECTRIC  CURRENTS

If two charged conductors are connected by means of a wire, it is observed that in

general the wire and the region about it acquire new and remarkable properties. The wire itself becomes warmer, and the region about it becomes a magnetic field. A wire which possesses these properties is said to have an electric current flowing through it. For methods of detecting the presence of an electric current in any circuit see GALVANOMETER.

There are many methods of producing electric currents but the most important ones from a practical point of view are the three following: the method of Galvani and Volt a—the so-called voltaic cell; the method of Faraday—cutting lines of magnetic force — exemplified in the ordinary dynamo; and the thermo-electric method discovered by Seebeck.

THE VOLTAIC  CELL

Volta discovered in 1800 that, by joining in series two conductors of the first class (see ELECTROLYSIS) and one conductor of the second class, he could obtain a continuous electric current at the expense of t;he chemical energy stored up in the liquid and the metals. Two conductors of the second class, together with one of the first class, will also produce a continuous current. Such a combination as either of the preceding is called a voltaic cell. A typical cell of this kind is one in which a strip of zinc is immersed in a solution of zinc-sulphate and a copper strip in a solution of copper-sulphate, the two solutions being separated by a porous partition. It is then found that, on joining the copper and zinc terminals with a wire, the positive electricity flows along the wire from the copper to the zinc. Accordingly, the copper is called the positive pole and the zinc the negative pole of the battery. Since, however, inside the cell the current flows from the zinc to the copper, the zinc-plate is called the positivee lectrode and the copperplate the negative electrode. (See ELECTROLYSIS.)

There is a great variety of cells; some adapted to one purpose, some to others. The one just described is known as the daniell cell. In the storage-cell the electrodes are of lead and lead-peroxide respectively, and the electrolyte is dilute sulphuric acid. The lead-plate is the negative one; the lead-peroxide is the positive one. When the circuit is closed, the lead-peroxide plate gives up a part of its oxygen, while the lead-plate, the negative electrode, becomes oxidized, until finally the two electrodes become very much alike and the current, therefore, becomes less and less, other things being the same. The battery is now said to be discharged. For two electrodes which are alike never give a current when immersed in any one electrolyte. But in order to put the cell again in good working shape, it is necessary only