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ELECTRICITY

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ELECTRICITY

to pass an electric current through the cell in a direction opposite to that obtained when the cell was in use. This process is known as charging the cell. The charging simply restores the oxygen to the positive plate, and removes the oxygen from the negative plate, leaving it spongy, metallic lead.

The following two features distinguish this from most other voltaic cells: The plates are prepared, i.e., made chemically different, by electrical means. They are capable of yielding currents which are enormous when compared with those from other forms of batteries.

INDUCTION OP  ELECTRIC  CURRENTS

Faraday in 1831 succeeded, by a beautifully simple method, in producing electric currents without the use of the voltaic cell and without the use of heat.

If we consider any closed circuit (say a piece of copper wire bent into the form of a circle or square) placed in a magnetic field, we may think of it as containing or inclosing a number of lines of force, as a fence might inclose a group of trees. In Fig. 4 six lines of force are represented as inclosed by the wire-circle. If by any means the magnetic field were made twice as strong, we should represent this change by drawing the lines of force twice as close together, so that now the circle would inclose twelve instead of six lines. Or, if the copper-circle were carried parallel to itself to another region where the magnetic field were not so strong, then we should describe this change by drawing fewer lines of force through the circle. Faraday's discovery was simply this: "When, by any means whatever, the total number of lines of force passing through any circuit is changed, an electric current is produced in that circuit.'' Such a current is called an induced current. This discovery of Faraday's is frequently, and perhaps fairly, considered the most important one ever made in electrical science; since upon this principle are constructed the dynamo, the telephone, the transformer, etc. See articles on these three subjects.

THERMO-ELECTRIC CURRENTS

Seebeck in 1821 discovered that, when a closed circuit is made up of two different metals and the two junctions of the metals are kept at different temperatures, an electric current flows around the circuit. The source of energy which supplies this current is the heat absorbed at the hot junction. The presence of such an electric current as this in a circuit is one of the most deli-

cate tests known for a difference of temperature between the two junctions.

EFFECTS  OF   ELECTRIC   CURRENTS

1. Chemical Effects. (See ELECTROLYSIS.) One of the chemical effects of an electric current has been employed to  define the size of the unit of current,  for the international congress of electricians at Chicago in  1893   agreed to define the unit-current as one which deposits silver at the rate of 0.001118  grams per second. This unit is called an ampere. See AMPERE.

2. Heating  Effects. Everyone who  has seen an incandescent lamp knows that an electric current in passing through a carbon filament may raise its temperature to such a point that the carbon becomes white-hot. Joule, the English physicist, showed that  the   amount   of   heat   developed   between any two points in a conductor varies directly as the square of the current, directly as   the   time   and   directly   as   a   constant depending upon the shape, size and chemical composition of the wire. Let us denote the heat developed by H, the current by C, the time by t and the constant for any conductor by R.    Then by properly choosing this constant we may write:

H = C2R*.

The constant R thus defined is known as the electrical resistance of the wire. The resistance of a wire may then be considered as a quantity which determines how much heat will be developed in it in a given time by a given current. The size of the unit of resistance is that offered by a column of mercury of uniform cross-section, at O°C, when its length is 106.3 centimeters and its mass is 14.4521 grams. This unit is called an ohm.

3. Magnetic  Effects. About  every  wire conveying   a  current   is   a  magnetic   field. This field may be represented by drawing closed lines of force including the current. It is these lines of force which magnetize a piece of iron placed in a coil of wire carrying a current. The behavior of the electromagnet and the telegraph-sounder depends upon this  principle. An exceedingly  important inference from this magnetic effect is the fact that an electric current, in any movable   circuit,   always   sets   itself   so   as to include as many lines of force as possible. This is the principle of the instrument which  Lord   Kelvin   devised   in   1867   for receiving messages over the Atlantic cable. It is, indeed, the principle of all galvanometers with movable coils and of all electric motors.

OHM'S LAW

A dynamo or a voltaic cell may be considered as an instrument for driving an electric current through a circuit or, in short, as an instrument for producing electrical pressure. Ohm, a German mathematician, showed in 1827 that

Magnetic Line of Force inclosed by an Electric Current