V. Electromagnetism


The movement of a compass needle, near a conductor through which a current is flowing, indicates the presence of a magnetic field around the conductor. When currents flow through two parallel conductors, the magnetic fields of the conductors attract each other when the current flow is in the same direction in both conductors, and repel each other when the flows are in opposite directions. The magnetic field caused by the current in a single loop or wire is such that if the loop is suspended near the earth, it will behave like a magnet or compass needle and swing until the wire of the loop is perpendicular to a line running from the north and south magnetic poles of the earth.

The magnetic field about a current-carrying conductor can be visualized as spreading radially outward from the conductor in the same manner as ripples created when a stone is dropped into water. The direction of the magnetic lines of force in the field is counterclockwise when observed in the direction in which the electrons are moving. The field is stationary about the conductor so long as the current is flowing steadily through the conductor.

When a conductor moves so as to cut the lines of force of a magnetic field, the field acts on the free electrons in the conductor, displacing them and causing a potential difference and a flow of current in the conductor. The same effect occurs whether a magnetic field is stationary and the wire moves, or the field moves and the wire is stationary. When a current begins to flow in a conductor, a field moves out from the conductor. This field cuts the conductor itself and induces a current in it in the direction opposite to the original flow of current. With a conductor such as a straight piece of wire this effect is very slight, but if the wire is wound into a helical coil, the effect is much increased because the fields from the individual turns of the coil cut the neighboring turns and induce a current in them as well. The result is that such a coil, when connected to a source of potential difference, will impede the flow of current when the potential difference is first applied. Similarly, when the source of potential difference is removed, the magnetic field "collapses," and again the moving lines of force cut the turns of the coil. The current induced under these circumstances is in the same direction as the original current, and the coil tends to continue the flow of current. Because of these properties, a coil resists any change in the flow of current and is said to possess electrical inertia, or inductance. This inertia has little importance in DC circuits, because it is not observed when current is flowing steadily, but it has great importance in AC circuits. See Alternating Currents, below.


VI. Conduction in Liquids and Gases

When an electric current flows in a metallic conductor, the flow is in one direction only, inasmuch as the current is carried entirely by electrons. In liquids and gases, however, a two-directional flow is made possible by the process of ionization. In a liquid solution, the positive ions move through the solution from points of high positive potential to points of low positive potential; the negative ions move in the opposite direction. Similarly, in gases, which may be ionized by radioactivity, by the ultraviolet rays of sunlight, by electromagnetic waves, or by an electric field of high potential gradient, a two-way drift of ions takes place to produce an electric current through the gas. See Electric Lighting.


VII. Sources of Electromotive Force


To produce a flow of current in any electrical circuit, a source of electromotive force or potential difference is necessary. The available sources are as follows: (1) electrostatic machines, which operate on the principle of inducing electric charges by mechanical means ; (2) electromagnetic machines, in which current is generated by mechanically moving conductors through a magnetic field or a number of fields ; (3) voltaic cells, which produce an electromotive force through electrochemical action ; (4) devices that produce electromotive force through the action of heat ; (5) devices that produce electromotive force by the action of light ; and (6) devices that produce electromotive force by means of physical pressure, for example, the piezoelectric crystal.