For an electromagnet, the strength of the magnetic field depends on how much current flows in the turns of the coil. The coil serves as a bar magnet with opposite poles at the ends.
The strength of an electromagnet is specified in SI units of ampere-turns. The product of amperes (A) and turns (t) indicates the magnetomotive force (mmf) of the coil. The cgs unit of mmf is the gilbert (Gb).
The field intensity, H, of a coil specifies the mmf per unit length. The units of H are A · t/m and the oersted.
The permeability of a material indicates its ability to concentrate magnetic flux.
Demagnetization of a magnetic material is also known as degaussing.
Current in a straight conductor has an associated magnetic field with circular lines of force in a plane perpendicular to the conductor. The direction of the circular field is counterclockwise when you look along the conductor in the direction of electron flow.
The left-hand rule for determining the polarity of an electromagnet says that when your fingers curl around the turns in the direction of electron flow, the thumb points to the north pole.
Motor action is the motion that results from the net force of two fields that can aid or cancel each other. The direction of the resultant force is always from the stronger field to the weaker field.
Generator action refers to induced voltage. For N turns vind = N(dφ/dt), where dφ/dt stands for the change in flux (φ) in time (t). The change is given in webers per second. There must be a change in flux to produce induced voltage.
Lenz's law states that the direction of an induced current must be such that its own magnetic field will oppose the action that produced the induced current.
The switching contacts of an electromechanical relay may be either normally open (NO) or normally closed (NC). The contacts are held in their normal or resting positions by springs or some gravity-actuated mechanism.
The movable arm on a relay is called the armature. The armature is magnetic and has contacts that open or close with other contacts when the relay is energized.
The pickup current of a relay is the minimum amount of relay coil current that will energize the relay. The holding current is the minimum relay coil current required to keep a relay energized.
