GLOSSARY

ACTUATOR  - The part of a switch that is acted upon to cause the switch to change contact connections; e.g., toggle, pushbutton, and rocker.

AMMETER  - A meter used to measure current.

ARC EXTINGUISHER  - The part of a circuit breaker that confines and divides the arc that occurs when the contacts of the circuit breaker open.

ARMATURE  - In a relay, the movable portion of the relay.

BREAK  - In a switch, the number of breaks refers to the number of points at which the switch opens the circuit; e.g., single break and double break.

BURNISHING TOOL  - A tool used to clean and polish contacts on a relay.

CONTINUITY  - An uninterrupted, complete path for current.

DAMPING  - The process of smoothing out oscillations. In a meter, damping is used to keep the pointer of the meter from overshooting the correct reading.

D'ARSONVAL METER MOVEMENT  - A name used for the permanent-magnet moving-coil movement used in most meters.

DIRECT SHORT  - A connection between two points in a circuit, such as between a component and ground.

ELECTRODYNAMIC METER MOVEMENT  - A meter movement using fixed field coils and a moving coil; usually used in wattmeters.

ELECTROMAGNETISM  - The relationship between magnetism and electricity.

ELECTROSTATIC METER MOVEMENT  - A meter movement that uses the electrostatic repulsion of two sets of charged plates (one fixed and the other movable). This meter movement reacts to voltage rather than to current and is used to measure high voltage.

FERRULES  - The cylindrical metallic ends of a cartridge fuse.

FREQUENCY METER  - A meter used to measure the frequency of an ac signal.

GALVANOMETER  - A meter used to measure small values of current by electromagnetic or electrodynamic means.

HOT WIRE METER MOVEMENT  - A meter movement that uses the expansion of a heated wire to move the pointer of a meter; measures dc or ac.

IN-CIRCUIT METER  - A meter permanently installed in a circuit; used to monitor circuit operation.

LOADING EFFECT  - The effect of a voltmeter upon the circuit being measured which results in an inaccurate measurement. Loading effect is minimized by using a voltmeter with an internal resistance many times higher than the resistance of the circuit being measured.

MAGNETIC TRIP ELEMENT  - A circuit breaker trip element that uses the increasing magnetic attraction of a coil with increased current to open the circuit.

MEGGER  - Common name for a megohmmeter.

MEGOHMMETER  - A meter that measures very large values of resistance; usually used to check for insulation breakdown in wires.

METER  - A device used to measure an electrical quantity; e.g., current, voltage, and frequency.

METER MOVEMENT  - The part of a meter that moves.

MOVING-IRON METER MOVEMENT  - Same as moving-vane meter movement.

MOVING-VANE METER MOVEMENT  - A meter movement that uses the magnetic repulsion of the like poles created in iron vanes by current through a coil of wire; most commonly used movement for ac meters.

MULTIMETER  - A single meter combining the functions of an ammeter, a voltmeter, and an ohmmeter.

NONTRIP-FREE CIRCUIT BREAKER  - A circuit breaker that can be held ON during an overcurrent condition.

OHMMETER  - A meter used to measure resistance.

OUT-OF-CIRCUIT METER  - A meter which is not permanently installed in a circuit. Usually portable and self-contained, these meters are used to check the operation of a circuit or to isolate troubles within a circuit.

PARALLAX ERROR  - The error in meter readings that results when you look at a meter from some position other than directly in line with the pointer and meter face. A mirror mounted on the meter face aids in eliminating parallax error.

POINT BENDER  - A tool used to adjust the contact spacing on a relay.

POLE  - (1) One end of a magnet. (2) The number of points at which current can enter a switch; e.g., single pole, double pole, and three pole.

POLE PIECE  - A piece of ferromagnetic material used to control the distribution of magnetic lines of force; i.e., concentrate the lines of force in a particular place or evenly distribute the lines of force over a wide area.

RANGES  - The several upper limits a meter will measure as selectable by a switch or by jacks; e.g., a voltmeter may have ranges of 1 volt, 2.5 volts, 10 volts, 25 volts, and 100 volts.

RECTIFIER  - A device used to convert ac to pulsating dc.

RELAY  - An electromagnetic device with one or more sets of contacts which changes position by the magnetic attraction of a coil to an armature.

RELUCTANCE  - The resistance of a magnetic path to the flow of magnetic lines of force through it.

ROTARY SWITCH  - A multicontact switch with contacts arranged in a circular or semi-circular manner.

SENSITIVITY  - (1) For an ammeter: the amount of current that will cause full-scale deflection of the meter. (2) For a voltmeter: the ratio of the voltmeter resistance divided by the full-scale reading of the meter, expressed in ohms-per-volt.

SHORT CIRCUIT  - An unintentional current path between two components in a circuit or between a component and ground which is usually caused by a malfunction in the circuit.

SHUNT RESISTOR  - A resistor in parallel. In an ammeter, shunt resistors are used to provide range capability.

SNAP-ACTING  - Changing position quickly with the aid of a spring.

SOLENOID  - An electromagnetic device that changes electrical energy into mechanical motion; based upon the attraction of a movable iron plunger to the core of an electromagnet.

SWITCH  - A device used to open or close a circuit.

TEST EQUIPMENT  - A general term applied to devices used to test electrical and electronic circuits.

THERMAL TRIP ELEMENT  - A circuit breaker trip element that uses the increased bending of a bimetallic strip caused by increased current to open a circuit.

THERMAL-MAGNETIC TRIP ELEMENT  - A single circuit breaker trip element that combines the action of a thermal and a magnetic trip element.

THERMOCOUPLE METER MOVEMENT  - A meter movement that uses the current induced in a thermocouple by the heating of a resistive element to measure the current in a circuit; used to measure ac or dc.

THROW  - In a switch, the number of different circuits each pole can control; e.g., single throw and double throw.

TRIP-ELEMENT  - The part of a circuit breaker that senses any overload condition and causes the circuit breaker to open the circuit.

TRIP-FREE CIRCUIT BREAKER  - A circuit breaker that will open a circuit even if the operating 5 mechanism is held in the ON position.

TROUBLESHOOTING  - The process of locating and repairing faults in electrical or electronic equipment.

VOLTMETER  - A meter used to measure voltage.

WAFER SWITCH  - A rotary switch in which the contacts are arranged on levels. Each level is electrically independent but mechanically connected by the shaft of the switch.

WATT-HOUR METER  - A meter used to measure electrical energy. 

WATTMETER  - A meter used to measure electrical power.

LAWS OF EXPONENTS

The International Symbols Committee has adopted prefixes for denoting decimal multiples of units. The National Bureau of Standards has followed the recommendations of this committee, and has adopted the following list of prefixes:

To multiply like (with same base) exponential quantities, add the exponents.

In the language of algebra the rule is a^m X aⁿ = a^m+n

To divide exponential quantities, subtract the exponents. In the language of algebra the rule is

To raise an exponential quantity to a power, multiply the exponents.

In the language of algebra (x^m)ⁿ = x^mn.

Any number (except zero) raised to the zero power is one. In the language of algebra x⁰ = 1

Any base with a negative exponent is equal to 1 divided by the base with an equal positive exponent. In the language of algebra x^-a = 1/x^a

To raise a product to a power, raise each factor of the product to that power.

To find the nth root of an exponential quantity, divide the exponent by the index of the root. Thus, the nth root of a^m = a^m/n.

SCHEMATIC SYMBOLS