Definitions [Handbook of Power Resistors (1959)]


Adjustable Resistor--An adjustable resistor is a resistor so constructed that its resistance can be readily changed.

Adjustohm--A Vitrohm enameled resistor with a bare side and clamp for adjustment.

Alternating Current--An alternating current is a periodic current the average value of which over a period is zero. The equation for alternating current is the same as that for a periodic current except that lo= 0* Ambient Temperature - Ambient temperature is the temperature of the surrounding cooling medium, such as gas or liquid, which comes into contact with heated parts of the apparatus. Ampere--The ampere is the constant current which, maintained in two parallel rectilinear conductors of infinite length separated by a distance of 1 meter, produces between these conductors a force equal to 2 x 10^-7 mks (meter-kilogram-second) units of force per meter of length.

Armature Resistor-A resistor connected in series with the armature of a motor either to limit the inrush current on starting, the gradual short circuiting of which brings the motor to normal speed, or to regulate the speed by armature-voltage control.

Axiohm--A Vitrohm resistor with axial lead terminals.

Barohm-- Light-weight continuous-duty high-current resistor consisting basically of a channeled resistance alloy ribbon of uniform cross-section, formed in a series of flat turns secured at the middle of their short axes to specially treated asbestos-cement sup port board.

ASA Standard

Ward Leonard Trade Name

Convection-Convection is the motion resulting in a fluid owing to differences of density and the action of gravity.

Creepage Distance--Creepage distance is the shortest distance between conductors of opposite polarity or between a live part and ground as measured over the surface of the supporting material.

Current-Limiting Resistor--A current-limiting resistor is a resistor inserted in an electric circuit to limit the flow of current to some predetermined value.

Note: A current-limiting resistor, usually in series with a fuse or circuit breaker, may be employed to limit the flow of circuit or system energy at the time of a fault or short-circuit.* Dielectric Strength-The dielectric strength of an insulating material is the maximum potential gradient that the material can withstand without rupture. It is usually specified in volts per unit thickness.

Dielectric Tests--Dielectric tests are tests which consist of the application of a voltage higher than the rated voltage for a specified time for the purpose of determining the adequacy against breakdown of insulating materials and spacings under normal conditions.*

Direct Current-A direct current is an unidirectional current in which the changes in value are either zero or so small that they may be neglected. A given current would be considered a direct current in some applications, but would not necessarily be so considered in other applications.*

Discohm--Compact low-wattage non-inductive Vitrohm resistor having a flat circular ceramic base on which resistance wire is set and embedded in fused-on vitreous enamel.

ASA---Standard Ward Leonard Trade Name

Edgeohm--High-current resistor made of an alloy resistance ribbon wound on edge forming an oval shaped coil supported by grooved insulators which space adjacent turns and insulate them from the support bars. Support bars are secured to steel end pieces forming a sturdy resistor suitable for continuous-and-intermittent-duty applications.

EIA--EIA is the Electronic Industries Association.

Electromotive Force-The electromotive force is the agency causing the flow of current in a circuit. It is the electrical pressure ( or drop) measured in volts.

Fan-Duty Resistor--A fan-duty resistor is a resistor for use in the armature or rotor circuit of a motor in which the current is approximately proportional to the speed of the motor.

Farad--The farad is the capacitance of an electric con denser in which a charge of one coulomb produces a difference of potential of one volt between the poles of the capacitor.

Ferrule Resistor-Ferrule resistors are resistors supplied with ferrule terminals for mounting in standard fuse clips.

Field Discharge Switch--A switch usually of the knife blade type having auxiliary contacts for connecting the field of a generator or motor across a resistor ( field discharge) at the instant preceding the opening of the switch.

Fixed Resistor--A fixed resistor is one designed to intro duce only one set amount of resistance into an electrical circuit.

Henry--The henry is inductance of a closed circuit in which an electromotive force of I volt is produced when the electric current traversing the circuit varies uniformly at the rate of I ampere per second.

Hot Spot--The point or location of maximum temperature on the external surface of a resistor is the hot spot.

Hysterset--Electronic method of control, including a special magnetic circuit, providing a high degree of power amplification especially adaptable for dimming purposes.

Inductance--Inductance is the (scalar) property of an electric circuit or of two neighboring circuits which determines the electromotive force induced in one of the circuits by a change of current in either of them.* Impedance-The impedance of an electric circuit is the apparent resistance of an A.C. circuit, being the combination of both the resistance and reactance and is equal to the ratio of the value of the EMF between the terminals to the current, there being no source of power in the portion under consideration. The unit of impedance is the ohm and is represented by Z.

Intermittent Duty--Intermittent duty is a requirement of service that demands operation for alternate intervals of (1) load and no-load; or ( 2) load and rest; or ( 3) load, no-load and rest; such alternate intervals being definitely specified.

Intermittent-Duty Resistor--An intermittent-duty resistor is one that is capable of carrying for a short period of time the high overload current for which it is designed without exceeding the specified temperature rise.

Loopohm--High-current resistor having a resistance alloy ribbon of uniform cross-section formed in a series of loops supported between two steel rods insulated with ceramic washers and bushings. Rods are bolted to pressed steel frames providing a sturdy relatively light-weight continuous-duty resistor unit.

Machine-Duty Resistor--A machine-duty resistor is a resistor for use in the armature or rotor circuit of a motor in which the armature current is almost constant.

Megohm-A megohm is a unit of resistance and is equal to one million ohms.

MIL Resistors--MIL resistors are resistors built in accordance with Joint Army-Navy specifications.

Multi-Section Resistor--A multi-section resistor is a resistor having two or more electrically independent sections.

N.E.C.-The National Electrical Code is the standard of the National Board of Fire Underwriters for electric wiring and apparatus as recommended by the National Fire Protection Association and approved by the American Standards Association.

NEDA-NEDA is the National Electronic Distributors Association.

NEMA - NEMA is the National Electrical Manufacturers Association, a non-profit trade association, supported by the manufacturers of electrical apparatus and supplies. NEMA is engaged in standardization to facilitate understanding between the manufacturers and users of electrical products.

Nominal Diameter--Nominal diameter as applied to tubular Vitrohm resistors is the diameter of the ceramic tube expressed in inches and/or fractions thereof.

Nominal Length-Nominal length as applied to tubular Vitrohm resistors is the length of the resistor base or core expressed in inches and/or fractions thereof.

Non-Inductive Resistors--Non-inductive power resistors are those in which the inductance and distributed capacitance are reduced to an absolute minimum.

Ohm--The ohm is a unit of resistance and is defined as the resistance at 0°C of a column of mercury of uniform cross-section having a length of 106.3 centimeters and a mass of 14.4 grams.

Ohmmeter--An ohmmeter is an instrument for measuring electric resistance and is provided with a scale graduated in ohms.

Periodic Duty--Periodic duty is a type of intermittent duty in which the load conditions are regularly recurrent.

Periodic Rating--The periodic rating defines the load which can be carried for the alternate periods of load and rest specified in the rating, the apparatus starting cold and for the total time specified in the rating without causing any of the specified limitations to he exceeded.

Plaqohm--An non-inductive Vitrohm resistor having a flat rectangular shaped molded ceramic base on which the resistive element is set and embedded with fused-on vitreous enamel.

Power-Power is the time rate of transferring or transforming energy; the rate of doing work or expending energy.

Power Resistors--Power resistors are resistors capable of dissipating 5 watts or more.

Rating-A rating of a machine, apparatus or device is a designated limit of operating characteristics based on definite conditions.

Note: 1--Such operating characteristics as load, volt age, frequency, etc., may he given in the rating.

Note: 2--The rating of control apparatus in general is expressed in volts, amperes, horsepower or kilowatts as may he appropriate, except that resistors are rated in ohms, amperes and class of service.

Reactor-A reactor is a device used for introducing reactance into a circuit for purposes such as motor starting, paralleling transformers and control of current.* Rectifier-A rectifier is a device which converts alternating current to unidirectional current hy virtue of a characteristic permitting appreciable flow of current in only one direction.

ASA Standard--Ward Leonard Trade Name Resistance-Resistance is the ( scalar) property of an electric circuit or of any body which may be used as part of an electric circuit which determines for a given current the rate at which electric energy is converted into heat or radiant energy and which has a value such that the product of the resistance and the square of the current gives the rate of conversion of energy. In the general case, resistance is a function of the current, but the term is most commonly used in connection with circuits where the resistance is independent of the current.* Resistance Tolerance - The resistance tolerance of a power resistor is the extent to which its resistance may be permitted to deviate above or below the specified resistance. Resistance tolerance is usually expressed in percent.

Resistance Method of Temperature Determination

This method consists in the determination of temperature by comparison of the resistance of the winding at the temperature to be determined with the resistance at a known temperature.** Resistive Conductor-A resistive conductor is a conductor used primarily because it possesses the property of high electric resistance.*

Resistivity--The resistivity of a material is the resistance of a sample of the material having specified dimensions.

Resistor--A resistor is a device, the primary purpose of which is to introduce resistance into an electric circuit.* Resistor Core-The resistor core or base of a power resistor is the insulating support on which the resistive conductor is wound.

Rheostat-A rheostat is an adjustable resistor so constructed that its resistance may be changed without opening the circuit in which it may be connected.

* ASA Standard

* *NEMA Standard

Ribflex-Tubular resistor consisting of an alloy resistance ribbon, crimped and edge-wound on a ceramic core, the ribbon being securely and permanently fastened to the core by vitreous enamel.

Ribohm-Resistor element made of a resistance alloy ribbon formed into a shallow channel shape and then formed into a vee, the ribbon being flat where the bends occur. Ends of the vee-shaped element are also flat and punched for securing the terminal assembly. Units are generally mounted on an impregnated asbestos composition faceplate.

RETMA--RETMA is the Radio-Electronics-Television Manufacturers Association. ( See EIA.) Screw-Base Resistors-Screw-base resistors are power type Vitrohm resistors equipped with Edison-type screw-base terminals for quick interchangeability.

Short-Time Rating--The short-time rating is the rating that defines the load which can be carried for a short and definitely specified time, the machine, apparatus or device being at approximately room temperature at the time the load is applied.

Single-Wound Resistor - A single-wound resistor is a resistor that has only one layer of resistance wire or ribbon wound around the insulating base or core.

Still Air--Still air is considered air having no circulation except that created by the heat of the resistor which is being operated.

Stock Resistor.--Stock resistors are those popular size resistors available for immediate shipment.

Stripohm--A Vitrohm resistor consisting of a hollow ceramic core, oval in shape, about which resistance wire is wound and completely embedded in vitreous enamel.

Tapped Resistor-A tapped resistor is one with two or more steps.

Temperature Coefficient of Resistance--The temperature coefficient of resistance is a measure of the increase or decrease in resistance of a resistive conductor due to change in temperature.

* ASA Standard--Ward Leonard Trade Name

Temperature Rise--Temperature rise is the difference in temperature between the initial and final temperature of a resistor. Temperature rise is expressed in degrees C or F, usually referred to an ambient temperature. Temperature rise equals the hot spot temperature minus the ambient temperature.

Thermal Shock--Thermal shock consists of a sudden marked change in the temperature of the medium in which the device operates.

Thermocouple--A thermocouple is a device for converting heat energy into electrical energy and is a pair of dissimilar conductors so joined as to produce a thermo-electric effect. It is used with a millivolt meter to measure temperature rise in apparatus.

Thermometer Method of Temperature Determination

This method consists in the determination of the temperature by mercury or alcohol thermometers, by resistance thermometers, or by thermocouples, any of these instruments being applied to the hot test part of the apparatus accessible to mercury or alcohol thermometers.

Varying Duty--Varying duty is a requirement of service that demands operation at loads, and for intervals of time, both of which may be subject to wide variation.

Vitrohm--Power-type vitreous enameled resistors, rheostats or resistance dimmers. A vitreous enamel developed by Ward Leonard.

Volt--The volt is a unit of electrical pressure, emf or potential difference. It is represented by E.

Watt--The watt is a unit of electric power. It is the power expended when one ampere of direct current flows through a resistor of one ohm.

Winding Pitch--Winding pitch is the distance from any point on a turn of a resistive conductor to the corresponding point on an adjacent turn measured parallel to the long axis of the winding.

* ASA Standard

**NEMA Standard--Ward Leonard Trade Name

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