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Glossary of Commonly Used Terms Relating to Reed Switch Products

The following glossary of definitions refer to the generally used terms relating to reed switch, reed sensor, reed relay, and electromechanical relay products. Some of the definitions have multiple names. The most popular name was chosen for this listing and is listed under that name. However, we have tried to list those other common names under the most popular name.



Actuation Time

Actuation Time is the time from initial energization to the first closing of open contact or opening of a closed contact, not including any bounce.

Ampere Turns (AT or NI)

Ampere Turns is the product of the number of turns in an electromagnetic coil winding times the current in amperes passing through the winding. AT usually defines the opening and closing points of contact operate conditions.


Armature is the moving magnetic member of an electromagnetic relay structure.
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Bias Magnet

Bias Magnet is a steady magnetic field (permanent magnet) applied to the magnetic circuit of a relay or sensor to aid or impede operation of the contacts.

Bias, Magnetic

Bias, Magneticis a steady magnetic field applied to the magnetic circuit of a switch.


Blade is used to define the cantilever portion of the reed switch contained within the glass envelope.


Bobbin is a spool, coil form or structure upon which a coil is wound.

Bobbinless Coil

Bobbinless Coil (self supporting coil) is a coil formed without the use of a bobbin.

Bounce, Contact

Contact Bounce is the intermittent opening of closed contacts occurring after initial contact actuation or closure of the contacts due to mechanical rebound, or mechanical shock or vibration transmitted through the mounting.


Break defines the opening of closed contacts.

Breakdown Voltage

Breakdown Voltage is that voltage at which an arc or break over occurs between the contacts.

Breakdown Voltage, Pre-ionized

Pre-ionized Breakdown Voltage is the voltage level at which the voltage breaks down across the contacts, after which, the voltage had been recently broken down across the contacts, creating an ionized state in the glass capsule. Usually the breakdown voltage in the pre-ionized state is a lower value and more repeatable. It is a truer measure of the breakdown voltage level.


Bridging is the undesired closing of open contacts caused by a metallic bridge or protrusion developed by arcing causing the melting and resolidifying of the contact metal.

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Changeover Contact

Changeover Contact (also referred to as a Form C or single pole double throw (SPDT)) has three contact members, one of them being common to the two contacts. When one of these contacts is open, the other is closed and vice versa.

Coaxial Shield

Coaxial Shield is an electrostatic shield grounded at both the input and output.


Coil is an electromagnetic assembly consisting of one or more windings of copper insulated wire usually wound on a bobbin or spool. When current is applied to the coil, a magnetic field is generated, operating the contacts of a Reed Relay or Electromechanical Relay.

Common Mode Voltage

Common Mode Voltage usually refers to a voltage level as measured between one or more lines and ground (common) or a current flowing between one or more lines and ground (ground).


Contact refers to the contact blades making up a Reed Switch or Electromechanical Relay.

Contact, Bifurcation

Contact, Bifurcation is a forked, or branching of contacting member so formed or arranged, as to provide some degree of independent dual contacting.

Contact, Break-before-make (Form C)

Contact, Break-before-make (Form C) defines the sequence in which one contact opens its connection to another contact and then closes its connection to a third contact.

Contact Force

Contact Force is the force which two contact points exert against each other in the closed position under specified conditions.

Contact Form

Contact Form describes the type of contacts used for a given design or applications (ex. 1 Form A, 1 Form B, etc.)

Contact, Form A

Contact, Form A is a single pole single throw (SPST) normally open (N.O.) switch.

Contact, Form B

Contact, Form B is a single pole single throw (SPST) normally closed (N.C.) switch.

Contact, Form C

Contact, Form C is a single pole double throw (SPDT) where a normally closed contact opens before a normally open contact closes.

Contact, Form D

Contact, Form D is a single pole double throw where the normally open contact closes before normally closed contact opens (continuity transfer).

Contact, Form E

Contact, Form E is a bistable contact that can exist in either the normally open or normally closed state. Reversing the magnetic field causes the contacts to change their state.

Contact, Current Rating

Contact, Current Rating is the current which the contacts are designed to handle for their rated life.

Contact, Gap

Contact, Gap is the gap between the contact points when the contacts are in the open state.

Contact, Make-before-break (Form D)

Contact, Make-before-break (Form D) defines the sequence in which one contact remains connected to a second contact while closing on a third contact and then the second contact opens its connection.

Contact, Rating

Contact, Rating refers to the electrical load-handling capability of relay contacts under specified conditions for a prescribed number of operations.

Contact, Reed

Contact, Reed defines a Reed Switch whereby a glass enclosed, magnetically operated contact using thin, flexible, magnetic conducting leads or blades as the contacting members.

Contact Resistance

Contact Resistance is the electrical resistance of closed contacts; measured at their associated contact terminals after stable contact closure.

Contact Seal

Contact Seal refers to a contact assembly sealed in a compartment separate from the rest of the relay.

Contact Separation

Contact Separation is the distance between mating contacts when the contacts are open.

Contact, Snap Action

Contact, Snap Action describes the crisp closure and opening of contacts at or around the operate points where the contact resistance remains constant and stable.

Contact, Stationary

Contact, Stationary is a member of a contact combination that is not moved directly by the actuating system.

Contact Tip

Contact Tip is the point at the end of a contact where the contacts come together when closed.

Contact Transfer Time

Contact Transfer Time (in a Form C switch) is the time during which the moving contact first opens from a closed position and first makes with the opposite throw of the contact.

Contact Weld

Contact Weld is a fusing of contacting surfaces to the extent that the contacts fail to separate when intended.

Contact Wipe

Contact Wipe occurs when a contact is making the relative rubbing movement of contact points after they have just touched.

Contacts, Mercury Wetted

Contacts, Mercury Wetted are contacts that make closure via a thin film of mercury maintained on one or both contact surfaces by capillary action.

Control Voltage

Control Voltage is another name for the voltage applied across the coil of a relay and refers to that point where the relay will operate.


Crosstalk is the electrical coupling between a closed contact circuit and other open or closed contacts on the same relay or switch, expressed in decibels down from the signal level.


Current is the rate of flow of electrons in a circuit measured in amperes (unit A).

Current, AC

Current, AC is alternating current flow from positive to negative.

Current, DC

Current, DC is current flow in one direction.

Current, Carry

Current, Carry is the amount of current that can safely be passed through closed switch contacts.

Current, Inrush

Current, Inrush is the surge of current a load may draw at initial turn on and may be many times greater than the steady current draw.

Current Leakage

Current Leakage is that parameter measuring the unwanted leakage of current across open contacts and/or leakage current between the coil and contacts.

Current Rated Contact

Current Rated Contact is the current which the contacts are designed to handle for their rated life.

Currentless Closure

Currentless Closure refers to contacts closing with no voltage existing or current flowing at the time of closure.


Cycling refers to the minimum number of hours during which a relay may be switched between the off state and the on state at a fixed, specific cycle rate, load current, and case temperature without failure.

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De-energize is the act of removing power from a relay coil.

Dielectric Strength

Dielectric Strength or Breakdown Voltage is the maximum allowable voltage, usually measured in DC Volts or Peak AC, which may be applied between two specified test points such as input-output, input-case, output-case and between current-carrying and non-current-carrying metal members.


Dropout refers to maximum value of coil current or voltage at which a Reed Switch or Relay resumes its natural condition.

Dropout Value

Dropout Value is the measured current, voltage or distance when the contacts open.

Duty Cycle

Duty Cycle is the percentage of time on versus time off or duty cycle = Ton/Toff.

Dynamic Contact Resistance

Dynamic Contact Resistance is the repetitive measurement of contact resistance measured 1ms to 3 ms after contact closure.

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Electrostatic Shield

Electrostatic Shield is a copper alloy material terminated to one or more pins and located between two or more mutually insulated elements within a relay which minimizes electrostatic coupling between the coil and Reed Switch in a Reed Relay.


Energization is the application of power to a coil winding of a relay.

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Frequency, Operating

Frequency, Operating represents the rate or frequency at which contacts be switched on and off.

Frequency Response

Frequency Response is the frequency at which the output signal decreases by 3 dB from the input signal.

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Gap, Magnetic describes the nonmagnetic portion of a magnetic circuit.

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Hermetic Seal

Hermetic Seal is an encapsulation process where the contacts are sealed in a glass to metal seal in the case of a Reed Switch. In the case of a relay, the contacts and coil are sealed.

Holding Current

Holding Current is the minimum current required to maintain closed contacts.

Holding Voltage

Holding Voltage is the minimum voltage required to maintain closed contacts.


1. The lag between the value of magnetism in a magnetic material, and the changing magnetic force producing it; magnetism does not build up at the same rate as the force, and some magnetism remains when the force is reduced to zero. Also, the difference in response of a device or system to an increasing and a decreasing signal.
2. Hysteresis is also referred to the difference between the operate voltage and the release voltage and can be expressed as a percentage of release/operate.

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I/0 Capacitance

I/0 Capacitance is the capacitance between the input and output terminals or between the coil and contacts.

I/0 Isolation Voltage

I/0 Isolation Voltage refers to the voltage value before voltage breakdown occurs. It is the same as breakdown voltage.


Impedance refers to the resistance in ohms composed of DC resistance, inductive reactance, and capacitive reactance added vectorally in an RF circuit.

Insulation Resistance

Insulation Resistance is the DC resistance in ohms measured from input to output or across the contacts. Measurement is usually done by applying 100 Volts to one of the points to be measured and the other is connected to a picoameter.

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Latching Relay

Latching Relay is a relay that maintains its contacts in the last assumed position without needing to maintain coil energization. To change the state of the contacts, the magnetic field must be reversed.

Leakage Current

Leakage Current is the current flow from input to output or across the contacts when the contacts are in the open state.

Load, Contact

Load, Contact is the electrical power encountered by a contact set in any particular application.

Load Power Factor

Load Power Factor is the phase angle (cos) between load voltage and load current in an electrical circuit caused by the reactive component of the load.

Load Voltage

Load Voltage refers to the supply voltage range at the output used to normally operate the load.

Low Thermal Relay

Low Thermal Relay is a Reed Relay designed specifically to switch very low microvolt or nanovolts signals without distorting their signal level.

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Magnetic Flux

Magnetic Flux is the total magnetic induction, or lines of force, through a given cross section of a magnetic field.

Magnetic Interaction

Magnetic Interaction is the undesired effect when relays are mounted in close proximity, the flux produced when the coils are energized affects the pickup and dropout values of the adjoining relays. This either increases or decreases both pickup and dropout values. The direction of the parameter shift is determined by whether the stray flux aids or bucks the flux produced by the coil of the relay under consideration. Problems may result from bucking flux raising the pickup voltage close to the coil drive voltage or by aiding the flux of sufficient magnitude that the relay will not drop out when its drive is removed. To calculate the change in pull-in voltage and dropout voltage, multiply the percent change shown by the relayÂßs nominal voltage. For example, if the percent change in pull-in voltage is 14% for a 5V nominal relay, the pull-in voltage will increase by 0.7 volts (see relay application notes).

Magnetic Pole

Magnetic Pole is the end of a magnet, where the lines of the flux coverage, and the magnetic force is strongest (north or south pole).

Magnetic Shield

Magnetic Shield is a thin piece of ferromagnetic metal surrounding a relay to enhance its magnetic field internally while reducing the stray magnetic field external to the relay.

Magnetostrictive Force

Magnetostrictive Force usually refers to the force produced on the contacts with current flowing and the coil energized. Here the magnetic field of the coil and the magnetic field produced by the current flowing through the contacts interact with each other producing a torsional force.


Make refers to the closure of open contacts.

Mechanical Shock, Nonoperating

Mechanical Shock, Nonoperating is the mechanical shock level (amplitude, duration and wave shape) to which the relay or sensor may be subjected without permanent electrical or mechanical damage (usually during storage or transportation).

Mechanical Shock, Operating

Mechanical Shock, Operating is the mechanical shock level (amplitude, duration and wave shape) to which the relay or sensor may be subjected without permanent electrical or mechanical damage during its operating mode.

Miss, Contact

Miss, Contact is the failure of a contact mating pair to close in a specified time or with a contact resistance in excess of a specified maximum value.

MOV (Metal Oxide Varistor)

MOV (Metal Oxide Varistor) is a voltage-sensitive, nonlinear resistive element. MOVÂßs are clamp-type devices that exhibit a decrease in resistance as the applied voltage increases. They are usually characterized in terms of the voltage drop across the device while it is conducting one milliamp of current. This voltage level is the conduction threshold. The voltage drop across an MOV increases significantly with device current. This factor must be taken into consideration when determining the actual protection level of the device in response to a transient.

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Normally Closed (N.C.) Contacts (Form B)

Normally Closed (N.C.) Contacts (Form B) represents a state of contacts before any magnetic field is applied to them in which they exist in the closed state.

Normally Open (N.O.) Contacts (Form A)

Normally Open (N.O.) Contacts (Form A) represents a state of contacts before any magnetic field is applied to them in which they exist in the open state.

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OHM’s Law

OHM’s Law the following is a table of common electrical conversions.

Operate Time

Operate Time or (contact operate time or Pull-in time) is the total elapsed time from the instant power is applied to the energizing coil until the contacts have operated and all contact bounce has ended.

Operating Temperature Range

Operating Temperature Range is the normal temperature range in which a Reed Switch, Sensor, or Relay will successfully operate.


Output is the portion of a relay which performs the switching function required.

Output Capacitance

Output Capacitance is capacitance across the contacts.

Output Offset Voltage

Output Offset Voltage or thermal offset usually measured in microvolts is voltage existing across closed contacts in the absence of any signals. The voltage which appears at the output of the isolation amplifier with the input grounded.


Overdrive is the amount of voltage or ampere turns applied after the exact point of closure of contacts is reached. Contact resistance is usually measure with 100 % overdrive.

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Permeability is a characteristic of a magnetic material which describes the ease of which it can conduct magnetic flux.

Pickup Value

Pickup Value refers to the measure of current or voltage applied to a relay when the contacts just close.

Pickup Pulse

Pickup Pulse is a short, high-level pulse applied to a relay; usually employed to obtain faster operate time.

Pole, Double

Pole, Double is a term applied to a contact arrangement to denote two separate contact combinations, that is, two single-pole contact assemblies.

Pole, Single

Pole, Single is a term applied to a contact arrangement to denote that all contacts in the arrangement connect in one position or another to a common state.

Pressure, Contact

Pressure, Contact refers to the force per unit area on the contacts.

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Rating, Contact

Rating, Contact is the maximum rating of the allowable voltage and current that a particular contact is rated to switch.

Reed Relay

Reed Relay is a relay that uses a glass-enclosed hermetically sealed magnetic reed as the contact members.

Reed Switch or Reed Sensor

Reed Switch or Reed Sensor is a switch or relay using glass-enclosed magnetic reeds as the contact members which includes mercury-wetted as well as dry contact types.

Relay, Antenna

Relay, Antenna switching is a special RF relay used to switch antenna circuits.

Relay, Close Differential

Relay, Close Differential is a relay having its dropout value specified close to its pickup value.

Relay, Crystal Can

Relay, Crystal Can defines a relay housed in a hermetically sealed enclosure that was originally used to enclose a frequency control type of quartz crystal.

Relay, Current Sensing

Relay, Current Sensing is a relay that functions at a predetermined value of current typically used in telecommunications as a line sense relay.

Release Time

Release Time or Dropout Time refers to the time from initial de-energization to the first opening of a closed contact time.


Reluctance is the resistance of a magnetic path to the flow of magnetic lines of force through it.


Reset refers to the return of the contacts to their normal state (initial position).

Resonant Frequency

Resonant Frequency is the tendency of the contacts to resonate at certain frequencies determined by their size and makeup.


Retentivity is the capacity for retaining magnetism after the magnetizing force is removed.

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Saturation exists when an increase of magnetization applied to a magnetic material does not increase the magnetic flux through that material.


Sensitivity refers to the pull-in of a Reed Switch usually expressed in ampere-turns.

Shield, Electrostatic

Shield, Electrostatic is the grounded conducting member located between two or more mutually insulated elements to minimize electrostatic coupling.

Slew Rate

Slew Rate is the rate of change in output voltage with a large amplitude step function applied to the input.

Small Signal Bandwidth

Small Signal Bandwidth is the frequency range from DC to a frequency where the signal strength is down 3 dB from its original signal strength.

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Thermal Offset

Thermal Offset usually measured in microvolts is the voltage existing across closed contacts in the absence of any signals.

Thermal Shock Nonoperating

Thermal Shock Nonoperating is the temperature shock induced into a group of Relays, Switches or Sensors to determine their robustness.

Turn Off

Turn Off or Dropout Time refers to the time from initial de-energization to the first opening of a closed contact time.

Turn On

Turn On or (contact operate time or Pull-in time) is the total elapsed time from the instant power is applied to the energizing coil until the contacts have operated and all contact bounce has ended.

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see MOV.

Vibration, Nonoperating

Nonoperating Vibration is the vibration level and frequency span to which the relay may be subjected without permanent electrical or mechanical damage.

Voltage, Nominal

Nominal Voltage is the typical voltage intended to be applied to the coil or input.

Voltage, Peak AC

Peak AC Voltage is the maximum positive or negative voltage swing of an alternating current signal or power supply.

Voltage, Peak to Peak AC

Peak to Peak AC Voltage is the maximum positive threw negative voltage swing of an alternating current signal or power supply. Vp-p =2Vp when no DC offset is present.

Voltage, RMS

RMS Voltage is the Root Mean Square of the positive and negative voltage swing of an alternating current signal or power supply.

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Winding refers to the electrically continuous length of insulated wire wound on a bobbin, spool or form.

Winding, Bifilar

Bifilar Winding represents two windings with the wire of each winding alongside the other, matching turn for turn.

Wipe, Contact

Contact Wipe refers to the sliding or tangential motion between two mating contact surfaces as they open or close.
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