AMPERE-TURN:
A measure of the magnetic field strength required to operate a reed switch. In practice, a few milliamps pass through a 5,000 or 10,000 turn coil. (EX: .003 ampere x 5,000 turns = 15 ampere-turns) Common abbreviations include AT and NI.

PULL-IN:
The required magnetic field strength to operate the contacts of a reed switch. Equivalent terms: Pick Up, Operate, Activate, Make.

DROP-OUT:
The value of magnetic field strength at which the contacts of a reed switch deactivate or return to their "at rest" state. Equivalent terms: Release, Break.

DIFFERENTIAL:
The difference between the Pull-In and Drop-Out of a reed switch expressed in Ampere-Turn; usually identified by the symbol . A differential is typically specified when the contacts must Drop-Out if the magnetic field strength is reduced a specified amount.

STANDARD TEST COIL NARM RS-421A:
Reed switch sensitivity is specified in ampere-turns, which is the product of current and the number of wire turns in a specified coil. There are four standard coils which meet EIA.NARM RS-421A and MIL-S-55433 requirements and their specifications are detailed on this web site.

FORM A (SPST-NO):
An industry abbreviation for a Single Pole Single Throw Normally Open Contact. The contacts of a Form A reed switch close in the presence of a magnetic field.

FORM B (SPST-NC):
An industry abbreviation for a Single Pole Single Throw Normally Closed Contact. The contacts of a Form B reed switch open in the presence of a magnetic field.

FORM C (SPDT):
An industry abbreviation for a Single Pole Double Throw set of contacts. A form C reed switch has two contacts - one that opens and one that closes - in the presence of a magnetic field. Equivalent term: Change over reed switch.

DWELL:
In a Form C switch, the difference in field strength (expressed in ampere-turns) between the value when the Common reed breaks the Normally Closed contact and the value when the Common reed makes the Normally Open contact.

MAXIMUM SWITCHING VOLTAGE:
Maximum allowable open-circuit voltage permitted across the contacts during switching to prevent excess contact wear out.

MAXIMUM SWITCHING CURRENT:
Maximum recommended closed circuit that
can be switched to avoid excess heat and
contact damage.

MAXIMUM SWITCHING POWER:
The maximum recommended switching power (in watts) that the contacts can switch and still achieve full rated life. The switching power is calculated by multiplying the open-circuit voltage across the switch by the closed circuit current that will flow through the contacts (EX: 24 volts x .100 amps = 2.4 watts). Although maximum recommended current and voltage are both given, the maximum power rating usually requires limiting the actual values. Catalog power ratings are for resistive type loads.

VOLTAGE BREAKDOWN (HOLDOFF):
The open-circuit voltage that causes arching across the contacts. Application voltages should be below voltage breakdown value.

INITIAL CONTACT RESISTANCE:
Maximum resistance of new contacts measured according to NARM standards. All switches are measured for contact resistance at 5 AT overdrive.