Glossary of Terms - I

Ice Point

Obsolete term in thermometry, the temperature of a mixture of melting ice and water at normal atmospheric pressure, defined as 0°C (32°F, 273.2°K) now replaced in lPTS-68 by the triple point of water to which the temperature of +0.0100°C has been assigned.

Immersion Depth

That length of a temperature probe, which is exposed to the temperature to be measured.


Resistance to electrical flow in an AC circuit. It is designated by the symbol Z and is expressed in ohms.

Inductive Reactance

The force which acts as a resistance in an inductor to limit the flow of current. This force creates a leading power factor in AC circuits.

Input Breakdown

Maximum continuous or instantaneous voltage that can be applied from either input terminal to the servo chassis.

Input Resistance

Resistance measured across the input terminals with signal leads disconnected.

Insulation Resistance

Sometimes called "leakage resistance" the resistance measured between specified insulated portions of a temperature transducer when a specified DC voltage is applied. If insulation resistance should get very low, i.e., below 0.5 megohms, errors may result due to shunting of an RTD. A circuit in which any spark or thermal effect produced normally or in a specified fault condition is incapable, in test conditions prescribed NFPA Standard 493-1975, of causing ignition of a specified mixture of gas or vapor in air in its most easily ignited concentration.


International Practical Temperature Scale, Revised 1968.


International Practical Temperature Scales Revised 1990. The currently accepted thermodynamic temperature scale.

Iron Vane

A technique for measuring AC or DC current using mutual repulsion of magnetic fields. Two parallel iron vanes, one fixed and one rotatable, are mounted inside a coil of wire; the moveable vane is supported on jewel pivots, with retaining torque springs and an attached pointer. Electric current passing through the wire coil induces identical magnetic fields in both vanes and the mutual repulsion of these fields causes the movable vane to rotate away from the fixed vane. The rotation is proportional to the strength of the fields, which is proportional to the current flowing in the field coil. Also described as "movable iron", "moving vane", or "moving iron."


A thermal layer, usually of such low mass that it does not affect the thermal stability or equilibrium of a device.