**Reluctance **

The opposition that a magnetic circuit presents to the passage of magnetic lines through it.

**Reactance**

The opposition to the flow of an alternating current by an inductive or capacitive element in the circuit.

**Resistance**

Resistance is defined as the measure of opposition to the motion of electrons due to their continuous collisions, with the atoms of the conductor

**Rheostat**

A potentiometer or a rheostat is an instrument used to measure and compare potentials without drawing any current from the circuit.

**Retentivity**

The ability of a magnetic material to retain its magnetism.

**Residual Magnetism**

The magnetism remaining after the external magnetizing force is removed.

**Radian**

In a circle, the angle included within an arc equal to the radius of a circle. Numerically, the angle is 57.3^{o}.

**RMS Value**

The value of alternating current that produces the same heating effect as the corresponding value of direct current. For sine wave currents, the RMS or effective value is 0.707 times the peak value.

**Resonance**

The condition that occurs when a circuit’s inductive and capacitive reactances are balanced.

**Reactive power**

The product of voltage and out-of-phase components of alternating current.

**Real power**

The average power, or active power, or the real part of the complex power.

**Reluctance**

The ratio of the magnetomotive force to the magnetic flux through any cross-section of the magnetic circuit.

**Reluctance power**

The component of the power delivered by a synchronous generator representing the effects of generator saliency.

**Reset**

The action of a relay as it makes designated responses to decreases in the input.

**Reset value**

The maximum value of an input quantity reached by progressive decreases that will permit the relay to reach the state of complete reset from the pickup.

**Residual voltage**

The generated voltage due to the residual ﬂux in the ﬁeld poles even when the ﬁeld circuit remains unexcited.

**Root-mean-square value**

The square root of the average of the square of the value of the function taken throughout one period.

**Rotor**

The rotating member of a machine, with the shaft.

**RPM**

RPM is the abbreviation for Revolutions per Minute. This is usually the rotational speed of things like motors.

**Relay**

A relay is an electromechanical device that can be used to control multiple loads with one control circuit.

**Reactive Power **

A measure of an inductor’s ability to receive and return energy in a circuit.