**Time Invariance **

In time invariant system, there are no changes in system structure as a function of a time t.

**Tesla**

The SI unit of magnetic flux density; equal to 1 Weber per square meter.

**Time Constant**

In a capacitor, the time required for a voltage to reach to 63.2 % of the steady-state or full-charge values.

in an inductor, the time required for a current to reach 63.2 % of full or steady-state value.

**Thevenin’s Theorem **

Thevenin’s Theorem states that any network having two open terminals can be replaced by a single voltage source V_{TH} in series with a single resistance R_{TH}. Voltage V_{TH} drives any load connected across these terminals with R_{TH} in series.

**Twin-Lead Transmission Line**

A type of transmission line comprised of two parallel conductors covered by a solid insulator.

**Terminal characteristics**

A plot of the terminal voltage versus the load current.

**Three-phase transformer**

A transformer consisting of three pairs of windings used to transform a set of three-phase voltages from one voltage level to another.

**Turns ratio**

The ratio of the primary winding turns to the secondary winding turns.

**Tachometer or Tach**

Test equipment designed to measure the speed of things like shafts that are rotating. The rotating speed is usually reported as RPMs, Revolutions per Minute.

**True power**

The power lost or dissipated in a circuit typically as heat.

**Timer**

A relay that will delay the switching of the contacts by a predetermined time.