# ثابت الزمن

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{{#category_index:T|time constant}}
**1**. The time taken for the current in a circuit having a steady emf to reach a definite fraction of its final value after the circuit is closed. The fraction is (1–1/e)=0.632. **2**. The time taken for the current to decay to 1/e=0.368 of its value after the emf is removed. Also called **decay constant** or **relaxation time. 3**. *AGC time constant* (q.v.). **4**. A time over which readings are averaged to remove statistical fluctuations, as with nuclear-log readings.