1. Definition of 1 Ohm:
   1 Ohm is the electrical resistance between two points of a conductor when a current of 1 ampere flows through it due to a potential difference of 1 volt across the conductor.

Mathematically:
R = V / I
so, if V = 1 volt and I = 1 ampere, R = 1 ohm

2. Device used to measure electric circuits:
   The device used to measure current is an ammeter,
   to measure voltage is a voltmeter,
   and to measure resistance (like ohms) is an ohmmeter.

To measure resistance in an electric circuit, an ohmmeter is used.