The device used to measure electric current in a circuit is called an ammeter. An ammeter is specifically designed to measure the flow of electric current in a circuit and is connected in series with the circuit. It is important to note that an ammeter should always be connected in series, as connecting it in parallel would result in a short circuit.

Option 2, a motor, is not the correct answer. A motor is a device that converts electrical energy into mechanical energy and is used to produce motion or rotation. It is not used for measuring electric current.

Option 3, a voltmeter, is also not the correct answer. A voltmeter is a device used to measure the potential difference or voltage across a component or between two points in a circuit. It is connected in parallel to the circuit.

Option 4, a generator, is not the correct answer. A generator is a device that converts mechanical energy into electrical energy by electromagnetic induction. It is not used for measuring electric current.

In conclusion, the correct answer is option 1, an ammeter, as it is the device specifically used for measuring electric current in a circuit.