The correct answer is Voltmeter.

 Key Points

• A voltmeter is an instrument used to measure the potential difference in an electrical circuit.
• A voltmeter is always connected in parallel in a circuit to measure the potential difference between two points.
• Unit of potential difference - Volt
• A galvanometer can be converted into a voltmeter by connecting a large resistance in series with it.
• The voltmeter was invented by Hans Oersted.

  Important Points

Ohmmeter

• It is an instrument used to measure electrical resistance.
• Electrical resistance opposes the flow of electric current in a circuit.
• Unit of resistance - Ohm
• Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points.
• V=IR, where (R) is the proportionality constant for resistance.

Thermometer

• It is an instrument used to measure temperature.
• The thermometer was invented by Galileo Galilei.
• The first mercury thermometer was invented by Gabriel Fahrenheit.

Gyroscope

• It is an instrument used for measuring or maintaining the orientation and angular velocity.
• The gyroscope was invented by Léon Foucault.