Correct answer: Voltmeter
Explanation:
A voltmeter is an electrical measuring instrument used to measure the potential difference (voltage) or electromotive force (emf) between two points in an electric circuit.
Other options:
- Ammeter: measures electric current.
- Calorimeter: measures heat.
- Barometer: measures atmospheric pressure.
Hence, the correct answer is: Voltmeter.
Three very long parallel wires carrying current as shown. Find the force acting at 15 cm length of middle wire : 
