Correct Answer: Ammeter : Current

Solution : Given:
Barometer : Pressure (This depicts the relation between the device and its uses)

First option: Thermometer : Volume; Thermometer is to measure the temperature not volume.
Second option: Scale : Seconds; Scales are often used to measure temperature, weight, and distance but not seconds.
Third option: Voltmeter : Heat; A voltmeter is an instrument used to measure voltage but not heat.
Fourth option: Ammeter : Current; Ammeter is used to measure current.

So, only the fourth option follows the same pattern as followed by the given pair. Hence, the third option is correct.