Which of the following instrument will be used to measure a small current of very high frequency?

Option 3 : Thermocouple type instrument

Thermocouple:

• A thermocouple is defined as a thermal junction that functions based on the phenomenon of the thermoelectric effect, i.e the direct conversion of temperature differences to an electric voltage.
• Thermocouple instruments are RF (radio frequency) type indicating instruments. 
• These instruments can be used for the measurement of AC signals up to 50 MHz frequency.

Thermocouple instrument:

• The instrument which uses the thermocouple for the measurement of the temperature, current, and voltage such type of instrument is known as the thermocouple instrument. 
• It is used for both the AC and DC measurement.
• The current whose magnitude is to be measured passes through the junction of the thermocouple.
• The current produces the heat in the heater element and the thermocouple induces emf in the output terminals.
• This emf is proportional to the temperature and RMS value of the current.

Note:

• Electrodynamic ammeter: Used for both Both AC and DC measurement.
• Moving coil galvanometer: Used for only DC measurement.
• Induction type instrument: Used for only AC measurement.