Understanding Instruments for High Frequency Current Measurement

The question asks about an instrument used to record a small amount of current, specifically at high frequencies. Let's look at the options provided and evaluate their suitability for this task.

We need an instrument that can respond accurately to rapid changes or high frequencies in the current and can measure small values.

• Dynamometer type instruments: These work on the principle of the force between magnetic fields produced by coils. They can measure both AC and DC and indicate the RMS value of AC. However, their accuracy can be affected at very high frequencies due to factors like inductance and eddy currents. They are also typically less sensitive for measuring very small currents compared to other methods.
• Induction type instruments: These instruments work based on the principle of induction and eddy currents. They are primarily used for AC measurements, like energy meters (kWh meters) and sometimes for current or voltage. They are generally designed for power frequency applications and are not suitable for measuring current, especially small currents, at high frequencies.
• Thermocouple type instruments: These instruments utilize the heating effect of current. The current flows through a heater element, which heats up a thermocouple junction. The thermocouple generates a small voltage proportional to the temperature difference, which is then measured by a sensitive DC instrument (like a galvanometer or millivoltmeter). Since the heating effect ($\propto I^2R$) is independent of the frequency of the current, thermocouple instruments can accurately measure both AC and DC currents, and importantly, they are suitable for high-frequency measurements (like RF currents) as long as the heater element's skin effect is accounted for or minimized. By using a sensitive DC instrument to measure the thermocouple output, they can also be used to measure small currents.
• Permanent magnet moving coil (PMMC) type ammeter: This instrument works based on the force on a current-carrying coil in a permanent magnetic field. It measures the average value of the current. For AC, especially high-frequency AC, the average value is zero. Due to the inertia of the moving coil, it cannot follow rapid variations and will show a zero reading for high-frequency AC. Therefore, it is not suitable for measuring high-frequency currents.

Based on the principles of operation and their limitations at high frequencies and for small current measurements, the thermocouple instrument is the most appropriate choice among the given options for recording a small amount of current with high frequency.