SOLUTION

To measure the voltage and current in the radio frequency range, a suitable instrument is an Electrothermic type.

Electrothermic type Instrument

The principle of operation of those instruments is based on the properties of certain circuit elements which get heated when current flows through them. The average effect is dependent upon the mean square value of current and therefore, these instruments are used to indicate the true r.m.s. values. Consequently, the reading obtained is virtually free from frequency and waveform errors and also errors due to stray magnetic fields. The accuracy is equally good on a.c. as well as d.c. circuits.

The main significance of such an instrument is that it can be used to measure current at very high frequencies without any errors. So obviously these instruments are used for current measurement at the frequency well above the range of moving Iron and electrodynamometer type Instrument. These instruments can be used for Precision voltage measurement at moderate frequencies.

The electrothermic instruments are classified into the following three categories:

(i) Hot-wire instruments

(ii) Thermocouple instruments

(iii) Bolometers

Hot-wire instruments

• The thermoelectric element used in hot wire-type instruments is made up of platinum-iridium.
• The sensitivity of electrothermic instruments is higher than electrodynamometer-type instruments.

Advantages and Disadvantages—

1. Not affected by frequency hence can be used at a higher frequency (more than 50 MHz), these indicate RMS value irrespective of the waveform.
2. Electrothermic type measures the voltage and current in the radio frequency range.
3. Not affected by the magnetic field.
4. Can be used AC & DC measurements.
5. Measurement in an electrothermic instrument is independent of the waveform.
6. The instruments have disadvantages like instability due to stretching of wire, sluggish response, high power consumption, and inability to withstand overloads and mechanical shocks.

Thermocouple Type instruments

• These instruments are generally used upto 500 V and at a frequency of more than 50 MHz. At frequencies more than 50 MHz the skin effect is dominant and can cause an error. But this can be minimized by using Tubular Conductor. This is preferable for current I > 3A.
• The thermoelectric instruments give the same calibration for AC & DC, therefore, these instruments can be calibrated by DC and can be used for AC measurement that is why these instruments are also called transfer instruments.
• At low frequencies, the voltmeter acts as a precision instrument.

Bolometer

Such an instrument is based on the fact that when current passes through the circuit element, it gets heated. Due to this dimension changes and resistance of the circuit elements changes.