Hot Wire Instrument Construction, Working Principle, Advantages & Applications

Hot wire instruments are widely used for the measurement of alternating current (AC) and direct current (DC), especially at high frequencies. These instruments operate based on the thermal effect of electric current, where the passage of current through a fine wire causes it to heat and expand. This expansion is translated into a mechanical movement, which is then indicated on a calibrated scale.

What Is a Hot Wire Instrument?

A hot wire instrument is an electromechanical measuring device that converts electrical energy into heat and uses the expansion of the heated wire to indicate the value of current or voltage. These instruments are primarily used for true RMS measurement of current, independent of waveform shape.

Construction of Hot Wire Instrument

A typical hot wire instrument consists of the following essential parts:

Fine Platinum–Silver Alloy Wire:
A thin wire of high resistivity and low temperature coefficient (usually platinum or platinum–iridium) is stretched between two fixed points.
Tension Spring and Pulley System:
The wire is connected through a tension spring and pulley system to maintain constant tension and ensure smooth movement.
Pointer and Scale Mechanism:
The expansion of the hot wire is transmitted through a silk or phosphor bronze thread over pulleys to a pointer, which moves over a calibrated scale.
Enclosure:
The entire assembly is enclosed in a metallic case to protect it from air currents and temperature variations.

Working Principle of Hot Wire Instrument

The working of the hot wire instrument is based on the heating effect of electric current. When current passes through the fine wire:

Heat Generation:
The current flowing through the wire causes power loss proportional to $I^2R$ , where $I$ is current and $R$ is resistance.
Thermal Expansion:
The generated heat increases the temperature of the wire, causing it to expand in length.
Mechanical Movement:
The expansion moves a tension spring and silk thread, resulting in the deflection of the pointer.
Indication:
The pointer moves over the scale calibrated in terms of current or voltage.

Mathematical Expression

The heat generated per second in the wire is:

H = I^2 R

Since the deflection of the pointer $\theta$ is proportional to the expansion of the wire, and expansion is proportional to temperature rise, we can write:

\theta \propto I^2

Thus, the instrument has a square-law response — meaning it is more sensitive to higher currents.

Types of Hot Wire Instruments

1. Hot Wire Ammeter

Used to measure current (both AC and DC). The current to be measured flows directly through the fine wire, and the deflection indicates the RMS value.

2. Hot Wire Voltmeter

Used to measure voltage. A high resistance is connected in series with the fine wire to limit the current flow corresponding to the voltage being measured.

Advantages of Hot Wire Instruments

True RMS Measurement: Measures RMS value irrespective of waveform.
AC and DC Compatibility: Can measure both AC and DC quantities.
Frictionless Operation: No iron or moving coil parts, ensuring smooth and stable motion.
Frequency Independent: Accuracy remains consistent over a wide frequency range (up to several MHz).
Simple Construction: Fewer mechanical components make the design robust and reliable.

Disadvantages of Hot Wire Instruments

Slow Response: Time lag due to heating and cooling of the wire.
Low Sensitivity: Requires significant current to produce measurable heat.
Non-Uniform Scale: Scale is cramped at the lower end and extended at higher values.
Temperature Error: Ambient temperature variations can affect readings.
Fragility: The fine wire is delicate and may break due to overcurrent.

Precautions While Using Hot Wire Instruments

Do not overload beyond rated capacity.
Avoid mechanical shocks or vibrations.
Allow sufficient time for stabilization before taking readings.
Keep the instrument away from external heat sources.
Use in vertical position for accurate deflection.

Applications of Hot Wire Instruments

Measurement of Radio Frequency (RF) Currents and Voltages
Testing of High-Frequency AC Circuits
Calibration of RF equipment
Laboratory experiments for waveform analysis
Measurement of non-sinusoidal waveforms

Key Points to Remember

Deflection ∝ $I^2$ , not directly proportional to current.
Indicates RMS value, suitable for both AC and DC.
Sensitive to frequency and waveform variations only at extremely high ranges.
Must be used with protective resistance in voltmeter form.

Hot Wire Instrument MCQ-

1. The working principle of a hot wire instrument is based on —

A. Magnetic effect of current
B. Heating effect of current
C. Chemical effect of current
D. Electrostatic effect of current

✅ Answer: B. Heating effect of current
Explanation:
When current flows through the fine wire, it produces heat due to Joule’s law (I²R).
This heat causes the wire to expand, and that expansion moves the pointer on the scale.
So, its operation is purely based on the heating effect of electric current.

2. A hot wire instrument can be used to measure —

A. Only DC current
B. Only AC current
C. Both AC and DC current
D. Only voltage

✅ Answer: C. Both AC and DC current
Explanation:
Since heating depends on I², the direction of current (AC or DC) doesn’t matter.
That’s why the hot wire instrument works with both AC and DC and reads RMS value of current.

3. The deflection in a hot wire instrument is proportional to —

A. Current (I)
B. Voltage (V)
C. Power (I²R)
D. Current squared (I²)

✅ Answer: D. Current squared (I²)
Explanation:
The wire expands because of the heat generated, which depends on I²R.
Since resistance (R) is constant, the deflection ∝ I², not directly to I.
Hence, it indicates RMS current for AC.

4. The wire used in a hot wire instrument is usually made of —

A. Copper
B. Aluminum
C. Platinum-iridium alloy
D. Nichrome

✅ Answer: C. Platinum-iridium alloy
Explanation:
A platinum-iridium alloy is used because:

It has low temperature coefficient (stable resistance),
High melting point, and
Good mechanical strength even at high temperatures.

**5. Which of the following statements about hot wire instruments is false?**

A. They are free from magnetic errors.
B. They have a quick response time.
C. They measure RMS value of AC.
D. They can be used on both AC and DC.

✅ Answer: B. They have a quick response time.
Explanation:
Hot wire instruments have a slow response because the wire takes time to heat up and cool down.
All other statements are correct — they’re non-magnetic, measure RMS, and can be used on both AC and DC.

6. What is the main disadvantage of a hot wire instrument?

A. It is affected by magnetic fields.
B. It cannot be used for AC.
C. It has slow response and poor overload capacity.
D. It cannot measure RMS value.

✅ Answer: C. It has slow response and poor overload capacity.
Explanation:
Because of the thermal lag and delicate nature of the thin wire, it reacts slowly and can be damaged by large overloads.

7. Hot wire instruments are mainly used for measuring —

A. Very high frequency current
B. Low current in DC circuits only
C. RMS current in AC and DC circuits
D. Instantaneous value of AC

✅ Answer: C. RMS current in AC and DC circuits
Explanation:
Since heat generated is proportional to I² (and RMS value represents the same heating effect), the hot wire instrument naturally measures RMS current.

8. Why is a spring used in a hot wire instrument?

A. To increase the wire temperature
B. To provide restoring force
C. To reduce heat loss
D. To measure temperature directly

✅ Answer: B. To provide restoring force
Explanation:
The spring pulls the wire back to its normal position as it cools, ensuring that the pointer returns to zero and deflection remains proportional to current.

9. Hot wire instrument gives accurate results at —

A. Low frequency
B. Very high frequency
C. DC only
D. Only when magnetic fields are present

✅ Answer: A. Low frequency
Explanation:
At low frequency (or DC), the heating is uniform and steady.
At very high frequencies, skin effect and heating variations may cause small errors.

10. The hot wire instrument scale is —

A. Uniform
B. Non-uniform
C. Linear
D. Logarithmic

✅ Answer: B. Non-uniform
Explanation:
Since deflection ∝ √(Temperature rise) and temperature rise ∝ I²,
the scale becomes non-uniform — compressed near the beginning and more spread at higher readings.