Hot Wire Ammeter- Construction, Working

A hot wire ammeter is an instrument that measures the electric current through a wire by heating and expanding it. These instruments have a slow response time and poor accuracy, but they can measure the radio-frequency current. Also, the hot wire ammeter can measure the actual RMS for an applied AC.


The hot wire ammeter has components that heat the electric wire and attachments that measure the expansion of the heated wire.

Image Source:

Components of Hot Wire Ammeter

  1. Wire Element: A wire AB is stretched between endpoints A and B, made of high-resistivity materials like nichrome or constantan. The wire is wrapped around cylinder D, connected to the middle point of AB. Cylinder D rotates around its axis with a pointer ‘P’ moving along a graduated scale. A small resistance ‘r’ is connected in parallel to wire AB to ensure low total resistance. The outer terminals T1 and T2 are connected to points A and B.
  2. Support Structure: The wire is fixed on a supporting structure that permits its expansion and movement when it heats up due to the current passing through it. It is essential that the supporting structure is designed in a way that is highly sensitive to even the slightest changes in the length of the wire.
  3. Current-Carrying Circuit: The instrument measures the current flow by passing it through T1 and T2. This causes the wire AB to heat up, and the temperature rise is proportional to the square root of the average of the square of the current (I2rms). As the wire length increases due to temperature, its tension reduces. The cylinder attached to the wire moves slightly, and the pointer deflects along the scale due to the tension in the spring on the other side. When the ammeter is disconnected from the circuit, the wire contracts to its original length, and the pointer returns to zero.
  4. Calibration Mechanism: The wire’s movement is utilized to activate a pointer or needle on a calibrated scale. The scale is labeled based on the current being measured. The calibration is usually performed by adjusting the wire’s initial position or tension.
  5. Enclosure: The protective case encloses the entire assembly, shielding delicate components from external influences and air currents that could affect current measurement accuracy.
  6. Power Supply: In some designs, a separate power supply may be used to ensure the wire is at a constant temperature, which can improve the accuracy and stability of the instrument.
  7. Insulation: The wire and its support structure are typically insulated to prevent short circuits and ensure efficient heat utilization for measurement purposes.

Working Principle of Hot Wire Ammeter

Thermal ammeters, also known as hot-wire ammeters, measure current by heating a wire. Here’s how they work:

  1. Current Flow Through the Wire: When an electric current flows through a wire, it faces resistance which impedes the flow of current. According to Ohm’s Law, the amount of resistance is directly proportional to the current passing through the wire. This can be expressed as V = I * R, where V represents voltage, I represents current, and R represents resistance.
  2. Heating Effect: The hot-wire ammeter works by utilizing the heating effect of the current passing through the wire. As the current flows through the wire, its resistance causes it to heat up. The amount of heat produced is directly proportional to the square of the current passing through it, as given by the formula P = I^2 * R, where P refers to power.
  3. Temperature Rise: When a wire is heated, it expands in length proportionally to the heat generated, which is related to the current flowing through the wire.
  4. Mechanical Movement: The wire expands mechanically, moving a calibrated pointer to indicate current.
  5. Calibration: The ammeter is calibrated in such a way that the pointer deflection corresponds to the amount of current passing through the wire. By observing the pointer position on the calibrated scale, the user can determine the current in the circuit.


Hot-wire ammeters are not as commonly used as other types of ammeters, but they find their application in specific situations where their unique properties are useful. Here are a few examples of the applications of hot-wire ammeters:

  1. Low Current Measurement: Hot-wire ammeters are highly sensitive to small changes in current and are used to measure low currents, making them ideal for high-precision applications, especially in the microampere or milliampere range.
  2. Laboratory Experiments: Hot-wire ammeters are sometimes used in laboratory setups and experiments where the goal is to measure small currents accurately. Due to their sensitivity and ability to measure low current levels, they are valuable in educational and research environments.
  3. Calibration Standards: Hot-wire ammeters are often used as calibration standards in labs to verify and adjust the accuracy of other measuring devices.
  4. Measurement of Pulsed or Fluctuating Currents: Hot-wire ammeters respond quickly to fluctuating currents, making them suitable for pulsed current measurements.
  5. Specialized Electronics Testing: In certain electronic testing applications, particularly those involving low-level signals or sensitive electronic components, hot-wire ammeters may be used to measure currents without significantly loading the circuit being tested.
  6. High Resistance Circuits: Hot-wire ammeters often have high internal resistance, which minimizes their impact on the circuit they’re measuring. This is particularly useful when measuring currents in high-resistance circuits.
  7. Historical and Educational Purposes: Although modern electronic measuring instruments are widely used today, hot-wire ammeters are occasionally utilized in classrooms to demonstrate the principles of current measurement and the heating effect in conductors.


The hot-wire ammeter is a device that measures both AC and DC electrical currents. It works by using a wire that expands when it is heated by an electric current. This expansion is proportional to the square of the RMS value of the AC or DC current being measured. In other words, the more current that passes through the wire, the more it expands, and this expansion can be used to determine the strength of the current.

Leave a comment