Hot-wire ammeters and voltmeters are types of analog instruments used for measuring electric current (ammeters) and voltage (voltmeters). They operate on the principle that the current passing through a wire will cause it to heat up, which in turn affects its resistance. This change in resistance can be calibrated to provide a measurement of the current or voltage being measured. Here's an overview of their construction and working:
Construction:
Hot-Wire Ammeter:
A hot-wire ammeter consists of a fine wire made of a material with a relatively high temperature coefficient of resistance, such as nickel-chromium (Nichrome) alloy.
The wire is mounted in a suitable housing, often in the form of a coil or zig-zag pattern to maximize its length within a limited space.
One end of the wire is connected to the instrument terminal through which the current to be measured passes.
The other end of the wire is connected to a spring mechanism or some other means to keep the wire taut.
A pointer attached to the wire moves over a calibrated scale to indicate the measured current.
Hot-Wire Voltmeter:
The construction of a hot-wire voltmeter is similar to that of a hot-wire ammeter.
The primary difference is that the wire used in the voltmeter has a higher resistance and is more delicate since it needs to handle smaller currents compared to an ammeter.
The voltmeter's wire is also connected across the terminals of the circuit to measure the voltage drop across it.
Working:
Hot-Wire Ammeter:
When current flows through the hot-wire ammeter's wire, the wire heats up due to the Joule effect (IĀ²R heating).
As the wire heats up, its resistance increases. This change in resistance causes the wire to elongate slightly.
The elongation of the wire is converted into a rotary motion through the spring mechanism, and this motion is transferred to the pointer on the calibrated scale.
The pointer indicates the measured current on the scale, and the user can read the current value directly from the scale.
Hot-Wire Voltmeter:
Similar to the ammeter, the hot-wire voltmeter's wire heats up when voltage is applied across it.
The wire's resistance change is proportional to the voltage applied.
The elongation of the wire is converted into a motion of the pointer on the calibrated scale, which indicates the measured voltage.
It's important to note that hot-wire instruments are relatively slow and have limited accuracy compared to modern digital instruments. Additionally, they are affected by factors such as ambient temperature, and their use is somewhat restricted to certain applications.
Modern digital instruments like digital multimeters have largely replaced hot-wire ammeters and voltmeters due to their higher accuracy, faster response times, and greater convenience.