Hot wire measuring mechanism
A Hitzdrahtmesswerk is used for display of streams , converting it into a corresponding of the current pointer deflection.
The current to be measured flows through a hot wire , which heats up when the current passes through ( Joule heat ). This causes this wire to expand. A thread attached to the wire is pulled by a spring over a roller, which sits on the pointer axis, according to the stretching. The rotation of the roller causes the pointer to deflect. After switching off the current, the wire cools down again and pulls the pointer back to the zero position.
The heating and thus the expansion of the wire is proportional to the square of the rms current . Due to the geometric relationships, the pointer deflection is non-linear , and so is the scale. Since the heating is independent of the direction of the current, this measuring mechanism is suitable for direct current as well as alternating current . It can also be used at high frequencies and is suitable for measuring the effective current.
However, it is not suitable for small currents because the heating is too low here. It has been replaced by heat-sensitive components (e.g. PTC resistors ).
In practical use, the hot wire measuring mechanism has now been almost completely replaced by digital measuring devices , apart from a few applications for measuring high-frequency currents .
A disadvantage of the hot wire measuring mechanism is its inertia, it only reacts slowly to changes in the current. On the other hand, it was precisely this inertia - and the fact that the pointer, in contrast to moving- coil and moving-iron measuring mechanisms, for example, can not oscillate freely - a reason why hot-wire measuring mechanisms were used for decades, especially in vehicle fittings .