In a Dynamometer type instrument, the operating magnet field is created by two air-cored fixed coils which are placed very nearer to each other thus operating magnetic field moves the moving coil which is mounted on the spindle of the instrument and thus deflecting torque is produced and moves the pointer of the instrument from zero position.
Working Principle of Dynamometer:
By connecting a high resistance in a series of suitable values, the ammeter can be used as a voltmeter. An ammeter can’t be connected across the load terminals, as the resistance of the ammeter is very low. If the ammeter is connected across the line or load, the ammeter will be burnt out due to the flow of a large current through the ammeter.
The voltmeter is always connected across the line or load terminals as the resistance of the voltmeter is very high. If the voltmeter is connected in series with the load or line, then the voltmeter will give a very low reading. At that time, the voltmeter record the potential difference or voltage between the two terminals of the line at which the voltmeter is connected. The voltmeter can be used as an ammeter by connecting a very low resistance in parallel with the voltmeter of requisite value.
If a permanent magnet moving coil type ammeter or voltmeter is connected to the AC supply by mistake, then there will be no deflection as the pointer of the instrument would tend to move in the opposite direction for each successive reversal of the alternating current. The last commonly used watt meter is the dynamometer type, which has a uniform scale and can be used both in AC and DC circuits. The symbol of the instrument is similar to that of dynamometer type ammeter and voltmeter.