SOLUTION

Permanent Magnets Moving Coil “PMMC” Instrument 

• These instruments are used either as ammeters or voltmeters and are suitable for d.c. work only.
• This type of instrument is based on the principle that when a current-carrying conductor is placed in a magnetic field, the mechanical force acts on the conductor.
• The coil placed in the magnetic field and carrying the operating current is attached to the moving system.
• With the movement of the coil, the pointer moves over the scale to indicate the electrical quantity being measured. This type of movement is known as the D’Arsonval movement.
• The PMMC type instrument can be used for measuring the DC value.

Dynamometer type  and the Moving iron instrument can be used to measure the A.C and D.C

• Whatever the direction of the current in the coil of the instrument, the iron vanes are so magnetized that there is always a force of attraction in the attraction type and repulsion in the repulsion type of instruments.
• Thus, moving iron instruments are unpolarised instruments i.e. they are independent of the direction in which the current passes. Therefore, the instruments can be used on both ac and dc.