What is a Permanent Magnet Moving Coil (PMMC)?

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What is a Permanent Magnet Moving Coil (PMMC)?

A Permanent Magnet Moving Coil (PMMC) meter – also known as a D’Arsonval meter or galvanometer – is an instrument that allows you to measure the current through a coil by observing the coil’s angular deflection in a uniform magnetic field.

A PMMC meter places a coil of wire (i.e. a conductor) inbetween two permanent magnets in order to create stationary magnetic field. According to Faraday’s Laws of electromagnetic induction, a current carrying conductor placed in a magnetic field will experience a force in the direction determined by Fleming’s left hand rule.

The magnitude (strength) of this force will be proportional to the amount of current through the wire. A pointer is attached to the end of the wire and it is put along a scale.

When the torques are balanced the moving coil will stop, and its angular deflection can be measured by the scale. If the permanent magnet fiel is uniform and the spring linear, then the pointer deflection is also linear. Hence we can use this linear relationship to determine the amount of electrical current passing through the wire.

PMMC instruments (i.e. D’Arsonval meters) are only used for measuring the Direct Current (DC) current. If we were to use Alternating Current (AC) current, the direction of current will be reversed during the negative half cycle, and hence the direction of torque will also be reversed. This results in an average value of zero torque – hence no net movement against the scale.

Despite this, PMMC meters can accurately measure DC current.


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