Moving Iron instruments

As the name suggests , there is iron piece in these instruments ..the movement of this iron piece gives the necessary reading.

These instruments are of two types i.e. attraction and repulsion.

In the attraction type, there is a iron and a coil ..when circuit current flows through the coil , the iron moves and a pointer is attached to the iron piece which gives reading on the scale.

In the replusion type , there are two iron piece and a coil . When circuit current flows through the coil and set up magnetic field ..than two iron pieces repel each other ..pointer is attached to one of the iron ..that gives the reading .

So basically moving iron instruments are of two types .. Attraction type and replusion type.

Attraction type moving iron instruments :

Components : a oval shaped iron piece , coil , Pointer , scale , control and balance weight , damping part

Construction : There is a coil, an oval shaped soft iron piece on the spindle on which pointer is attached , controlling and damping part

These type of instruments are mostly used as ammeter to measure the current and as voltmeter to measure the voltage.

In this instrument, there is a coil through which current to be measured is passed or current to be proptional to the voltage to be measured is passed. Than as current passes through the coil than coil sets up a magnetic field.

The soft iron piece attached to the spindle moves inwards the coil and as the current increases through the coil than magnetic field strength becomes more and more than soft iron piece moves more and more towards the coil and comes closer to the center of the coil where magnetization is maximum.

As the soft iron piece is moved towards coil , the pointer attached to it also moves through a scale .the pointer movement or deflection is directly proportional to the current flowing through the coil.

Replusion type moving iron instruments

Construction: A fixed coil , two rod(bar) shaped soft iron piece (one iron piece is fixed and another iron piece is movable and is attached to the spindle of instrument, controlling and damping part

Working: When current to be measured is passed through the coil of this instrument, than a magnetic field is setup in the instrument which magentizes both the iron piece equally and with same polarity. So both the iron piece repel each other so moving iron piece moved away from the fixed iron piece ..hence pointer attached to the spindle also moves in this way. More the current through the coil , more is the strength of the magentic field and more the movement of moving iron or hence pointer through the scale which gives necessary reading i.e. value of the current .

Advantages and disadvantages of moving iron instruments

  • Cheap
  • Robust
  • Reliable
  • Works on both AC and DC

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