Attraction Type Instruments

Parts

• Flat Fix Coil or Electromagnet
• Moving Iron
• Spindle
• Pointer with Balancing Weight
• Scale Card
• Control Spring
• Air Damping Chamber & Damping Vane

Working Principle

Construction

In this type of instrument, a flat coil (electromagnet) is fixed in one place and used. Just in front of the space between the coils, a piece of oval-shaped soft iron is fitted on the spindle. A pointer, control spring and air damping arm are also fitted with this spindle. As shown in below (fig.a & b).

Working Method 

A current passing through a coil creates a magnetic field that moves through the space between the coils. This magnetic field attracts the iron (disc) mounted on the pivoted spindle, which moves the pointer on the spindle scale to indicate the reading. Irrespective of the direction of the current in the coil i.e. on the side of the piece of iron whether the magnetic field is "N" end or "S" end. In both cases, the force of attraction of the iron piece will act. Hence these instruments are used on both AC and DC. A single spring is used for control torque and mostly the air fraction method or vane chamber method is used for damping torque. The air fraction method is shown in (Fig.a & b) above. In these instruments, the deflecting torque is directly proportional to the square of the current. Therefore, their scale is non-linear or non-uniform.