Electrodynamometer Type Wattmeter:
Construction: It essentially consists of the fixed and moving coil. The fixed coil is split into two equal parts which are placed close together and parallel to each other. Moving coil is pivoted in between two fixed coils.The fixed and moving coils are exited / connected as shown in figure. The moving coil is attached to the moving system so that under the action of deflecting torque the pointer moves over the scale. Controlling torque is provided by springs and damping torque is provided by air friction damping.
Working: When the instrument is connected in the circuit operating current flow through the coils due to this mechanical force exists between the coils. The result is that moving coil moves the pointer over the scale to give reading. When direction of current reverses, then it reverses the direction of current of fixed as well as moving coil so that the direction of deflecting torque remains unchanged hence these instruments can be used for measurement of A.C. & D.C. power.