These instruments are used on both AC and DC supplies. Theseprovide accurate readings only when frequency and supply voltages are constant. These wattmeters consists of two laminated electro magnets and one of them is excited by the current with exciting winding. This winding is connected in series of the. The other magnet is excited by the current which is proportional to the. Its exciting coil is connected in parallel with the circuit. The winding of first magnet carries line current and the other coil carries voltage and is highly inductive. This coil is connected to the supply and the flux lags 90behind the voltage. Theis produced and is proportional to the power in the load circuit. The torque acts on the disc and force is produced in the disc. The disc starts moving and theis attached on it with aand it shows reading on its scale.