An Improved Type of D-C Wattmeter of the Shunted Type

While wattmeters for high current values of a-c power are universally used in connection with instrument transformers, d-c power is ordinarily measured by using separate ammeters and voltmeters on account of the difficulty in making a heavy-current d-c wattmeter. Although large direct currents are conveniently measured by use of the shunt and millivoltmeter, shunted-type d-c wattmeters have in the past been somewhat unsatis-factory. An improved d-c wattmeter is described which corresponds in accuracy and sensitivity to the usual permanent-magnet moving-coil ammeter and operates from the usual ammeter shunt.