The theory, construction and use of an electrostatic wattmeter suitable for the measurement of the losses in magnetic materials subjected to alternating magnetization are described. Details are also given of amplifiers for use in conjunction with the wattmeter, particularly at low frequencies and low flux-densities.Selected experimental results are given to illustrate the use of the equipment with a variety of magnetic materials, over a frequency range of 25 c/s to 25 kc/s.