A modified Schering bridge which, whilst simple in operation, will measure the power factors of insulating oils with a high order of accuracy has been designed and constructed. The power factors of insulating oils are often very low, e.g. 0.00005 or less at room temperature, and considerable care must therefore be taken to avoid small errors which may arise from deficiencies in the electrical circuit, the oil test cell and the measuring technique. An oil test cell available commercially has been modified for use in an oil bath, so that accurate temperature stabilization is rapidly attained, and a method of cleaning and filling the cell has been evolved. Details of cell corrections are given. The procedure described in the paper enables the 50-c/s power factor of an oil to be measured with an error of approximately ±3% ±0.000015.