The main purpose of this paper is to describe the construction of a differential electronic stabilizer which has been designed and constructed by the author to provide a supply unaffected by fluctuations in the mains voltage, for testing a.c. instruments. A simple mathematical theory is developed of a bridge consisting of two lamps and two resistors, which is an essential part of the apparatus, and experimental evidence of the accuracy of the theory is given. Applications of the stabilizer to the testing of d.c. and a.c. indicating instruments and of light- and heavy-current integrating meters are described and an account is given of the use of stabilized voltages for a.c. potentiometry. A suggestion is made for the calibration of a d.c./a.c. transfer voltmeter.