A voltage transformer for use in the measurement of small voltages

In a paper by Mr. R.S.J. Spilsbury and the present author, ¿ apparatus for providing a voltage-drop of about 2 volts propotional to and in time phase with an alternating current is described. The 2 volts is obtained from a ¿non-inductive¿ resistance connected either in the main-current circuit or in the secondary circuit of a high-precision current transformer, the primary winding of which is connected in the maincurrent circuit. The problem is to measure this 2 volts to a high degree of accuracy. Voltages of the order of 100 volts may be measured to an accuracy of about 1 part in 10 000 by means of a suitable electrostatic voltmeter, and this paper describes the performance of a transformer for transforming 2 volts up to 100 volts. The core is of mumetal, and the ratio of transformation is shown to be constant to 1 part in 10 000 over a wide range of frequency and applied voltage. The phase displacement between secondary and primary voltage is less than 2 minutes at rated voltage. The testing of the transformer necessitated the development of a special bridge and apparatus, which is described.