Balance Methods in Alternating Current Measurement

Abstract

(1) Owing to the comparatively low inherent sensitivity of alternating-current instruments, balance or opposition methods of measurement are more desirable and of wider application in alternating-current testing than in direct-current work. (2) The application of balance methods to alternating-current tests is a greater problem than in direct-current work, because of the necessity of having to oppose the measured quantity in phase position as well as in magnitude; and because of the difficulty of precisely detecting when a condition of balance is obtained. (3) A suitable source of current or voltage for opposition tests consists in the phase-shifting tranformer, to which is added a potentiometer type of rheostat. (4) A satisfactory detector is found in the D'Arsonval galvanometer or the electrodynamometer, so arranged that the indicator oscillates at an easily observable periodicity. A condition of balance exists when the oscillations have been reduced to zero. (5) In current measurement, use is made of a transformer having three windings, the measured and opposing current flowing in two of these windings, and the difference, as derived from the third, reduced to a zero value, when the ratio between the active currents becomes the turn ratio of the transformer. With this arrangement it is equally easy to measure extremely small or extremely large currents. (6) In voltage measurement a simple potentiometer arrangement of resistance is used, the ratio being set to a suitable value at the beginning of the test.