A High Sensitivity Power Factor Bridge

This paper describes an a-c. bridge of high precision, used for power factor measurements in one of the dielectric investigations being carried on at the Johns Hopkins University. The bridge, a modified form of the Schering bridge, possesses several novel features and advantages. The detecting instrument is a moving coil a-c. galvanometer; its field excitation is supplied from a phase shifting transformer, which permits obtaining independent ratio and phase angle balances. A shielded transformer electrostatically isolates the galvanometer from the bridge circuits. This bridge is completely shielded and guarded and the analytical theory of the resulting mesh connection is given in the paper. The mathematical treatment is general and may be applied to any similar network. A full discussion is given of the effects upon the balance relation of various sources of error, for example, as failure to balance guard circuits properly and the presence of leakage resistance between measuring and guard circuits. Experimental verification of the equations is presented. Power factors of specimens of cable material have been measured ranging from 0.00007 to 0.16 with a maximum variation of ± 0.000005.