A Method of Measuring the Magnetic Properties of Small Samples of Transformer Laminations

This paper describes equipment developed for measuring the magnetic properties of small pieces of transformer laminations when energized with a 60-cycle magnetic field with or without super-imposed direct-current polarization. A Maxwell bridge circuit was used in such a manner that the adjustable amount of magnetic gradient H which was applied to the sample remained unchanged by any manipulation of the bridge parameters in making a balance, thus permitting a direct-reading calibration of H. Bridge parameters were direct reading in dynamic permeability for a specified cross-sectional area and in a factor Δ from which the power loss per unit volume could be computed. Second-order correction factors are evaluated in the Appendix. A visual null-balance detector of the directional type was devised to be sensitive uniquely to either the reactive or the resistive balance control of the bridge. This was accomplished by the use of a polarized modulation bridge together with a phase-shifting network and facilitated balancing operations to a considerable degree. The necessary sensitivity and selectivity were obtained with a degenerative amplifier terminated in a current-limiting network which eliminated all necessity of monitoring an adjustable gain control.