Homer Fay

Magnetic Hysteresis Loop Tracer Using Operational Amplifiers

A magnetic hysteresis loop tracer has been built to measure the magnetic properties of small cylindrical samples of nonconductive ferromagnets. Applied fields of 10 000 Oe peak amplitude are obtained from a water cooled solenoid driven at 60 Hz, and the change in flux is detected with a coil on the sample. Two operational amplifiers convert the input variables to analogs of the local field H and the induction B or magnetic intensity I. The sample area and demagnetizing factor are used as parameters, and a correction is made for the excess coil area. Tests indicate that correct loops are produced except when the demagnetizing factor and the excess coil area are both large. Application to the rapid determination of the magnetic properties of hard ferrites is illustrated. The apparatus may also be used to magnetize and demagnetize samples and measure the permeabilities in both states.

Electro-Optic Modulation of Light Propagating near the Optic Axis in LiNbO 3

The passive and the electrically induced birefringences and retardations in LiNbO3 crystals have been calculated for all directions of propagation near the optic axis and for a field direction in the x–y plane. In two special directions, parallel to the induced orthorhombic principal axes, the passive and induced retardations are exactly additive. In other directions the retardation is not a linear function of the field. By rotation about these axes, any desired degree of optical biasing may be obtained without degradation of the electro-optic modulation. Experiments with rotation for a quarter-wave bias have shown that useful modulation may be achieved without compensator waveplates.

Characterization of potassium tantalate-niobate crystals by electrooptic measurements☆

Abstract The electrical and optical properties of crystals of potassium tantalate-niobate are compared with the properties expected for the ideal perovskite structure. Melt-pulled crystals obey the Curie-Weiss law but exhibit dielectric and optical anisotropy. The striations that are observed perpendicular to the growth direction are found to consist of both permanent lines and bands and regions of variable phase retardation. Local variations in the electrooptic effect indicate that the latter are caused by local changes in the dielectric constant. A variation in Ta/Nb of as little as 0.5 mole % could have produced these aberrations. The quadratic electrooptic effect in KTN is found to be isotropic.

Harmonic Bridge for Measurement of Nonlinear Electric and Electrooptic Properties of Crystals

A harmonic bridge circuit has been developed for measuring linear and nonlinear electric coefficients and quadratic electrooptic coefficients in high dielectric perovskite crystals. Third and fifth harmonic components are added to the fundamental audio‐frequency driving voltage. The bridge is balanced when the charge or displacement of the crystal is sinusoidal. The harmonic content of the driving voltage may then be measured on a harmonic analyzer, from which measurements the electric coefficients of the crystal may be calculated. The same sinusoidal displacement may be used to display the electrically induced phase retardation intensity pattern on the oscilloscope. The effective electrooptic coefficients are readily determined from the oscillographs. The advantages of the harmonic bridge method are illustrated by measurements of the electric and electrooptic coefficients of a crystal of potassium tantalate niobate (KTN).

Density separation of solids in ferrofluids with magnetic grids

Abstract Nonmagnetic solids in a superparamagnetic ferrofluid are subjected to body forces proportional to the intensity of magnetization of the fluid and the gradient of the magnetic field. An apparent density of the fluid can be defined from the force equations, and since the apparent density can be much larger than the true density, it is possible to levitate or float dense objects. Mixtures of solids with a density greater than the apparent density sink while lower density solids float. In practice it is difficult to create a uniform gradient over a large volume and “single gap” magnetic separators require very large magnets or have a limited throughput. To overcome that problem, “multiple gap” magnetic grids have been designed. Such grids consist of planar arrays of parallel bars of alternating polarity, driven by permanent magnets. When immersed in ferrofluid, magnetic grids create nonuniform field gradients and apparent densities in the fluid. However, both analysis and experimental measurements sh...