A Magnetostriction Filter

A rod of monel metal supported at its mid-point and fitted with two coils, one on each end, shielded from each other, and supplied with a polarizing magnetic field, resonates sharply at its natural longitudinal frequency to a voltage impressed on one coil, and induces a voltage in the other only while vibrating. It is thus a very selective band-pass filter. The attenuation is more than sixty decibels off resonance and about twenty at resonance. The band is about seventy cycles wide, thirty decibels up from the minimum, when the resonant frequency is 20,000 cycles. The effect of heat treatment and the design of the coils and shields are discussed. Actually two rods in series, coupled by a vacuum tube are used. This gives a curve only seventy cycles wide, as much as sixty decibels up from the minimum. The attentuation is found to be low at frequencies below about 8,000 cycles and there is a rather sharp minimum at 16,000 cycles due to some mode of vibration other than the longitudinal one. These defects, however, may easily be remedied by the use of a high-pass filter of conventional design.