Rictometer System for Calibrating Transmitters of Remote Indicating Compasses

Abstract

Calibration of the sensor (transmitter), of a remote indicating compass system, must be carried out in a magnetic field whose magnitude and direction simulate closely those of the earth at a specified point. Major disturbances of the earth's magnetic field by electrical equipment occur in most laboratories. Where conventional Helmholtz calibrating schemes are used, accuracy can be achieved only by lengthy averaging of repeated measurements and exercising extreme care. This paper describes a measurement system called a rictometer, which permits precise transmitter calibration with much greater ease than previously. The rictometer consists of a system of shields which reduces the earth's horizontal field component from 200 milligauss to about 13 of a milligauss. Two pairs of Helmholtz coils within the shielding system restore the horizontal component of the earth's field. By proper selection of currents in each of these pairs of coils,this horizontal component can be rotated in discrete steps through 360° for calibration purposes. The rictometer allows the experimenter to calibrate a compass transmitter readily with a precision of plus or minus 0.1 deg in normal laboratory fields. Deliberate immersion of the rictometer in a 50 gauss field causes a maximum transmitter calibration change of plus or minus 0.15°. Thus, the system should prove valuable for reliable sensor calibration under most adverse field conditions.