The design of second-harmonic detector heads and their application to the reading of digital information and the measurement of low speeds of rotation

Two applications of the second harmonic principle are outlined and the design of suitable detector heads is detailed. The first application relates to the detection of binary recorded information on magnetic tape. The detector head described provides an output signal which is a direct measure of the flux Unking the head at any instant and does not depend for its output signal on relative motion between the head and the source of flux. The design and construction of such a head is considered and the results of an approximate analysis of its operation are compared with the actual performance of a practical head. The head is capable of resolving digits at packing densities in excess of 100 per inch. A double head arrangement consisting of a writing head separated from a flux-sensitive head by a digit separation is also described. This makes it possible to record digits at a constant packing density irrespective of tape speed and also can be used to provide character by character checking of the information being recorded. The second part of the paper considers a detector head suitable for detecting a d.c. magnetic field of the order of 10 millioersteds in space in the presence of another d.c. field at right angles and several orders of magnitude greater. A theoretical analysis of the operation of the device is presented and the results are in good agreement with the performance of a practical head. The detector head is intended to measure the `cross field¿ generated when a cylindrical conducting rotor is rotated in a primary d.c. field. This cross field is proportional to the angular velocity of the rotor. The combination provides a basis for a commutatorless and brushless d.c. tachogenerator having a very low noise content.