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Jotes on Wear of Magnetic Heads
G. A. DEL VALLE and L. W. FERBER
Since physical pressure (under contact) is essential between a magnetic head and magnetic media to provide high-quality sound, it is of extreme importance to determine the operating life of the head. This paper describes the methods used and the results obtained in establishing the wearability of the recordreproduce heads on RCA magnetic projectors.
SING PRESENT-DAY striped film and 12 standard sound film speed of 36 fpm, isre were three things which could be i ried in designing a magnetic recording lad for a maximum life expectancy. riese were: (1) the magnetic material i: pole pieces of the head; (2) the force ] tiding the head against the sound track; ;.d (3) the shape and surface condition the head.
The magnetic materials universally
ed for magnetic recording heads are
the high nickel-iron alloy group, one of
lich is used in the record-reproduce
ads on RCA magnetic projectors.
liese materials are soft physically and
(jnnot be hardened by heat-treating;
;jid the electroplating of hard or wear
sistant materials on these alloys affects
je important magnetic properties for
pich they were chosen. When used
a record head the pole pieces must
esented on October 10, 1952, at the •ciety's Convention at Washington, D.C., G. A. del Valle and L. W. Ferber, who ad the paper, Radio Corporation of nerica, RCA Victor Div., Engineering oducts Dept., Camden 2, N.J. "his paper was received on September 1 2, 52.)
have the following characteristics: the hysteresis and eddy-current losses must be reasonably low, because of the high-frequency bias voltage; the magnetic retentivity must be low in order to avoid a noisy system resulting from the application of a unidirectional magnetic bias to the film ; and the saturation value must be high enough so that the magnetic coating on the film will saturate first. When used as a reproduce head the pole pieces must exhibit a low retentive force to assure that little noise will be introduced into the system, and in addition the permeability for low-flux densities must be high in order to obtain efficient operation at the low values of flux obtained from the film.
Experience has shown that a prerequisite for a high-quality recording system is to have the magnetic heads (especially the reproduce head) in intimate contact with the magnetic coating. Because of irregularities on the surface of the magnetic coating on the film, and the friction in the pivots of the arm which holds the head, it is necessary to apply a force to the head assembly sufficiently great so that even under the most adverse conditions the head will contact
April 1953 Journal of the SMPTE Vol. 60
501