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1950 MAGNETIC SOUND FILM 507
Bulk Wiping
The magnetic film stock is generally kept for the sake of convenience in the form of reels or spools and the erasing or wiping is usually effected by unwinding the film, drawing it past a magnetic erasing head, and then rewinding it on to a further spool.
However, as large reels up to a length of one thousand feet or more are usually employed in practice, it will be appreciated that a considerable time is needed to carry out the erasing operation while there is always the danger of insufficient demagnetization at the joints due to loss of contact with the erasing head and the necessity of paying particular attention to the demagnetization of them.
It would therefore be highly advantageous if the recorded signals on a whole reel could be erased in a single operation without the
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Fig. 9. Laboratory model of apparatus for wiping magnetic film in bulk.
necessity of the reel being rewound, that is, so that the magnetic signals are erased while the film itself is maintained in a reeled, i.e., bulk form.
Such means for the bulk wiping of magnetic sound records have been devised and consist of a turntable upon which the reel is rotated in mutual combination with the translatory movement of an electromagnet which produces an erasing field.
Figure 9 illustrates the original laboratory model of the apparatus which has been evolved. The film is placed on the turntable which is then rotated by means of the handle through internal gearing. A lead screw forming part of the drive mechanism also drives an electromagnet toward and then, upon reversal of the direction of rotation