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Using Magnetic Recorders
When first you handle a magnetic tape recorder, you will probably be surprised at how simple it all is. Usually it is built into a case of about the same size as a portable gramophone. A hinged lid gives easy access to the two reels of tape which generally are arranged horizontally, side by side.
The tape measures J inch wide and just over \\ thousandths of an inch thick. In this country, the popular size of a reel is 7 inches in diameter and carries 1,200 feet of tape. This lasts half an hour, running at the popular standard speed of 1\ inches per second. Only half the width of the tape is used at one passage, however, and by interchanging spools you can run the tape back again and record for a further half hour. This type of machine is called a twin-track recorder.
The Mechanism
A capstan roller draws the tape over a recording head consisting of a coil of wire wound round an iron core. This forms an electromagnet, in the core of which there is a narrow gap only half a thousandth of an inch across. Usually the gap is filled with brass and is barely visible. Nevertheless the force of the electromagnet is concentrated along the line of the gap and the iron oxide is magnetised locally as it passes over.
In addition to the recording impulses, the recording head is supplied with a supersonic bias. This is an electric current alternating about sixty thousand times a second. Its job is to "shake up" the coating on the tape so that the recording impulses can more readily affect it. Without the bias, the
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