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)URCES OF SOUND CURRENT 525
faultlessly smooth d. c. (29) If it is drawn from dry batteries, high resistance test phones should he connected directly across the battery terminals. I f any noise is heard in the phones the battery has become too noisy for use with a photo-cell, and must be replaced.
The Disc Reproducer
(30) The phonograph pick-up, or reproducer, is a miniature generator in which the physical motion that generates current is reciprocating, rather than rotary as in the case of a power-supply generator. The needle tracking the groove of the record vibrates from side to side at the same frequency with which irregularities of the groove follow each other. The distance through which the needle vibrates varies according to whether the irregularities" of the groove are large or small.
Attached to the needle, inside the reproducer, is a small piece of iron. Lines of magnetic force originating in a steel magnet within the reproducer flow through this piece of iron and also through a coil of wire. When the needle vibrates, the piece of iron attached to the needle holder vibrates with it, and the strength of the magnetic held, as it passes through the coil of wire, is varied accordingly. This change in magnetic strength generates an alternating current in the wire. The frequency, of this current corresponds to the frequency of the needle's vibration, and its strength to the distance through which the needle moves.
Some of the highest grade pick-ups contain oil as a "damping" medium. The needle-holder assembly is generally resonant at some frequency of audible sound, and will therefore tend to vibrate more strongly at that frequency than at any other. The oil counteracts this tendency, but makes projection room repair of the pick-up distinctly difficult. It is not easy to replace the oil even if the right oil is at hand, and for that reason it is good practice to return such pick-ups to the factory even for simple repairs, rather than open them in the theatre.