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SPEAKERS 587
of the voice coil is, as explained, solely to oppose or to reinforce that field.
(4) The dynamic speaker is made to produce sound at high volume, and in consequence its diaphragm or armature must move through a relatively great distance, even when loaded by a horn or a cone of large diameter. Manufacturers have not found it practical to provide a permanent steel magnet of such strength that it will continue to exercise an even influence upon the moving diaphragm over appreciably wide distances. Instead, an electro-magnet is used for speakers intended to reproduce sound at high volume. The winding of the electromagnet so used is the "field coil" of the dynamic speaker.
The voice coil of the same speaker commonly consists of a very few turns of wire, firmly fixed to the d'aphragm. Alternating current is supplied to this coil through the output transformer of an amplifier. The voice coil therefore produces an alternating magnetic field, while the field coil, being supplied with d c, possesses a steady field. During every cycle of alternating current that flows through its windings, the voice coil will be alternately attracted toward and repelled by the magnetic field of the e1ectro-magnet. Hence the voice coil, and the diaphragm attached to it, will vibrate around the central, or neutral, position which they normally occupy by virtue of the mounting arid inherent elasticity of the diaphragm. The extent and frequency of their vibration will be governed (since the field of electromagnet does not fluctuate) solely by the amperage and frequency of the voice coil current.
(5) The field coil, which is the winding of the electromagnet of a dynamic speaker, must be powered with relatively smooth d. c. If a. c. were used a powerful 60-cycle hum would be heard. If rough or pulsating d. c were used the pulsations would be heard in the sound. Perfectly smooth d. c. is not necessary, partly because the field coil of any speaker makes a very good filter choke, and partly because irregularities in the speaker field supply are not amplified (while irregularities in the d. c. supply to a photo-cell, for example, are amplified tremen