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528 RICHARDSON'S BLUEBOOK OF PROJECTION
a marked difference in the meter readings the granules in one of the buttons are probably packed.
Excessive current through a carbon microphone will pack and ruin it. Minute sparking between the carbon granules will weld them together so firmly that only new carbon will restore the instrument.
Condenser and dynamic microphones are sometimes used in the larger theatres. The former is simply a condenser, one plate of which is constructed as a diaphragm. When that plate vibrates, changing the distance between itself and the other plate, the condenser undergoes a vibratory change in its capacitance, which is to say, in the quantity of charge it is capable of holding. A minute charge-discharge current flows in and out of the condenser through the wires that connect its plates with a d. c. source. This current is quite as weak as photoelectric cell current. Therefore condenser microphones operate with a small amplifier mounted inside the same casing.
The dynamic microphone is made in several forms, but essentially it consists of a metal diaphragm (or metal diaphragm carrying a coil of wire) that vibrates in a magnetic field. In other words, it is based on the same principle of construction and operation as the disc reproducer. Exactly as in the case of the disc reproducer, alternating current is generated by virtue of motion in a magnetic field.
Various forms of the dynamic microphone have come into increasing favor for theatre work, such as reinforcing vaudeville, "crooning" by the organist, etc. An interesting and popular variation is the "ribbon" or "velocity" mike, which consists of a very thin duralumin ribbon so mounted that it is free to vibrate in the field of a steel magnet. Sound waves approaching the edge of the ribbon exert equal pressure against both its faces and hence do not Cause it to vibrate. This microphone is highly "directional," responding only to sounds that originate in the areas in front of it or behind it, and remaining "dead" to sounds that have their source at either side, or directly above or below.