We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
SOURCES OF SOUND CURRENT
527
Microphones
(32) The common, or carbon-type, microphone consists of a carefully weighed quantity of carbon granules between two metal plates, one of which is a diaphragm free to vibrate in response to sound vibrations in the neighboring air. The effect of the vibration is to vary the pressure on the carbon and thereby change its resistance to the flow of current. A steady direct current passes through the microphone. When the diaphragm vibrates the amperage increases and decreases in rhythm with the frequency of the sound that causes the vibration, and in proportion to its intensity. The commonest microphone circuit is that for the "double-button mike/' shown in Figure 124. The granules in one "button" are compressed while those in the other are released from pressure. The resulting fluctuations in the current through the transformer primary generate an alternating current of corresponding frequency and strength in the secondary.
Fig. 124. — Circuit of a double-button carbon microphone.
(33) The carbon granules of this microphone may "pack" or cling together as a result of rough handling or of age. In that case they will not respond to vibration of the diaphragm. Gentle shaking will sometimes cure this condition. In severe cases it is incurable in the theatre.
Packing can be detected by reading the current through both buttons with a milliammeter. and this test should be applied from time to time as a precaution. I »< >t h buttons should have the same resistance, and if there is