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56 AMATEUR TALKING PICTURES
current can range from 2 to 20 volts — much the same as for the Amplion microphone previously described. Generally speaking, however, when the microphone is coupled relatively close to the transformer a voltage of 6 will be found satisfactory. As a matter of fact, in using all types of microphone the voltage which should be applied is really immaterial, since it is the flow or current consumption which is really important. In the case of the Pentrovol this should be of the order of 12-15 milliamps., the resistance of the microphone being approximately 300 to 400 ohms. If a long lead has to be employed then naturally the voltage would be higher.
At D and E are two microphones manufactured by C. Fredk. Adolph & Co., who specialize in work for home recording. They are priced at 25s. and 12s. 6d. respectively, and represent the product adopted by most of the manufactures of home recording equipment. The same company possess many other types of microphone, particularly suited to the needs of the amateur. At F is a neat "Adolph" microphone transformer, and at(?a control unit embodying transformer, connection terminals, and rheostat.
The leads running from the microphone to the transformer should preferably be shielded, or in other words run in either lead -covered cable or metal braided cable. The metallic covering of either of these should be earthed preferably to the earth connection of the amplifier or radio set.
In long distance transmission, and by long distance we imply something in the order of 20 or 30 ft., the leads should be taken direct from the microphone across the distance, and then coupled to the transformer which should be adjacent to the amplifier.
Volume control of the microphone takes place between the secondary of the transformer and the amplifier, as shown in Fig. 26. In using this volume control it should be borne in mind that its object is to modify the recording level according to the requirements of the particular sound being recorded, and is not for keeping the output of the" amplifier within distortionless limits. With the microphone volume control set at a maximum the amplifier volume