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54
AMATEUR TALKING PICTURES
distinction is so definite that some manufacturers list their microphones as "announcing" apart from general purpose.
Fig. 25 shows several microphones, etc. At A is the "S-J" microphone marketed by Messrs. Claude Lyons, which is of the single button type and of 100 ohms resistance. It is stated to handle frequencies from 70 to 3,000 cycles, and is obtainable in three sensitivities, viz. A very sensitive, B medium (standard), and C damped. For general purposes the medium sensitivity is to be preferred. The price of this microphone is 75s. The writer has employed this microphone for a certain amount of recording work and found it quite satisfactory from the point of view of voice, although it will not produce best quality results on music.
The Amplion microphone shown at B is a larger model priced at £5, and is 4 in. in diameter and very strongly constructed. Primarily it is an announcing microphone, but its characteristic is sufficiently good for music. It is not as sensitive as some of the cheaper microphones, but possesses the advantage that it can be handled during recording if necessary, providing care be taken. It is usually operated with a step -up 13 to 1 transformer, and with a polarizing voltage up to 20 volts. In using a microphone of any description the battery voltage is kept to a minimum consistent with good results. A higher voltage tends to give increased loudness of signal, but if too high may damage the microphone.
At C we have the Igranic Pentrovol microphone on stand. This is probably one of the finest carbon microphones on the market, and is capable of producing really excellent results. It is listed at £7 7s., but for serious work is really worth the outlay. With it can be supplied a control unit which embodies a series of connections for microphones, battery pick-up if required, amplifier, an ammeter showing the microphone current, and a volume control for regulating.
The transformer which should be used with the Pentrovol microphone should have a step-up ratio of 10 to 1 or higher, and a primary winding with an impedance of the order of 1,500 ohms at a frequency of 50 cycles. The energizing