Experimental Electricity
��Practical Hints for the Amateur
���Wireless Communication
��A Microphonic Amplifier
By Arthur Ellison
��THE amplification of radio signals is receiving much attention. For this purpose nothing is superior to the audion amplifier, but the price of this apparatus places it beyond the reach of many amateurs.
The microphone amplifier herein de- scribed will do remarkable work when properh- adjusted, but it 7)utsl be kept free from vibration. The easiest way to do this, and perhaps the best, is to place several layers of felt under the base, or, if possible, to mount it firmh- on a solid brick wall. Small, soft rubber feet under the in- strument are also of some assistance in cutting out undue vibration.
In the accom- panying drawing at Fig. I is shown the electromagnetic microphone com- plete. It consists of two I ,ooo-ohm tele- phone receivers mounted on a base in a vertical po- sition.
The recei\'er shells aredrilled and tapped toadmit two small machine screws which fasten them to the brass angles. The support
���Fig. 1. The Electromagnetic Microphone Complete and Mounted in Position
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��on the left is a plain L with a hole in the bottom part to clamp it to the base, but the other L is somewhat different in shape and is shown in detail in Fig. 40. As will be readily understood, it is slightly shorter than the first named, and is turned up at the end so the thumb screw will act on it. A slot is cut in the bottom part so that the recei\'er may be moved forward or backward by operating the thumbscrew, and a spring is arranged to push it back normally.
In the center of each of the dia- phragms is mounted a small carbon button. These are only about J^ in. in diameter and are shown in detail in Fig. 2. A conical hole is drilled in one face and a slot cut with a fine saw on one side. A very thin flexible wire is laid in this slot and solder poured in to hold it there. By twisting the saw while cutting the slot, the slot will be made wider at the bottom, and the solder will hold the wire in place.
The carbon but- tons are fastened to the center of the
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