This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The owner of an amateur wireless outfit often has reason to regret that he cannot let some of his friends listen to a message at the same time as he himself. The magnifier described in the following permits all those present in the room to hear the message, provided, of course, they are able to interpret the Morse alphabet by sound.
A very simple means, making the message audible at a distance of about 10 ft., is to attach a phonograph horn, or a horn of cardboard or metal, to the telephone receiver, but a much better arrangement can be made as shown in the diagram, in which A represents the antenna or aerial; B, the detector, and C, the receiver. Procure a small microphone, D, placing its mouthpiece closely against the receiver - for the sake of clearness the two are separated in the diagram - and connect the former with a battery, F, of two or three dry cells, in series with the microphone of an ordinary telephone transmitter provided with a large horn. The effect obtained by this simple means will be surprising.
Ill: The Phonograph Horn as It is Connected to a Detector for Transmitting the Messages