An interesting and unusual set of equipment, these small portable telephones were used to provide inter-communication within vehicles or over other short lines when circumstances required rapid and simple setting-up and disconnection. Consisting of a small handset which can be disassembled easily for storage in its portable leather case, the other components include the combination microphone/receiver and a separate receiver equipped with mounts for a set of fabric head straps, which were not present with this unit.
The design of the handset is unusual as the electro-acoustic unit is used as microphone and receiver. The unit is mounted in a normal receiver housing with an acoustical path from the speaker's mouth to the rear of the diaphragm by means of an extended horn-type mouthpiece. The horn is a die cast aluminum alloy, plated with several very thin alternate layers of copper and nickel, then coated with Glycerylphthallate lacquer. The weight of the complete handset is approximately 10 ounces. The spare receiver weighs 7 ounces. Originally made with two different types of connecting plugs on the ends of the line cords, this set has been altered for whatever reason and these plugs were removed.
On the exterior of the leather carrying case is a pouch for the brass tool used for grounding the unit. The lid of the carrying case is marked in black ink by the manufacturer and the carrying straps are present. And, although a few areas have some crazing and are becoming somewhat fragile, they are intact and still quite supple.
As the name implies, the microphone of a sound-powered telephone derives its energy directly from the sound waves and is a reversive to the original principle of the telephone, in which a receiver unit was used as a microphone.
A rare and unusual piece of Japanese communications equipment rarely seen on the market.
Approximate dimensions of case are Length 7" Width 2.5" Height 6"