The Wireless Age, April, 1915, page 489:
Marconi's Wireless Telephone
WITHIN a few months it is expected that the Marconi Wireless
Telegraph Company will be ready to announce the commercial wireless telephone, long predicted and confidently awaited.
The short distance Marconi wireless telephone now being developed for commercial use has a guaranteed working range of 50 kilometres (about 31 miles) between ships at sea carrying aerials 100 feet high and 200 feet span. This working range has been considerably exceeded in tests, during which it was also determined that the telephone can be set up, all connections made and the whole arrangement be put in working order in a half hour.
The telephone transmitter consists of a specially constructed Marconi valve, shunted with condensers and self-induction coils in such a manner that a continuous stream of oscillations is produced. The frequency of these oscillations is controlled by means of variable ebonite condensers, shown in the illustration in front of the transmitting valves. The oscillations produced by the valve being continuous and of constant amplitude give no sound in the receiver, even if the latter is placed but a hundred yards away.
The variation required for transmitting speech is produced by means of a microphone, in the use of which two methods can be adopted. The simpler method gives remarkably clear speech of better quality than is obtained with the wire telephone, and the more complex method considerably stronger speech, equal in quality to the wire telephone. The advantages of the second method are that no special care need be taken to speak loudly into the microphone and that this instrument and the receiving telephone may be placed in any part of the ship--say, the chart room--while the set itself remains in the wireless cabin. A simple change-over switch, which may also be controlled from a distance, is arranged for switching from talking to listening.
An 80 ampere hour accumulator is provided for the low voltage current used to heat the filaments of the valves. Four cases of dry cells connected in series give the high tension (500 volts) current necessary through the vacuum of the transmitting valve. An extra case of batteries is supplied for emergencies or when the others have dropped in voltage. The usual value of the vacuum current being from 10 to 20 milliamperes it is sufficiently small to make practical the use of dry cells for intermittent purposes. With the addition of a telegraph key the set can at once be adapted for continuous wave telegraphy.