The Fullerphone

Contributed by James Patton

The Fullerphone was a portable DC line Morse telegraph, devised in 1915 by Captain (later Major General) A.C. Fuller of the Royal Engineers Signal Service. The important feature of the Fullerphone was that its transmissions were practically immune from being overheard, which made the system at the time very suitable for use in forward areas. In addition, the Fullerphone was very sensitive, and a line current of only 0.5 microampere was sufficient for readable signals. In practice, however, 2 microamperes were required for comfortable readings, and it could be worked over normal Army field lines up to 15-20 miles long. When superimposed on existing telephone lines, telephone and Fullerphone signals could be sent over the line simultaneously without mutual interference. Fullerphone signals were much clearer than those of a "Buzzer telegraph," as the start and end of a signal did not depend on the starting and stopping of a vibrating armature. Hence the potential speed was higher than that offered by the Buzzer telegraph.

Captain A.C. Fuller

The Fullerphone should not be compared with other DC telegraph systems and Buzzer telegraphs, since its operational principle differed considerably. The Fullerphone employed direct current in the line. By means of a chopping device and a filter circuit the current which flowed into the headphones of both transmitting and receiving Fullerphones was interrupted at an audible frequency (about 400 to 550 Hz). That means that no call could be received (or side tone be heard) unless the chopping device (also known as [the] Buzzer-Chopper) was working and properly adjusted. Therefore the Buzzer-Chopper was always required—whether transmitting or receiving. A filter combination of chokes and condensers prevented any variation in the line current during a signal and suppressed any audible frequency currents produced either by induction from other lines or by a buzzer or telephone speech on the line from passing through the headphones. It also ensured that the rise and fall of line current was comparatively slow and thus prevented "clicks" being heard in the receiver of a telephone set superimposed on the same line. Thus, the Fullerphone could not be overheard either by induction or earth leakage and could only be "tapped" by a similar instrument directly connected to the line. It was found that only with the use of very sensitive equipment, believed to be valve [vacuum tube] amplifiers, was it possible to overhear a Fullerphone, and then only when the listening earth was within 180 feet of the Fullerphone’s  earth contact.

Improved models of the Fullerphone were also used in WWII.

From The Imperial War Museum site (with minor editing and emphasis added)