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Fullerphone

Fullerphone Mk.V. This model is fitted in a wooden case ('Boxes, No. 2, Fullerphone, YA.6156') which has no provision for stowing the headphones in the lid. Headphones without a metal clip such as 'Receivers, headgear, C.L.R., double, No. 3' or 'Receivers, headgear, D.L.R., double, No. 5' (shown in photo) were normally used with this type of wooden case.

Reception.
If a DC voltage is applied between the L1 and L2 terminals and the circuit is momentarily closed at contact K, a current will flow through the choke coils CH1 and CH2, the headphones, contact K and the Morse key. When the circuit is broken at K the current cannot flow through the headphones but will flow into and charge the condensers C1, C2 and C3. If the circuit is again closed at K the condensers partially discharge through the headphones. Therefore when the chopper is working, an intermittant current at audible frequency flows through the headphones while the line current alternately flows through the headphones and into the condensers. The line current remains practically constant. Thus Morse code dots and dashes sent by the single current Morse key at the distant Fullerphone are received as short and long notes in the headphones while the current in the line is of much the same nature as that sent by a single current Morse set. The line current in this case, however, is very much smaller, and clicks are not perceptible due to the smoothing action of the filters in the Fullerphone.

Transmitting.
When transmitting, the Buzzer-Chopper is kept working as on reception. If the circuit is closed at contact K and the Morse key is pressed, a current from the battery flows through contact K, the headphones, the chokes CH1 and CH2, and the line to the distant Fullerphone. At the same time, the condensers C1, C2 and C3 are charged to the voltage of the battery. When the circuit is broken at contact K, no current can flow from the battery or through the headphones. But the condensers are now able to discharge and send a current to the line in the same direction as the battery current. The combined action of the chokes and condensers thus maintains a steady flow of current to the line as long as the key is pressed in spite of the fact that the battery current is being continually interrupted at contact K. This arrangement enables the sending operator to hear his own signals in his headphones.

Potentiometer.
The potentiometer is used for balancing out certain forms of DC earth potentials which may be picked up by the Fullerphone line circuit, notably on those where the earth is used as return line. Such a DC potential results in a current flow which is heard at the same frequency as the signals, thereby confusing them. The potentiometer is energised by an independent 1.5 volt dry battery and connected through a reversing switch (see the circuit diagram) so that a voltage can be impressed in the line equal and opposite to the voltage which is causing the interference. As the currents picked up will not necessary be the same at each end of the line, each Fullerphone station must adjust its own potentiometer to suit the earth current received. In normal operation, the potentiometer is not required and the reversing switch left in the centre position.

Overhearing
A filter combination of chokes and condensers prevents any variation in the line current during a signal, and prevents 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 ensures that the rise and fall of line current is comparatively slow and thus prevents clicks being heard in the receiver of a telephone set superposed on the same line. Therefore, the Fullerphone cannot be overheard either by induction or earth leakage, and can only be tapped by a similar instrument directly connected to the line. It was found that with the use of very sensitive equipment (believed to be valve amplifiers) it is possible to overhear a Fullerphone when the listening earth is within 180ft of the Fullerphone earth.

Used in two wars
Initial issues of Fullerphones were made up from converted field telephone sets. This type, however, appeared to be not the most successful. Towards the end of 1916, the Fullerphone was firmly established, and by 1918 most divisions had adopted Fullerphones for all their forward communication circuits. After the Armistice, more improvements and modifications of the instrument were carried out. The basic principle, however, was never changed. In 1939 a fully re-designed model, the Mk. IV, went into service. This can be considered as the most successful model, not only being more sensitive than its predecessors, it had also a simplified buzzer-chopper, and was easy to use as it carried no telephone set. Although the Fullerphone was devised as a non-overhearable signalling set for static warfare, it was again widely used during World War Two because of its capability to work simultaneously with a telephone over the same line and working through very long or leaky lines where telephone or telegraph traffic was impossible. In the South West Pacific, for example, the Australians made extensive use of the Fullerphone notably in New Guinea. During Word War Two cases arose where submarine cable circuits were available but the necessary terminal equipment was found to be totally destroyed or not immediately available. To ascertain to what extent Fullerphones could be used on submarine cables of various lengths, trials were carried out on the request of the War Department, by Cable and Wireless Ltd. The results exceeded all expectations, and ranges of up to 700 miles were obtained with faint but readable Morse signals at a maximum of 20 words per minute.