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Introduction
This article covers the history and technical description of
a British service Morse telegraph instrument, invented in 1915
by Captain A. C. Fuller and still in use in WW II.
1915 . . . the British and German armies were densely packed
in their trenches, at places only a matter of yards apart. Communication
was mainly by telephone and various Morse buzzer telegraph instruments,
connected by a single wire and earth return. During mid-1915
the Germans were extraordinarily well informed of Allied plans.
Espionage was suspected but an interned British civilian brought
back the information that induction from cables led to overhearing.
Experiments carried out within the Allied lines left no doubt
of the cause of the leakage.
 The solution to the overhearing problem came toward the end of
1915 from Captain (later Major General) A.C. Fuller, who invented
the Fullerphone, a small direct-current Morse telegraph instrument.
In October, Fuller brought two prototypes to 5 Corps in Flanders.
His invention was tested on a five-mile loop of cable, part of
which ran in the water-filled moat of Ypres with a 10-ohm leak
to earth. The prototypes worked well. They were obviously the
answer to the problem of overhearing by induction which had brought
the British Expeditionary Force almost to a standstill in terms
of signals.
Working principle of the Fullerphone
The Fullerphone is essentially a DC Morse telegraph instrument
with high sensitivity. Morse signals can he sent and received
by the same instrument, no send-receive switch being used, so
"break-in" working is possible. When sending, a very
small direct current flows through the line and the receiving
instrument. A readable signal is produced with a current of only
0.5 microampere!
To make the DC Morse signals audible, Fuller added an interrupter
for changing the steady current into an intermittent current,
suitable to produce a tone in a pair of headphones. The result
is that dots and dashes sent by the key at the far end are received
as short and long notes at the receiving end. Fuller completed
the circuit by inserting capacitors and LF chokes to keep the
current in the line relatively constant.
Features of the Fullerphone
Fullerphone signals cannot be overheard by induction or earth
leakage and can be tapped only by the direct connection of a
similar instrument to the line. The Fullerphone can be used simultaneously
with a telephone or buzzer telegraph on the line. Working via
leaky or very long cables is possible. However, the normal range
for reliable communication under field conditions is 25 to 35
miles. Much greater ranges are possible under special conditions
(e.g. open-wire lines in the desert) or by putting in a minor
circuit change.
Detailed Technical Description
At the sending end, when the Morse key is pressed and the circuit
is closed at X, a current flows from the battery, Morse key,
X contact of the interrupter, headphones, chokes, and line terminal
Ll, through the distant station and back via earth to the negative
of the battery. At the same time, the capacitors charge up to
the battery voltage.
 When the circuit is broken at X, no current can flow from
the battery. The tone in the sending operator's headphones, from
the 550-Hz chopping rate of the interrupter, stops. Energy stored
in the capacitors and chokes slowly discharges. The result is
that the sending operator can hear his own signal and judge the
strength of the line current. Should the line be cut or the distant
operator's interrupter fail, he is immediately notified by hearing
musical clicks instead of his own buzzer.
When, at the receiving end, the circuit is closed at X, a steady
current will flow to earth through the choke coils, headphones
and contact X of the interrupter. If the circuit is broken at
X, the current will charge the capacitors. When the circuit is
again closed at X, they partially discharge through the phones.
With a working interrupter X, an intermittent current is received
in the headphones, giving an audible note, while (if the chokes
and capacitors are suitable) the line current alternately flows
through the headphones and into the capacitors, and remains practically
constant and continuous on the line.
The arrangement of coils and capacitors not only prevents any
appreciable variation in the line current, but also prevents
any AC currents (such as produced by induction from other circuits)
passing through the operator's headphones. Owing to energy storage
in the capacitors and the self-induction of the choke coils,
the current in the line can rise only comparatively slowly. This
prevents the possibility of Morse signals being read by the enemy
from clicks and prevents these clicks interfering with telephony
carried over the same line.
Interrupter
The interrupter is a polarized buzzer, operating on 1.5 volts,
using one pair of contacts for driving and a second pair for
breaking or "chopping." The DC Morse signals of the
Fullerphone at a 550-Hz rate.
Interference
Difficulties in working the Fullerphone were almost invariably
due to interference from small currents being picked up by the
line from earth potentials, earth faults, or leakage from other
circuits. This interference could be balanced out very effectively
at the receiving end by producing a current of equal strength
but of opposite direction to the earth current. A potentiometer
R, dry cell B2, and reversing switch S2 were provided for this
purpose.
Models of Fullerphones
Initial issues of Fullerphones were made up from converted "DIII"
field telephone sets. This type was, however, not the most successful.
During WW I a variety of different models of Fullerphone were
provided; some of the variations were to use existing parts while
others reflected modifications to make the instrument more suitable
for use in the trenches.  After 1918 improvements and modifications
on the Fullerphone were carried out. The basic principle, however,
never changed. During 1939 a newly designed Mk IV model went
into service. It was more sensitive than its predecessors, with
a more easily adjustable buzzer/interrupter, and simpler to use
as it carried no telephone set. Small modifications, such as
radio-interference suppression of the buzzer and insertion of
a crash limiter across the phone jacks to limit line static,
were carried out on the Mk IV model.
In 1943 a tropicalized version was issued. This Mk V model had
a similar circuit but tropicalized components. The Mk VI Fullerphone,
the last type built, was completely tropicalized and immersion-proof.
The Italian Army copied the Fullerphone in the early Thirties,
producing an "apparado da campo per telegraphia inintercettabile."
The circuit is like the British version hut lacks the line-balancing
potentiometer.
Construction Details
 WW I models were mainly constructed on a steel frame, fitted
in a wooden box, covered with waterproof canvas. Fullerphones
Mk IV, V and VI were housed in an aluminum case, which, in turn,
slides on guides in a wooden carrying case. The MK VI slides
in a metal case with a watertight gasket on the lid, so that
the case, when closed, is immersion-proof.
Use of the Fullerphone in WW II
Though designed as a non-overhearable signal instrument for static
warfare in trenches, the Fullerphone was widely used during WW
II for its other features, viz., the ability to work simultaneously
with a telephone on the same line and the possibility of operating
through long and leaky lines where telephone traffic was impossible.
In the southwest Pacific, the Australians made extensive use
of the Fullerphone. It more than lived up to its reputation when
used in New Guinea. The result, a considerable saving of cables
at a time when men and supply were scarce, was fully recognized
by the forces.
 Cases arose where a submarine cable circuit was available but
the necessary telegraph terminal equipment was found totally
destroyed or was not immediately available. To ascertain to what
extent Fullerphones could be used on submarine cables of various
lengths, trials were carried out in 1943 by Cable & Wireless
Ltd. at request of the British War Department. The results exceeded
any expectation; ranges of up to 700 miles were obtained with
faint but readable Morse signals at a maximum of 20 words per
minute.
The Fullerphone on Display
The Royal Signals Museum at Blandford Camp, near Blandford Forum,
Dorset, U.K., is open to the public during weekdays. Most of
the Fullerphones mentioned above are on display.
Acknowledgements
I am indebted to Major Harfield and Mr. Evans of the Royal Signals
Museum at Blandford Camp for their help and for allowing me to
publish drawings.
References
The Fullerphone, Its Action and Use, HMSO 1917.
Signal Service Diagrams, 1917.
R. Priestly, The Signal Service (France), 1922.
Signal Training, Vol. 3, Pamphlet No. 3, HMSO, 1923.
Signal Training, Vol. 3, Pamphlet No. 21, HMSO, 1939
Signal Equipment Used by Enemy Nations, SRDE, 1944.
Australian Corps Of Signals, Signals, 1944.
"Jimmy," The Magazine of the Royal Signals in the Middle
East, 1944.
D. Signals monthly liaison notes, 1943-45
EMER's 632, 1945.
Nalder, British Army Signals in the Second World War, RSI, 1953.
various letters and documents in the Royal signals Museum.
L. Meulstee, Fullerphone, HRSA Newsletter,
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