i877-j 
Recent Advances in Telegraphy. 
35 7 
line. With an ordinary instrument this current would pro- 
duce a signal, but it is essential for duplex working that the 
sending from a station should not affeCl the receiving instru- 
ment at that station. Therefore, in order to counteract the 
effect of the signalling current on the instrument, Gintl em- 
ployed a second smaller battery with its local or “ compen- 
sating ” circuit, and whenever the signalling current passed 
through the receiving instrument into the line he also caused 
a current from this second or compensating battery to pass 
in an opposite direction through the instrument, so as to neu- 
tralise or balance the effeCl of the signalling current. For this 
purpose the instrument was wound in opposite directions by 
two wires, one long and thin, represented by the full line, 
the other short and thick, represented by the dotted line. 
The long wire was in circuit with the sending battery and 
the line ; the short wire was in circuit with the small battery 
and the compensating circuit. A peculiar key having double 
points was used for sending, so that when it was depressed 
it closed both the line and compensating circuits. The sig- 
nalling current then rushed into the line by way of the long 
wire of the receiving instrument ; but although it thus 
passed through it, it failed to make the receiving instrument 
signalise, because the current from the compensating battery 
passed simultaneously in an opposite direction through the 
short wire. These two currents were accurately adjusted 
to balance each other in their effects on the needle or indi- 
cator of the instrument. The essential condition of having 
the receiving instrument so placed as to be undisturbed by 
the sending was thus fulfilled, while at the same time it was 
in circuit with the line, and therefore free to receive signals 
from the distant station. Gintl’s system had two serious 
defects : — In working the key in sending, the line was insu- 
lated for the moment when the lever was passing from the 
earth-contaCt, e, to the battery-contaCt, b. And the com- 
pensating battery was found to spend its strength quicker 
than the signalling battery. The harm arising from the first 
fault was that the received currents did not get freely to 
earth, and were therefore “ broken up.” The non-equiva- 
lence of the batteries, of course, destroyed the balance of 
currents on the receiving instrument, and “ false ” signals 
due to the sending were the result. 
Gintl’s plan, however, served as a stepping-stone to some- 
thing better. Herr Carl Frischen, a telegraph engineer of 
Hanover, in the year 1854, greatly improved it by making 
the receiving instrument differential , — tnat is to say, instead 
of winding it with two dissimilar wires, as Gintl had done, 
