There is thus a weak negative current flowing to line. When K1 alone is
worked, the current of E1 is reversed. When K2 is worked alone, c of K1
is transferred from b to a, and the strength of the negative current
going to line is increased through the increase of the electro-motive
force from E1 to 3.3 E1 for the whole battery is brought into play. When
K1 and K2 are depressed together, then the negative pole of E1 goes to
earth through Z1; and the positive pole of E2 to line through a of K2
and c1 of K1 and a positive current, due to the whole electro-motive
force 3.3 E1 goes to line. Hence the effect of working K1 is simply to
reverse the current, whatever its strength, while that of K2 is to
strengthen it, whatever its direction.
The resistance coil s, of 100° resistance, is called a spark coil,
because it prevents the high electro-motive force of the whole battery
from damaging the points of contact by sparking or forming an arc across
when signals are sent; and the resistance r2 is made approximately equal
to the combined resistance of E2 and the spark coil, so that the total
resistance of the circuit may not be altered by the working of the
apparatus.
Fig. 321. QUADRUPLEX TELEGRAPH.
A1 and B1 (Fig. 320) are the relays which are used to respond to the
changes in the currents sent by the keys K1 and K2 at the distant
station.
A, is a simple polarized relay wound differentially, each wire having a
resistance of 200 [omega], and so connected up as to respond to the working of
the reversing key K1 of the distant station. It acts independently of
the strength of the current, and is therefore not affected by the
working of the increment key K2. It is connected up so as to complete
the local circuit of the sounder S1 and the local battery l1 and forms
the receiving portion of the 'A' side.
B, is a non-polarized relay also wound differentially, each coil having
a resistance of 200 [omega]. It responds only to an increase in the
strength of the current, and therefore only to the working of the
increment key K2 of the distant station.
[Transcriber's note: In current usage upper case omega indicates ohms and
lower case omega denotes angular frequency, 2*PI*f.]