ack at
Right. This Employs the Kellogg Parallel Arrangement of Frames.]

_Testing--Called Line Busy._ If we now assume that the called line was
already busy, by virtue of being connected with at another section, the
test rings of that line would accordingly all be raised to a potential
above that of the earth. As a result, when the operator applied the tip
of her calling plug to a test thimble on that line, current would flow
from this test thimble through the tip of the calling plug and tip
strand of the cord and through one winding of the cord-circuit repeating
coil to ground. This would cause a slight raising of potential of the
entire tip side of the cord circuit and a consequent momentary flow of
current through the secondary of the operator's circuit bridged across
the cord circuit at that time.

_Operator's Circuit Details._ The details of the operator's talking
circuit shown in Fig. 347 deserve some attention. The battery supply to
the operator's transmitter is through an impedance coil _9_. The
condenser _12_ is bridged around the transmitter and the two primary
windings _10_ and _11_, which windings are in parallel so as to afford a
local circuit for the passage of fluctuating currents set up by the
transmitter. The two primary windings _10_ and _11_ are on separate
induction coils, the secondary windings _13_ and _14_ being, therefore,
on separate cores. The winding _15_, in circuit with the secondary
winding _14_ and the receiver, is a non-inductive winding and is
supposed to have a resistance about equal to the effective resistance to
fluctuating currents of a subscriber's line of average length. Owing to
the respective directions of the primary and secondary windings _10_ and
_11_, _13_ and _14_, the result is that the outgoing currents set up by
the operator's transmitter are largely neutralized in the operator's
receiver. Incoming currents from either of the connected subscribers,
however, pass, in the main, through the secondary coil _13_ and the
operator's receiver, rather than through the shunt path formed by the
secondary _14_, and the non-inductive resistance _15_. This is known as
an "anti-side tone" arrangement, and its object is to prevent the
operator from receiving her own voice transmission so loudly as to make
her ear insensitive to the feebler voice currents coming in from the
subscribers.

_Order-Wire Circuits._ The two keys _16_ and _17_, shown in connection
with the operator's talking