334 The N.Z. Journal of Science and Technology. [Nov. 
However, for the sake of clearness in this description systems dealing 
with only 10,000 lines, having the small groups of 100 lines each, will be 
discussed. 
Operation, 
When the subscriber first removes his telephone from T the hook, current 
flows out from the central office through a relay, through the subscriber’s 
line and telephone, and back to the battery at central office. This relay 
becomes energized, and in 
turn closes a local circuit, 
which starts several line- 
finders hunting for the 
particular line calling. 
The various mechanisms 
used in establishing con¬ 
nections are shown grouped 
in fig. 2. The switches are 
all of the rotating type, and 
are driven by means of the 
constantly running vertical 
shafts, which drive indi¬ 
vidual horizontal shafts, 
which are connected to each 
switch by means of mag¬ 
netic, clutches which are 
energized when a switch is 
to be rotated. 
A line-finder is shown in 
fig. 3. When this rotates it 
wipes connecting-brushes 
over the line - terminals 
shown at the back. 
A number of lines are 
accessible to a group of these 
line - finders (see left - hand 
bay of fig. 2), and when one 
of these mechanisms has 
become connected with the 
particular line calling, con¬ 
nection is continued from the 
subscriber’s line to another 
mechanism, known as a 
group - switch (fig. 4, left 
hand), which is to be used for selecting a trunk to the particular group 
of 1,000 in which the desired number appears. Attached to the circuit 
connecting the line-finder and the group-switch are a set of registers (fig. 5), 
and as the subscriber manipulates his dial in accordance with the digits of 
the particular number desired these registers revolve until they assume 
positions corresponding with these particular digits. These registers are 
used to record and store the number in such a way that the further move¬ 
ments of all the mechanisms necessary in completing the connection will 
be controlled by the register set. If, for analogy, we liken the automatic 
central-office equipment to a large machine-works which receives cus¬ 
tomers’ orders for all kinds of parts, we may compare the register sets to 
Fig. 2.—Switches mounted on base. 
