1918.] The N.Z. Journal of Science and Technology. 335 
that instead of having an operator at the exchange to complete the con¬ 
nection for the subscriber, machinery is used to accomplish this result, 
and this machinery is controlled by the subscriber himself from his telephone 
set by means of a dial. On this dial (fig. 1) appear a series of numbers 
from 0 to 9, and the face of the dial is rotatable to any number at the will 
of the subscriber. 
In operating such a system the subscriber removes his receiver, and 
thereby causes a certain machine in the exchange to become connected to 
his line. He then operates his dial, which controls the operation of one 
machine after another until connection is secured with the line of the 
particular subscriber wanted. This subscriber’s bell is then rung, and when 
he answers the connection is completed in such a manner that the two 
subscribers can converse. The replacing of the receiver on the switch-hook 
sends all the machinery back to normal, and leaves the subscriber's line 
in condition ready to be used again for other connections. 
The latest common-battery systems, as explained above in connection 
with the later design of magneto systems, are arranged with large multiples- 
of all the subscribers’ lines, so that any operator can connect a calling 
subscriber directly with any other subscriber in the exchange. Strange 
as it may seem, this arrangement does not work out well in automatic 
systems, and in order to obtain satisfactory operation first principles were 
reverted to, and connections are secured by working through trunks in 
conjunction with groups of predetermined size. 
Some systems are worked on what is known as the decimal basis. In 
such a system—say, for 10,000 lines—the subscribers’ lines are grouped 
into sets of 100 each ; again, 10 groups of 100 are arranged in a group of 
1,000, 10 of these in turn being grouped into the one lot of 10,000 lines. 
The method of securing a certain line is to have the calling subscriber, after 
removing his receiver, connected by means of trunks to the correct group 
of 1,000, then of 100, and then to find in this group of 100 the particular 
group of 10 desired, and finally to become connected with the particular 
unit which is the line desired. The controlling of this selecting is done 
by the subscriber in connection with his dial. In a system 10,000 lines, the 
lines would be numbered from 0000 to 9999, and it is necessary for the 
subscriber to operate his dial four times, each time corresponding to one 
of the four digits in the number desired. As mentioned, the connection is 
completed through trunks from one group to another. Now, in any given 
exchange there will be a number of connections in process of completion 
at the same time to any group of lines, also a number of completed con¬ 
nections in use. This necessitates a number of trunks being supplied from 
one group to another, and the number of these trunks depends upon the 
average number of times each subscriber calls per day and the average 
length of time each conversation lasts. 
The subscriber in manipulating his dial connects himself to a set of 
trunks each terminating in a machine associated with the group through 
which the connection is to be completed, and the exchange apparatus picks 
out an idle trunk from this set. 
Owing to engineering reasons, economy in trunks and machines is secured 
in a system by having the final group of subscribers’ lines larger than 100, 
and in the system now being installed in New Zealand these final groups 
consist of 200 lines each, and in this case the next grouping is 2,000 ; and 
when more lines are wanted the next general grouping is for 20,000, and 
so on. 
