1918.] The N.Z. Journal of Science and Technology. 337 
free trunk is obtained in this level which leads the subscriber’s connection 
to the particular group of 100 lines in which is to be found the 
desired line. This trunk in turn terminates on what is known as a “ final 
machine,” which is very similar to the group-switch, the only difference 
being that after selecting a particular level it selects some particular 
number in this level instead of hunting for a trunk. Its operation is, 
first, to have the trip-spindle set in accordance with the number as recorded 
on the tens register, and then to drag the particular set of brushes selected 
over the particular level of terminals in which is to be found the desired 
line. Instead of simply hunting over these terminals its forward progress 
is controlled by means of the units register, and when it has sent the units 
register home to its normal position its forward motion is arrested and it 
is connected to the line of the desired subscriber. At this stage of the 
connection a test must be made to determine whether the subscriber’s 
line is in use or whether it is free, as it may at this particular moment be 
connected to the line of some other subscriber in the exchange. If the 
line is found to be engaged an interrupted tone is given to the calling 
subscriber to notify him that the connection cannot at this time be com¬ 
pleted, and the calling subscriber should hang up his telephone and 
attempt the connection at a later time. However, if the line is.free when 
his test is made, current to ring the called subscriber’s bell is intro¬ 
duced on the called subscriber’s line, and, when the called subscriber 
answers, his ringing current is cut off, and battery for talking is con¬ 
nected to his line in such a way that the two subscribers can now converse 
with each other. 
When the conversation is completed and the subscribers restore their 
telephones to the hook the circuits through their lines are broken, and all 
the mechanisms used to complete the connection return to their home or 
normal positions. 
As will be readily appreciated from this brief outline of the progress 
‘of a connection through an automatic exchange, numerous different 
electrical conditions must be obtained to accomplish the different results 
desired, and these combinations of electrical conditions are controlled by 
the sequence - switch, which has been often termed the brains of the 
system. n. 
As shown on fig. 6, there are a number of sets of springs on this 
sequence-switch, the middle spring of the set being arranged to assume 
three different positions—No. 1, resting against the right-hand spring ; 
No. 2, touching neither right- nor left-hand spring ; and No. 3, resting 
against the left-hand spring. The position of this middle spring is deter¬ 
mined by the cam (G2) associated with the spring. It will be noticed that 
each one of these cams has its periphery cut in different shapes, and the 
shape of this cutting determines the position of the spring associated with 
the cam. When this cam is rotated about its axis it causes the spring to 
assume different positions, depending upon the form of the cams. The 
whole set of cams are fastened together on the shaft which goes through 
the middle, and this member is rotatable through any one of eighteen 
different positions. It may stop in any one of these positions, the cutting 
of the different cams in each one of these positions determining the 
relationship of the various springs on the sequence-switch. So it will be 
seen that a great number of different electrical combinations may be set 
up on any one of the sequence-switches, the circuits for these com¬ 
binations being led through the springs so as to be made through the 
22—Science. 
