338 The N.Z. Journal of Science and Technology. [Nov. 
right-hand spring, not made at all, or made through the left-hand spring 
on each set. 
At each stage in the progress of a connection one of these sequence- 
switch shafts moves to a new position and prepares the circuit combinations 
for the next step in the progress. The registers, which were mentioned 
before, are built on the same general principles as the sequence-switches, 
and operate in the same general manner. 
The method of rotating the various mechanisms was touched upon 
on a previous page. This system is constructed with what is known as 
power drive. In fig. 4, showing one machine, the spool under the middle 
of the brush-carriage is an 
electro - magnet, which when 
energized attracts the plate 
directly above the spool 
against the plate on the small 
horizontal shaft going through 
the middle of the electro¬ 
magnet. This small horizon¬ 
tal shaft is directly geared to 
the vertical shaft at the left 
of the bay of machines. The 
vertical shaft is in turn geared 
to a horizontal shaft running 
along the base of all the bays, 
and this latter shaft is con¬ 
stantly run by means of a 
small motor. It necessarily 
follows that when the electro¬ 
magnet is energized the two 
plates are brought together 
and the brush-carriage must 
revolve, and when the electro¬ 
magnet is de-energized the 
brush - carriage will remain 
wherever it may be at that 
given instant. The same 
method is adopted for driving 
the trip-spindle at the left of 
the group - switch, and for 
driving the sequence - switch 
shown at the right. By the 
adoption of this power drive 
in an automatic exchange 
great simplicity is secured, and at the same time absolute reliability in 
operation. In some of the earlier forms of automatic systems the mechanisms 
were caused to advance by means of what is generally spoken of as a 
step-by-step mechanism, which consisted of a small ratchet or dog arrange¬ 
ment being operated by an electro-magnet so that the mechanism advanced 
one notch every time the magnet became energized. 
Power Plant. 
The power for operating all switch-clutches, relays, &c., and for supplying 
the subscriber with current for talking, is obtained from a storage battery 
Fig. 6.—-Sequence-switch. 
