sand-line
exchange comprises six major switches. Of the six major switches, one is
termed a _secondary connector_, another an _interconnector_, and the
four remaining are termed the _primary portion_ of the division.

[Illustration: Fig. 404. Unit of Switching Apparatus]

Before taking up the operation of the switches, the mechanical nature of
the switches themselves will be described. The switches are built with a
contact bank cylindrical in form and with internal movable brushes
traveling in a rotary manner in circular paths upon horizontal rows of
contacts fixed in the cylindrical banks. For driving these brushes a
constantly rotating main power-driven shaft is provided. Between each
shaft and the rotating brushes of each major switch is an electric
clutch, which, by the movement of an armature, causes the brushes of the
switch to partake of the motion of the shaft and by the return of the
armature to come again to rest. The motion of the brushes of the major
switches, or cylinder switches, as they are frequently called because of
their form, is constantly in the same direction. They have a normal
position upon a set of the cylinder contacts. They leave their normal
position and take any predetermined position as controlled by the
magnets of the clutch, and, having served the transient purpose, they
return to their normal position by traversing the remainder of their
complete revolution and stopping in their position of rest or idleness.

The mechanical construction of each of the cylinder switches is such
that it may disengage its clutch and bring its brushes to rest only with
the brushes in some one of a number of predetermined positions. The
locations of the brushes in these positions of rest, or "stop"
positions, as they are called, may differ with the different cylinder
switches, according to the nature of the duty required of the switch,
and the total number of stop positions also may vary. The primary and
secondary connectors, the interconnector selectors, and the
interconnectors each have eleven stop positions; the rotary switch has
eight stop positions; the signal-transmitter controller has but two.

In the six cylinder switches making up a connective division and
required for any conversation, in a ten-thousand-line exchange some of
the switches are set to positions which are determined by the control of
the calling subscriber and represent by their selective positions the
value of some digit of the call