Figure 11-40.-Types of plumb bobs.
Figure 11-41.-Plumb bob, cord, and target.
plumb bobs are 6, 8, 10, 12, 14, 16, 18, and 24
OZ; the 12- and the 16-oz are the most popular.
Typical plumb bobs are shown in figure 11-40.
A plumb bob is a precision instrument and
must be cared for as such. If the tip becomes bent,
the cord from which the bob is suspended will not
occupy the true plumb line over the point
indicated by the tip. A plumb bob usually has a
detachable tip, as shown in figure 11-40, so if the
tip becomes damaged, it can be renewed without
replacing the entire instrument.
Each survey party member should be equipped
with a leather sheath, and the bob should be
placed in the sheath whenever it is not in use.
The cord from a plumb bob can be made more
conspicuous for observation purposes by the
attachment of an oval form aluminum target (fig.
11-41, view A). The oval target has reinforced
edges, and the face is enameled in quadrants
alternately with red and white. Also, a flat
rectangular plastic target may be used (fig. 11-41,
view B). It has rounded corners with alternate red
and white quadrants on its face. These plumb bob
string targets are pocket size with approximate
dimensions of 2 by 4 in.
Optical Plumbing Assembly
The optical plumbing assembly, or plummet,
is a device built into the alidade or the tribrach
of some of the instruments to center the
instrument over a point. Its working principle is
shown in figure 11-42. The plummet consists of
a small prismatic telescope with a cross wire or
Figure 11-42.-Optical plumbing assembly.
11-40
