An Introductory Course of Quantitative Chemical Analysis | Page 6

using the same balance.
Before making any weighings the student should seat himself before a
balance and observe the following details of construction:
1. The balance case is mounted on three brass legs, which should
preferably rest in glass cups, backed with rubber to prevent slipping.

The front legs are adjustable as to height and are used to level the
balance case; the rear leg is of permanent length.
2. The front of the case may be raised to give access to the balance. In
some makes doors are provided also at the ends of the balance case.
3. The balance beam is mounted upon an upright in the center of the
case on the top of which is an inlaid agate plate. To the center of the
beam there is attached a steel or agate knife-edge on which the beam
oscillates when it rests on the agate plate.
4. The balance beam, extending to the right and left, is graduated along
its upper edge, usually on both sides, and has at its extremities two
agate or steel knife-edges from which are suspended stirrups. Each of
these stirrups has an agate plate which, when the balance is in action,
rests upon the corresponding knife-edge of the beam. The balance pans
are suspended from the stirrups.
5. A pointer is attached to the center of the beam, and as the beam
oscillates this pointer moves in front of a scale near the base of the
post.
6. At the base of the post, usually in the rear, is a spirit-level.
7. Within the upright is a mechanism, controlled by a knob at the front
of the balance case, which is so arranged as to raise the entire beam
slightly above the level at which the knife-edges are in contact with the
agate plates. When the balance is not in use the beam must be
supported by this device since, otherwise, the constant jarring to which
a balance is inevitably subjected, will soon dull the knife-edges, and
lessen the sensitiveness of the balance.
8. A small weight, or bob, is attached to the pointer (or sometimes to
the beam) by which the center of gravity of the beam and its
attachments may be regulated. The center of gravity must lie very
slightly below the level of the agate plates to secure the desired
sensitiveness of the balance. This is provided for when the balance is
set up and very rarely requires alteration. The student should never

attempt to change this adjustment.
9. Below the balance pans are two pan-arrests operated by a button
from the front of the case. These arrests exert a very slight upward
pressure upon the pans and minimize the displacement of the beam
when objects or weights are being placed upon the pans.
10. A movable rod, operated from one end of the balance case, extends
over the balance beam and carries a small wire weight, called a rider.
By means of this rod the rider can be placed upon any desired division
of the scale on the balance beam. Each numbered division on the beam
corresponds to one milligram, and the use of the rider obviates the
placing of very small fractional weights on the balance pan.
If a new rider is purchased, or an old one replaced, care must be taken
that its weight corresponds to the graduations on the beam of the
balance on which it is to be used. The weight of the rider in milligrams
must be equal to the number of large divisions (5, 6, 10, or 12) between
the central knife-edge and the knife-edge at the end of the beam. It
should be noted that on some balances the last division bears no
number. Each new rider should be tested against a 5 or 10-milligram
weight.
In some of the most recent forms of the balance a chain device replaces
the smaller weights and the use of the rider as just described.
Before using a balance, it is always best to test its adjustment. This is
absolutely necessary if the balance is used by several workers; it is
always a wise precaution under any conditions. For this purpose, brush
off the balance pans with a soft camel's hair brush. Then note (1)
whether the balance is level; (2) that the mechanism for raising and
lowering the beams works smoothly; (3) that the pan-arrests touch the
pans when the beam is lowered; and (4) that the needle swings equal
distances on either side of the zero-point when set in motion without
any load on the pans. If the latter condition is not fulfilled, the balance
should be adjusted by means of the adjusting screw at the end of the
beam unless the variation is not more than one division on the scale; it
is