36 QUANTITATIVE AGRICULTURAL ANALYSIS
of a gram. In order that such weighing may be performed the
balance must be constructed with mathematical accuracy. The
three bearings are commonly of agate, ground to a very fine
"knife" edge and each resting upon a smooth block of the same
material. They must be so placed as to lie in the same plane
while weighing (the central bearing is usually slightly below the
plane of the end bearings, to allow for distortion of the beam
when loaded) and absolutely parallel. The moving parts are as
light as is consistent with the strength required to bear the rated
load and they are provided with a mechanism for arresting their
motion and for lifting the knife edges from their bearings. The
entire balance is enclosed in a glass case, which is kept closed
during the final adjustment of weights, so as to avoid inter-
ference of air currents.
These points will be made clearer by reference to Fig. 15, which
shows only the skeleton of the balance.
Weights.—Practically all weighing operations of analytical
chemistry are carried out by means of metric weights. A balance
is rated for a certain maximum load and the largest piece of the
set of weights should not be heavier than half this rated load.
A balance rated to carry 100 gm in each pan will thus require
a set of weights having a 50-gm piece as the largest piece of the
set. The smaller pieces will then be, in grams, as follows: one
20, two 10's, one 5, one 2 and three 1's. These will total 100 gm.
The fractional pieces (milligram pieces) are then apportioned as
follows: one 500, one 200, two 100's, one 50, one 20 and two
10's, with a movable " rider" on the right arm of the balance
beam to make another 10 mg, the beam being graduated so that
by shifting the rider, 0.1 mg fractions may be made. It will be
seen that these milligram pieces total 1 gm.
The Rider.—The reason for using a rider on the beam instead
of the very small weight pieces on the pan is largely one of
convenience. The rider may be adjusted with the balance
case closed and this facilitates the final adjustment. This
method also dispenses with the use of a large number of very
small weights.
The actual weight of a rider to be used on a given balance
will depend upon the manner in which the beam is graduated.
These graduations are to indicate a certain number of milM-