SM0486
l. Simple comparisons. The simple method of comparing two weights is to let
them hang from opposite ends of a lever which is supported and pivoted at its
center.
Exact measurements require that you know the ratio of the beam (lever)
arms; that you suspend the weights from equal, corresponding points from each of
the ends of the beam; and that the friction at the pivot point be reduced to its
lowest value.
m. Measurement methods.
Some of the methods used for weight measurements are
as follows:
(1) Direct method - Place the sample (unknown) weight on one of the pans
suspended from one of the beam (lever) arms, and the calibrated weights on the
other pan.
(2) Transposition method - Perform two successive direct
weighings,
reversing the sample and weight positions in the second weighing.
(3) Substitution method - Place the unknown in the right pan and
counterbalance with calibrated metal weights placed in the left pan.
Bring the
balance into equilibrium, first by the addition of weights to the left-hand pan and
second by the movement of the left-hand rider (metal accessory placed on the beam)
while the right-hand rider remains at zero. Replace the unknown in the right pan
with standard weights and move the right rider to the same equilibrium point as
before.
The weight of the unknown is the sum of the substituted weights and the
reading of the rider.
n. Let us examine each of the measurement methods listed to be sure that you
understand the principle used. In the statement which described the direct weight
measurement method, we are referring to the use of a balance such as the one
represented in Figure 3.
When the calibrated weights on the right pan
counterbalance the unknown mass on the left pan, the weights of the known and
unknown masses should be within 3 to 5 grams of each other. The condition whereby
the two weights are nearly equal is evidenced by the fact that the instrument
pointer no longer moves.
When the nearly balanced condition is reached, the pan
arrests (devices designed to restrict the movement of pans) are released. The pans
are released so that the right-hand rider on the beam shown in Figure 4 can be
manipulated until the pointer in Figure 3 remains between the extreme graduation
marks on the scale. After you bring the balance into approximate equilibrium, the
total weight on the left hanger (unknown mass) is the sum of:
(1) The weights on the right side.
(2) The effective weight of the rider or beam weight determined by its
position.
(3) Plus or minus the swing of the pointer interpreted in milligrams.
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