b. Weighing Techniques
(1) The scale and the pointer are used as an indicator which shows when the
masses on each of the pans are equal. When both masses are equal the pointer
would rest at the center of the scale. In practice, however, the rest point of the
pointer must be found while the pointer is still oscillating-not after it has actually
come to rest. The reasons for this are twofold. The most obvious one is the time
spent taking a measurement. The time required for the oscillations of a sensitive
balance to stop is excessive. More importantly, however, the precision of the
measurement taken with the pointer moving is much greater than the precision of
those taken with the pointer at rest. This is because during normal usage the knife
edge of the balance becomes slightly rounded. When the balance is moving this
friction only tends to dampen the oscillations of the balance. But as the balance
moves slower and slower this friction may tend to make the pointer stop before it
has come to the true rest point. To find the rest point of the balance while the
pointer is still oscillating, requires a simple calculation.
(2) One requirement for determining the rest point by calculation is that the
oscillations of the pointer be within the area of the index. When this requirement
is met, the pointer must be allowed to swing back and forth at least twice before
recording any indications. After the pointer has made the two initial swings we
record the readings from the index where the turning points occur. Remember that
the length of each swing of the pointer continuously decreases. It is therefore
important that the first and last readings are both obtained on the same side of the
index. For convenience we will say that both the first and last readings will be
taken with the pointer at a left turning point. In other words if the first reading is
taken at the pointer's left turning point the final reading must also be taken at a
left turning point, regardless of how many other readings are taken in between.
Let's consider an example. After the pointer has made its initial two swings we
determine its left turning point to be 2.2. Now the pointer swings to the right and
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