100

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n

la pure water this would be placed at 10 to bring the balance
into equilibrium. This would read 10 tenths, or 1. Three
other denominations of weights are provided, reading to the
second, third arid fourth decimal places, respectively.

Calibration. — This instrument, like the floating hydrometer,
must be used at the temperature for which its displacement of
water is known. Any plummet may be calibrated for use at
its rated temperature or at any other desired temperature by
weighing it dry, and again suspended in distilled water which
has been boiled to expel dissolved gases, then cooled to the desired
temperature. The difference between these weights represents
the weight of water displaced. If the displacement so found in
not exactly 5 gm, all specific gravity determinations are corrected
to take account of the deviation, thus:

X

e

where Sto is the true specific gravity ,*SV" the figure found experi-
mentally and d the water displacement at /°.
The weights for the Westphal balance ant calibrated by the
method described for analytical weights, page 41.
Use of the Westphal Plummet on an Analytical Balance.
The Westphal balance is a convenient and low priced piece of
apparatus for making specific gravity determinations with a fair
degree of accuracy. Determinations may be made* with a higher
degree of refinement by using the Westphal plummet with a
good analytical balance. The plummet in cleaned and dried,
then suspended from a hook on the left arm of the balance.
Weights are added to the right pan to counterpoise, then a
cylinder of the liquid to be tested is placed on a bridge over the
balance pan in such a way as to allow the balance to swing
unimpeded, but supporting the cylinder HO that the plummet is
entirely immersed in the liquid. A counterpoise in again effected
by removing weights from the right pan. The* difference be-
tween the two weights is the liquid displacement. Thin divided
by the water displacement, found as already deHeribed, given
t°
the specific gravity at ,o" The mechanical arrangement is shown
in Fig. 27.