Page 120 - Physics

Physics for Secondary Schools

Note that the volume of the bulb determines
the range of densities that can be measured.
This is largely because the bulb sinks so
as to push the surface of the liquid up to
the stem. The narrower the stem, the more
Figure 5.13: Plimsoll lines on a side of a ship sensitive the hydrometer. This means that
a small difference in density results to a
Hydrometre large change of the reading on the stem.

A hydrometre is an instrument used to Mode of action of hydrometre
determine the relative density of liquids. A hydrometre is made to fl oat in the liquid
It is consists of a cylindrical stem which
is graduated and a glass bulb at its end as whose relative density is to be measured.
shown in Figure 5.14. The bulb is fi lled It sinks to different levels depending on
with mercury or lead ballast to make it fl oat the relative density of the liquid in which
upright. The stem is made thin to allow a it is fl oating, as shown in Figure 5.15.
great change in height for a small change in
density. The graduations in the stem of the Stem
hydrometre start with small numbers at the
top and end with large numbers at the bottom,

since, it sinks more in less dense liquids. Methanol

Stem (T) Water
0.8
0.9
Bulb
1.0
1.1
Lead ballast
1.2
Bulb (B)
Figure 5.15: Hydrometre in water and in
methanol

Lead ballast The liquid whose relative density is to be
determined is poured into a tall jar and the
Figure 5.14: The Hydrometer hydrometre is gently lowered into the liquid

Structure of a hydrometre until it fl oats freely. The point where the
A practical hydrometre has the following surface of the liquid touches the stem of the
parts: hydrometre indicates the relative density of
(a) a hollow narrow graduated glass tube the liquid. Remember that since the density
or stem (T); of water is 1 g/cm , the density of the liquid
3
(b) a wide bulb (B); and in g/cm is numerically equal to its relative
3
(c) a lead or mercury ballast to make the density. Considering Figure 5.15, the
hydrometre fl oat upright in liquid. hydrometre sinks more in methanol whose

114
Student’s Book Form One

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