To compare the densities of two liquids by means of Hare’s apparatus
In chapter 9 we saw how the relative density of a liquid can be found by the use of a density bottle. The following method for measuring the relative density of a liquid is based on the pressure exerted by a liquid column. The apparatus consists of two vertical wide-bore glass tubes connected at the top by a glass T-piece. These tubes dip into beakers containing the two liquids of densities PI and P2 (Fig. 11.8 (a).
Some air is sucked out of the tubes through the centre limb of the T-piece and the clip closed. Removal of air causes a reduction of pressure inside, with the result that atmospheric pressure pushes the liquids up the tubes. The liquids rise until the pressures exerted at the base of each column are each equal to atmospheric pressure A certain amount of difficulty may arise when measuring the height of the li columns, owing to the meniscus which forms when a boxwood scale touches surface of the liquid. This may be overcome by the use of a bent wire attached to lower end of the scale, as shown in Fig. 11.8 (b). The scale is adjusted until the :- the wire is just level with the liquid surface. The scale reading of the liquid lev the tube is then taken, and added to the distance x between the tip of the wire the zero of the scale. Several pairs of values of hI and h2 are taken, entered in a suitable table an mean value of the ratio of the densities calculated.
Alternatively, we may plot a graph of h2 against h, and obtain the ratio fro