To measure the specific latent heat of steam (more accurate method)
The apparatus for this experiment consists of an inverted vacuum flask containing distilled water, the level of which can be observed and maintained as required by means of a funnel and side tube (Fig. 42.5).
The water inside is boiled by a heater coil supplied with a current of I in amperes at a p.d. of V in volts. After the water has been boiling for some time, a steady temperature state is reached and, under these conditions the rate of supply of heat
energy, VI in watts (i.e., joules per second) is equal to the rate of absorption of latent heat into the vapour. The vapour passes down the central glass tube projecting up into the boiler and is condensed back to water as it passes through a straight tube surrounded by a cold water jacket.
When steady temperature conditions have been reached, a dry weighed beaker is placed under the outlet tube and the time for about 20 g of water to collect is measured. The beaker and water are then weighed to find the mass of water condensed.
This is an example of a steady temperature state, continuous flow experiment, in ‘hich errors due to heat and vapour loss during the warming up period and heat o s during the boiling period have been almost entirely eliminated. The heat capacity of the apparatus itself does not enter into the calculation, and the use of a vacuum vessel reduces heat losses during the steady temperature state to a minimum.