A Direct Electrical Method of Determining Latent Heat of Evaporation at the Boiling Point

Extract

In this method the liquid of which the latent heat is to be determined is contained in a glass vessel which is suspended by a fine wire from the hook of a balance, and the amount of electrical energy is measured which is required for the evaporation of an observed mass. The electrical energy is supplied by a large current passing through a small resistance in the liquid itself. In order to reduce heat losses as much as possible, the vessel carrying the liquid and the resistance is completely surrounded by a doublewalled shield, which is filled by the saturated vapour of the liquid itself. The estimation of the electrical energy supplied necessitates tne measurement of the current, C (amperes), the resistance R (ohms), of the heating coil, and the time in seconds, T, during which the current flows. It is expressed by C²RT joules, and if M be the mass evaporated in gms., the latent heat expressed in joules per gm. becomes C²RT/M.