292 REPORTS ON THE STATE OF SCIENCE, ETC, 



transmission, and the current variations in the thermal condition of the oven 

 can, therefore, be determined. 



The observed efficiency varies according to the temperature at which the 

 coil is maintained. It is, therefore, necessary to lay down an arbitrary 

 temperature at which all observations shall be made. If that temperature 

 is high the resultant efficiency falls so low at low rates of combustion — even 

 in extreme cases to zero — that its value cannot be accurately determined. As 

 it is necessary to obtain a curve of efficiencies at all rates of combustion, it is, 

 therefore, necessary to keep the mean temperature of the coil low, so that 

 low rates of combustion may be included in the observations. 



The temperature of the coil is of very great importance in the case of an 

 ordinary gas oven. Where the temperature is low it enables the total heat of 

 combustion to be brought into account ; whereas when it is high the net heat 

 only is available. 



The use of gas enables much more accurate observations to be made, as 

 a close approximation of the steady state is easily attainable by careful regulation 

 of the rate of combustion. The latter can be controlled in the case of an oven 

 so that any desired rate of transmission is maintained. In this case the 

 principal difficulty is to determine how to deal with the difference between the 

 gross and the net calorific value of the gas. 



With a cold coil in the oven, a considerable part of the latent heat in tiie 

 water vapour in the products of combustion may be communicated to the coil. 

 The water appears as condensation on the surface of the coil, and drips down 

 into the base. Thus obviously a portion of the latent heat is utilised. When 

 the temperature is high there is no such condensation, and therefore no 

 utilisation of this fraction of the heat of combustion. Whether and to what 

 extent this latent heat is utilised in the actual cooking of an article of food 

 it is impossible to say — probably some portion in the earlier stages, and later 

 a re-evaporation of the same water originally condensed. 



It will be evident that the efficiency as observed with this coil calorimeter 

 will vary according to the surface area of the coil, or, what is the same thing, 

 according to the size of the oven in which the coil is placed. 



In order to compare, on a rational basis, ovens of different size it was 

 necessary to investigate the question of the variation of apparent efficiency 

 according to the size of coil used, and to specify the standard loading for ail 

 oven of any given size. It was found that when an oven is loaded to its 

 fullest practical capacity the surface of all tlie food is approximately half the 

 surface of the oven plates. This was, therefore, laid down as the standard 

 loading. 



The final efficiency therefore determined upon was the ratio between (1) the 

 heat communicated to a coil whose exterior surface was half the interior sur- 

 face of the oven when kept at a constant temperature, and (2) the amount 

 of heat in the quantity of standard fuel burnt per hour. This function varies 

 according to the rate of combustion, the efficiency increasing with increasing 

 combustion. Its value was determined for three different rates, and a curve 

 drawn through the three experimental points so obtained. 



Hot Plate. — The efficiency in this case is the ratio of the maximum hourly 

 amount of heat which can be communicated by the hot plate to ordinary cooking 

 vessels to that in the fuel consumed per hour. This efficiency is fairly constant 

 at all rates of combustion. The hot plate is covered with standard cooking 

 vessels, each containing a weighed quantity of water, the rate of rise in which 

 is determined. 



Hot-u-ater Supply.— This efficiency is the ratio between the heat communi- 

 cated to the water in a weighed and measured cylinder and that in the fuel 

 . consumed. 20 lb. of fuel are used for this test. 



