260 The N.Z. Journal of Science and Technology. [Jan-. 
A narrow calorimeter, 8 in. by 8 in. by 1 in., was constructed of polished 
tin, with one face covered with a dead black. The calorimeter was filled 
with water and placed successively at various positions near the stove, with 
its blackened face towards it. From the rate at which the temperature 
increased, an estimate could be made of the radiant heat, after suitable 
corrections for cooling and an integration to allow for the distribution of 
the radiation. With the blackened stoves used it was found that about 
8 per cent, of the heat-value of the coal was radiated. 
With regard to convection heat an estimate is more difficult, but. 
judging from various recorded experiments with other types of apparatus, 
it is probable that about 10 per cent, is thus given off. 
The heat contained in the gases escaping up the flue is wasted unless 
the range is placed against an inside wall, when it is conducted to the room 
behind the fire again. If the house is of more than one story some of this 
heat is utilized for heating the upper rooms. The average temperature of 
the flue-gases after leaving the stove is about 180° C. It is probable 
that under good conditions 15 per cent, of the heating-value of the original 
coal can be obtained from the flue-gases for heating various parts of the 
house. 
Neglecting the heat recoverable from flue-gases, it will be seen that 
under the most favourable conditions over 44 per cent, of the heat-value 
of the fuel can be utilized. Since in practice the various parts of the 
range are not generally in use simultaneously, the percentage of heat 
usefully employed will be less than this value, and may be reckoned as, 
say, 25 per cent, under good conditions. 
III. Comparison of Fuels for Oven Cootung. 
The question of the amount of heat given to material placed in an oven 
has already been discussed for the case of a Shacklock (coal) range. 
A similar method of measurement was employed in the cases of gas and 
electricity. 
In the case of gas, tests were made in two ranges, one made by Fletcher 
Russell and the other by the Wellington Gas Company. Cooking-tins 
containing water were placed on the shelves of the oven, and after a suit¬ 
able period of test the heat given to the water was determined as in the 
case with a Shacklock range. The amount of gas was measured by a 
specially calibrated meter kindly supplied by the Wellington Gas Company, 
and the calorific value was obtained by a Boy’s calorimeter. The results 
were sensibly the same for the two stoves, and varied from 17 per cent, 
to 39 per cent, according to the number of vessels placed in the stove and 
the rate of consumption of gas. Using the oven at practically its maximum 
capacity and normal flow of gas, we may take the figure 35 per cent, as 
giving the efficiency under conditions comparable with those used for the 
coal-range. 
Similar tests were made on an electric oven. The “ hot-point ” oven 
illustrated in fig. 1 was used. 
With the heaters both at “ high ” the efficiency was 77 per cent, and 
very consistent, but with only half the consumption of power the efficiency 
fell to 66 per cent. 
Comparing the three types of heating, we thus obtain the following 
values for the number of British thermal units actually used in the ovens 
for Id. It may be mentioned that the volumes of the three types of ovens 
were practically the same. 
