258 The N.Z. Journal of Science and Technology. [Jan. 
in which heat-energy is temporarily stored as electro-magnetic wave- 
vibrations of the so-called ether. When these latter vibrations have 
frequencies between 7-5 X 10 14 and 3-8 X 10 14 they also cause the sensation 
of light. 
Energy or work is generally measured in foot-pounds or ergs, and is 
convertible into heat, the ratio of work to heat being 774 ft.-lb. per B.Th.U., 
or 4*18 X 10 7 ergs per calorie. 
Electricity is a form of energy which is particularly easily transformed 
into either energy of motion or energy of heat. The unit of electrical 
energy is the watt. 
10 7 *239 
1 watt = 10 7 ergs = ——- —-= = *239 calorie = —— = *00095 B.Th.U, 
& 4*18 X 10 7 252 
1 Board of Trade unit (B.O.T. unit) — 1 kilowatt-hour = *00095 X 3600 
X 1000 = 3,420 B.Th.U. 
Fuel contains stored-up chemical energy which can be liberated as heat 
on complete combustion with free oxygen or with oxygen of the atmo¬ 
sphere. Thus the best coal can produce on complete combustion about 
14,500 B.Th.U. per pound, or 8,000 calories per gramme. A few calorific 
values are given : Best coal, 14,500 B.Th.U. per pound ; good dry coke, 
13,000 B.Th.U. per pound ; State coal as used in the following experiments, 
13,680 B.Th.U. per pound ; coal-gas (normal value), 500 B.Th.U. per cubic 
foot; coal-gas (average during experiments), 450 B.Th.U. per cubic foot. 
In comparing the efficiencies of various types of fuel from the consumer's 
point of view it will be necessary to specify the price per unit. In this paper 
the retail prices ruling in Wellington during October, 1920, will be taken. 
It will be an easy matter to deduce the figures for other basic prices. It 
will be convenient to express the figures as the number of British thermal 
units which it is possible to purchase for Id. We thus obtain the following 
results as the theoretical thermal units to be purchased for Id.:— 
Theoretical Heat-units for Id. 
Price per Unit. 
Calorific 
Value. 
B.Th.U. 
for Id. 
Coal 
4s. per hundredweight 
13,680 
32,000 
Gas 
8s. 4d. per 1,000 cubic feet . . 
450 
4,500 
Electricity 
lfd. per B.O.T. unit 
3,420 
2,280 
For the experiments to be described a quantity of well-mixed coal was 
purchased from the State coal-depot, and Dr. J. S. Maclaurin (Dominion 
Analyst) kindly determined for us the calorific value of an average sample. 
With regard to gas, some ninety determinations of calorific value were 
made during the course of the tests, and the value 450 B.Th.U. per cubic 
foot is the average of the results. 
It will be seen from the above table that Id. will buy fourteen times 
as many heat-units from coal and twice as many from gas as from electricity 
at the rates quoted. These figures refer, of course, to heat-units of total 
thermal capacity. In practice the various fuels will be used at an efficiency 
depending on the particular appliance used, and it is necessary to determine 
the fraction of the heat-units which is usefully employed. 
