CHAPTER VII
CALORIMETRIC PYROMETERS
General Principles.—If a piece of hot metal, of known weight and specific heat, be dropped into a known weight of water at a temperature t1, which rises to t2 in consequence, the temperature of the hot metal, t0, can be obtained by calculation, as shown by the following example:—
Example.—A piece of metal weighing 100 grams, and of specific heat 0·1, is heated in a furnace and dropped into 475 grams of water, contained in a vessel which has a capacity for heat equal to 25 grams of water. The temperature of the water rises from 5° to 25° C. To find the temperature of the furnace.
The heat lost by the metal is equal to that gained by the
water and vessel. Equating these,
100 × 0·1 × (t0 - 25) = (475 + 25) × (25 - 5)
from which t0 = 1025° C.
The above calculation, which applies generally to this method, depends for its accuracy upon a correct knowledge of the specific heat of the metal used. This value is far from constant, increasing as the temperature rises, and the result will only be correct when the average value over a given range is known.
The metal used in the experiment should not oxidise readily, and should possess a high melting point. Platinum is most suitable, but the cost of a piece sufficiently large would considerably exceed that of a thermo-electric or other outfit. Nickel is next best in these respects, and is now generally used for the calorimetric method, up to 1000° C. The specific heat varies to some extent in different specimens, but can be determined for the ranges involved in practical use. This may be done by heating a given weight to known temperatures and plunging into water, the result being obtained as in the foregoing example, t0 in this case being known and the specific heat calculated. From a series of such determinations, a curve may be plotted connecting specific heat and temperature range, from which intermediate values may be read off.