HEAT. 



Diffusivity. A well-known piece of apparatus used to illustrate 

 conductivity, was devised by Ingenhousz. In this a number of equal 

 metal bars are placed in a row, with their lower ends in a vessel into 

 which hot water can be put (Fig. 65). The rods may be smeared with 

 beeswax, and the rate of melting along the different rods compared. 

 But it is clear that the propagation of the high temperature along the 

 rods depends not only on their conductivity, but also on their specific 

 heats. If, for example, a cubic centimetre of one rod has twice the 

 heat capacity of the same volume of another, it requires twice the 

 quantity of heat to raise its temperature to the same extent, and if the 

 melting extends equally quickly along the two rods, the conductivity 

 must be twice as great in the one case as it is 

 in the other : that is, we must consider, not 

 only the conductivity, but the ratio 



Conductivity K 



FIG. 65. Ingenhousz's 

 Apparatus. 



Heat capacity per cubic centimetre ps' 



where K is the conductivity, p the density, and s 

 the specific heat. This quantity is termed the 

 diffusivity of the substance. We may regard it 

 as the conductivity for temperature as distin- 

 guished from the conductivity for heat. 



Another quantity which plays an important 

 part in researches : on conductivity is the 

 emissivity of a surface. We may define this as 

 the heat lost by the surface per square centimetre 

 per second, per degree of excess of temperature 

 of that surface above the surroundings. 

 Measurements Of Conductivity. From the definition of conduc- 

 tivity we have 



quantity of heat passing through a sq. cm. per sec. 

 slope of temperature perpendicular to the area 



and if we can separately determine numerator and denominator, we 

 obtain It. 



It would appear then that the simplest way to measure conductivity 

 would consist in catching the heat passing through some known area 

 and measuring at different instants its rate of flow, and at the same 

 instants observing the temperatures at two points near the area to give 

 the temperature-slope. But while it is easy to measure the total quan- 

 tity of heat which a body has gained in a given time, i.e. the difference 

 between its gain and loss, it is quite another matter to measure the rate 

 at which it is gaining at any instant. Indeed, the difficulty is not unlike 

 that which a merchant might experience in trying to estimate his actual 

 rate of profit at any instant. He may succeed in finding his exact profit 

 or loss during a year, buo he can hardly trace all the transactions and 

 estimate the profit or the loss which is accruing in any one minute. And, 

 again, while various methods of measuring the temperature at a point 

 may be successful, it is not always easy to carry out the measiirement 

 without affecting and even diverting the flow of heat. Through these 

 difficulties there is no measurement in which more widely different 



