EXPERIMENTAL METHODS AND APPLIANCES 23 1 



the Calorimeter and the other t in a larger vessel v filled with 

 water. Under ordinary circumstances the temperatures at 

 the two junctions are the same, and the deflection of the 

 sensitive Galvanometer in circuit is zero. The distance of the 

 scale from the Galvanometer is so adjusted that a difference 

 of temperature of i° C. in the two vessels produces a de- 

 flection of 50 mm. It is thus easy to measure a rise of 

 temperature in the Calorimeter as small as 0-02° C. 



Elimination of loss of heat by radiation. — In calorimetric 

 measurements special precaution has to be taken in guarding 

 against loss of heat by conduction and by radiation. Loss 

 by conduction is avoided by suspending the calorimetric 

 vessel by a silk thread in the recess of a sliding block of wood. 

 The loss from radiation is eliminated as follows. The 

 duration of the incident light is in all cases about three 

 minutes only, and the resultant rise of temperature in the 

 Calorimeter is f. The water in the Calorimeter is, to begin 



t° 

 with, at a temperature of - below that of the room. During 



the first half period of the experiment the Calorimeter 

 is at a lower temperature than that of the room, and 

 therefore absorbs heat from its surroundings ; during the 

 second half, when its temperature is rising above that of 

 the room, it is losing heat. The gain and loss being equal 

 there is no resultant loss. This will be clear from the follow- 

 ing concrete example. The rise of temperature in the Calori- 

 meter after three minutes' exposure to light is, generally 

 speaking, i° C, which corresponds to a deflection of 50 mm. 

 of the Galvanometer. The temperature of the water at the 

 beginning is adjusted half a degree below the temperature 

 of the room, indicated by a deflection of — 25 mm. of the 

 Galvanometer. The incident radiation is allowed to fall on 

 the Calorimeter for three minutes with the resulting rise of 

 temperature through i° C, indicated by an increase of 

 galvanometric deflection from — 25 to + 25 mm. The 

 maximum difference of temperature between the water in 

 the Calorimeter and the temperature of the room is only 



