66 



conduct heat very easily, thus making the ideal case described above 

 very far from bein^ realised. 



If a source of heat were introduced into the vessel while a regu- 

 lated stream of air was passed through, only a part of the heat 

 liberated could be used to raise the air- temperature ; the remainder 

 would [)ass into the surrounding water l)y conduction. 



It was to be expected that, when a ^letinite source of heat was 

 present, a maximum ditTerence of temperature between the in- and 

 out-streaming air would arise at"ler some time; with the given rate 

 of passage of the air this ditTci-ence of temperature caused by this 

 source of heat, could not become greater. A calculation as to how 

 great this maximum difference of temperature would be for different 

 amounts of heat, would be very complicated, if not entirely imjms- 

 sible. Fur this reason the sim[)lest way was to calibrate the a[)paratus 

 by introducing a source of heal of known maguitude. P^or this 

 purpose a manganin-wire was |)lace(l inside the ai)|)aratus over as 

 wide an extent as possible, in the place where later the gei-minatiug 

 wheat-grains were to be put. This wire formed a metallic contact 

 with two co|)per rods which jU'ojected above the lid of the appara- 

 tus. An electric current could be passed through the manganin-wire 

 by connecting these rods with the two poles of an accumulator. 

 The resistance of the manganiu-wire was accurately determined, whilst 

 a milliampèremeter, placed in the circuit, sei'ved to measure the 

 strength of the current. By taking the current from 1, 2, and 3 

 accumulators alternately, sources of heat of different magnitude could 

 be introduced into the apparatus. 



When in this way a source of heat of known magnitude occupied 

 the apparatus, air was i)assed through and at regular intervals the 

 (double) deflection of the spot of light on the scale was read till 

 this ultimately remained constant and therefore had reached a 

 maxiuium. These observations wei'e conducted at temperatures of 

 20°, 30°, and 40° of the sui-rounding water, and also therefore of 

 the entering air. 



These calibration-experiments showed : l*^"^ that the maximum 

 deflection of the spot of light, or in other words the difference of 

 temperature between the iu- and out-goiug air was roughly in 

 proportion to the source of heat which was placed in the apparatus, 

 2'"^' that this proportionality was maintained at a surrounding 

 temperature of 20°, 30°, and 40°, 3"' that the absolute magnitude 

 of the deflection was independent of this temperature. 4''', that a 

 deviation of 1 centimetre corres})onded to a derelopincnt of about 

 11.5 calories per hour. 



