126 HEAT. 



Experiment of Reynolds and Morby. Yet another mode of carrying 

 out the water-friction experiment has been described by Osborne 

 Reynolds and Morby (Phil, Trans., A. 190, 1897, p. 301). Having at 

 command a 100 h.-p. engine provided with a hydraulic brake, the idea 

 occurred to Professor Reynolds that this might be used to determine the 

 amount of work needed to raise the water in the brake from 32 F. to 

 212 F. The brake itself might be regarded as consisting of paddles 

 working in a water-stirring vessel so arranged that the couple exerted 

 in the stirring could be varied at will and measured at any value. The 

 water was delivered into the brake at 32 F., and was raised in it to 

 about 212 and then passed out, the rate of flow being regulated so that 

 the rise should be through about 180. The quantity flowing while a 

 given amount of work was done was measured. 



The time of running was 62 minutes with a speed of 300 revolutions 

 per minute, and various horse-powers and various quantities of water 

 were used, the total quantity of water rising in some experiments to 

 nearly half a ton. 



The final value obtained is that the mean specific heat of water 

 between 32 F. and 212 F. measured in foot-lbs. at Manchester is 

 776'94. In ergs and degrees centigrade it is 4'1832 x 10 7 . 



Griffith's Experiment. In 1883 E. H. Griffiths gave an account of 

 an experiment to determine the mechanical equivalent of heat by the 

 method of electrical heating (Phil. Tram., 184, A., 1893, p. 361). This 

 research was carried out with the greatest care in every detail, and the 

 original paper should be consulted for particulars and especially for the 

 method of temperature regulation. 



A coil was immersed in a calorimeter 8 cm. deep and 8 cm. wide, 

 containing various quantities of water up to about 250 gms. The 

 calorimeter was closed by an air-tight lid through which passed the ther- 

 mometer, the stirrer, and the wires to the coil. It was suspended in an 

 exhausted enclosure and the walls of this enclosure were double, the 

 cavity between being filled with mercury. A graduated tube led out 

 from the cavity so that it formed, practically, the bulb of a big ther- 

 mometer in the middle of which the calorimeter was suspended. Any 

 variations in the temperature of the enclosure could thus be detected. 

 The double-walled vessel was surrounded by water, and was kept as 

 nearly as possible at a uniform temperature in order that radiation loss 

 could be exactly allowed for. 



When a current was passed through the coil, if E was the fall of 

 potential in it (determined by comparison with a Clark's cell), and if 

 R was its resistance, both in electro-magnetic units, the rate of energy 

 supply was E 2 /R ergs per second. Measuring the heat developed in 

 any time the number of ergs per calory could be determined. 



The thermometer mercury-in-glass was compared with a platinum- 

 resistance thermometer and the indications of this were found first in 

 terms of the air thermometer and later in terms of the hydrogen scale. 

 Different temperature ranges were used between 15 and 25, and the 

 final result corrected to the hydrogen scale was very nearly 



4-2 x 10 7 {1 - -000266(* - 15)} ergs per gramme of water heated 



1 of the hydrogen scale, 

 where t is the temperature on the Centigrade scale. 



