Thermoelectric Circuit of Three Metals. 635 



of the three junctions being respectively 0°, the temperature 

 of the water-supply (read on Gr ), and 60° (read on Gr,). 



The thermostat for maintaining the temperature 0° was a 

 vessel of diameter 10 cm. packed with broken ice and filled up 

 with water *. The depth of the thermo-junction below the 

 surface of the ice was about 8 cm. 



The thermostat for maintaining a temperature of 20°, 60°, 

 or 80° was a closed metallic vessel of diameter 8 cm. and 

 depth 15 cm., nearly full of water. A powerful stirrer in the 

 base of the vessel, mechanically driven at the rate of 50 to 

 100 revolutions per minute, together with suitable break- 

 waters in the interior, ensured a uniform temperature. The 

 vessel was, so far as possible, thermally insulated. A coil 

 conveying an electric current projected into the interior of the 

 vessel ; the current was so regulated as to produce a tempe- 

 rature within half a degree of that desired for half-an-hour 

 previous to an observation of electromotive force, and within 

 one-tenth of a degree for five minutes previous to an obser- 

 vation. In most cases an observation was made while the 

 temperature was very slowly rising. The depth of the thermo- 

 junction, which was arranged in the close neighbourhood of 

 the thermometer-bulb, was about 8 cm. below the top of the 

 lid of the vessel f. 



The third thermostat was a vessel of diameter 7 cm. and 

 height 13 cm. Water was introduced at the base and taken 

 off at the brim. The thermo-junction and thermometer-bulb 

 were about 8 cm. below the water-level. 



The thermo-elements were in the first instance constructed 

 from soft wires of copper (believed to be pure), iron (believed 

 to be pure), and German silver, each of diameter about 1*5 mm. 

 For a second series of experiments these wires were drawn 

 to a diameter of 0*5 mm. 



The electrical connexions are shown diagrammatieally in 

 fig. 2. AB is a resistance of 10,000 ohms ; BB' an adjustable 

 resistance. CD is a resistance of 10,000 ohms ; EF a constant 

 resistance of approximately 100 ohms. Connexions are made 

 either between Xj Y l5 X 2 Y 2 , X 3 Y 3 , X 4 Y 4 , or between Y 3 Z 3 , 



* Ice and water is much more satisfactory for the maintenance of a 

 constant temperature than ice alone. 



f In control experiments the water in this thermostat was replaced by 

 paraffin, and in other experiments the hrst thermostat was modified by 

 the introduction of a small vessel of paraffin (well stirred) surrounding- 

 the thermo-junction and the base of the thermometer. The use of paraffin 

 in the latter case necessarily gave a somewhat less reliable value for the 

 temperature of the thermo-j unction. The results obtained agreed, within 

 the limits of experimental error, with the results of the experiments in 

 which water was used. 



2X2 



