202 MESSRS. W. R. BOUSFIELD AND W. ERIC BOUSFIELD 



of an alloy and adopted a mercury resistance of a novel type. It consists of a long 

 spiral glass tube containing mercury, into the ends of which are sealed platinum wire 

 electrodes. This spiral tube is connected with a thermometer tube provided with a 

 scale. The spiral resistance thus forms the bulb of a thermometer and a calibration 

 of resistance in relation to scale-reading enables the resistance to be accurately 

 ascertained from the scale-reading even when a heavy current is passing. Two such 

 thermometer-resistances are used in our apparatus, one as the heater, the other as a 

 standard resistance for ascertaining the current with the aid of a battery of standard 



f 



cadmium cells. 



The following are the principal points of the method used by us : 



(a) A considerable bulk of liquid is used (about 3 litres) which is contained in a 

 Dewar vessel and constantly stirred. 



(b) The Dewar vessel is placed in a water-bath of plate glass. The temperature of 

 the contents of the calorimeter is indicated by mercury thermometers which can be 

 read through the glass walls, and for all experiments, whether involving steady or 

 rising temperature, the temperature of the external bath is kept the same as that 

 of the contents of the calorimeter. 



(c) The mouth of the Dewar vessel is closed by means of a loosely fitting obturator, 

 consisting of a circular platinum box, containing water which can be heated to the 

 temperature desired by an electric heater. The obturator is kept at the fixed 

 temperature of 20 C. when the contents of the calorimeter are below 10 C., and 

 when the contents of the calorimeter are at any temperature above 10 C. the 

 obturator is kept 10 C. higher. The obturator contains suitable open vertical tubes 

 through which stirrer, shaft, thermometers, &c., can be passed. The heating effect 

 due to obturator, stirring, &c., amounts to less than one-half per cent, of the electrical 

 heating, and was determined by a separate series of experiments. 



(d) The contents of the calorimeter are heated electrically by means of a mercury 

 thermometer-resistance of 9 to 10 ohms furnished with a graduated scale, by means 

 of which the resistance of the mercury, when a current is passing, is accurately 

 known. 



(e) The whole current is also passed through a second mercury thermometer- 

 resistance of about 2 ohms resistance, which is in shunt with 9 cadmium cells and a 

 galvanometer and key. The current is regulated by means of a resistance in the 

 main circuit so as always to balance the standard cells. By this means the current is 

 accurately known, and was kept approximately constant. 



(/) There is also in the main current circuit a Kelvin ampere balance which 

 served as a useful check in regulating the current, and whose corrected readings 

 a^ived within about 1 part in 5000 with the current deduced from the standard cells 

 and thermometer-resistance. 



(y) The current used (alxmt 5 amperes.) was obtained from a dynamo driven from 

 power mains by an alternating current motor. The poles of the dynamo were 



