ANOMALOUS BEHAVIOUR OF DELICATE BALANCES, ETC. 



405 



by CAREY FOSTER. The two coils to be compared are the platinum wire spirals, whose 

 temperature may vary. The whole of the bridge, with the exception of the slide 

 wire, can be introduced into a short beam balance case. 



Using an Elliott "Century" 1)' Arson val galvanometer, having a resistance of 

 50 ohms, and a bichromate cell working through a suitably adjusted rheostat, it was 

 found that a movement of the slider equal to 0'3 mm. could be detected with 

 certainty. 



Taking the temperature coefficient of the platinum wire spirals as 0'004, and using 

 the data already given, it can be shown that the bolometer enabled us to measure 

 temperature differences rather smaller than 1/10,000 C. 



In the experiments which we now describe, the temperature differences were in the 

 majority of cases recorded to within 1/5000 C. only. 



Experiments with the Bolometer. 



(1) The bolometer was placed inside a Bunge balance case and arranged so that 

 the centres of the platinum spirals were as near as possible to the ends of the beam. 

 The case was then closed and left for a time, in order that the temperature within 

 might become normal. Bolometer readings were then commenced and continued for 

 a period of 19^- minutes : the observations were taken at intervals of 10 seconds, and 



O JO 60 9O 120 ISO IBO 2IO 240 Z7O 3OO J3036O390 



JO 60 90 I2O ISO 100 210 240 270 JOO 33O 36O 390 



Time in Seconds 



C 



0080 

 0060 

 OO40 

 OOZO 



zo'oooo 

 oooo 



0060 

 0040 

 OOZO 



zo'oooc 



0080 

 0060 

 OO40 

 OOZO 



zo'oooo 



<30 6O 90 120 ISO ISO 210 240 27O 300 330 36O 390 



reduced to their equivalent temperature values. On plotting the temperatures along 

 the Y-axis, and the corresponding time in seconds along the X-axis, the graph shown 

 in Fig. VIII. is obtained. For convenience this is divided into three sections. 



Experimenting in this way with the other balances, precisely similar results were 



