Currents arising from two' fine heated Platinum Wires. 943 



could be made directly to O001 cm. and by estimation to 

 O'OOOl cm. These openings. Y served for inspection of the 

 wires, and could be closed when desired. The axial position 

 of the wire A could be accurately determined by viewing A 

 by means of the microscope and rotating the chamber 

 through 180°. The axial position was ascertained by 

 securing that no radial displacement of A in the chamber 

 occurred during such rotation. The necessary axial adjust- 

 ment could be effected by suitable motion of the blocks E 

 carrying the spring supports Y, and the wire was secured 

 in the desired position by means of the screws Z. The wires 

 constituted two adjacent arms of a Wheatstone bridge, the 

 remaining two arms being made up respectively of a resist- 

 ance of 1000 ohms connected to the central wire of the pair, 

 and the other arm was capable of suitable adjustment so that 

 the bridge was balanced when the plane of the wires was 

 horizontal. The battery terminals were connected to the 

 bridge so that the maximum heating effect was produced 

 in the fine platinum wires. The bridge current was adjust- 

 able by means of a rheostat, and was maintained constant to 

 within 0*002 amp., readings being made by means of a 

 Siemens & Halske milliammeter. For any chosen distance 

 apart of the wires, galvanometer deflexions could be read 

 corresponding to various inclinations of the plane of the 

 wires to the initial horizontal position. The respective 

 resistances of the wires could be deduced from the drop of 

 potential across each wire as determined by means of a 

 Weston voltmeter inserted across each wire in turn. The 

 temperatures of the respective wires were deduced therefrom 

 using the respective values of their resistances at U C. and 

 the values of their temperature coefficient, determined by 

 means of a Callendar & Griffiths bridge. The comparative 

 values of their resistances were more readily and somewhat 

 more accurately measurable by disconnecting the- wires from 

 the bridge and employing the galvanometer in series with a 

 resistance of 12,000 ohms as a voltmeter. Galvanometer 

 deflexions were read in both directions by reversing the 

 current in the wires, and could be made with accuracy to 

 within half a scale division. * As previously explained*, the 

 galvanometer deflexions to which the respective resistances 

 are proportional were throughout corrected to a uniform 

 atmospheric temperature of 17° 0. In general, during the 

 course of the experiments detailed herein, the atmospheric 

 temperature did not vary by more than 1°0. 



* Phil. Mag. vol. xli. p. 722 (1931). 



