Heat from a Pair of fine heated Wires. 279 



between the ebonite blocks C. Care was taken that the 

 continuity of the inner surface of the flow tube was nowhere 

 disturbed by the insertion of the blocks, which were held 

 rigidly in place against the blocks C by means of nuts 

 screwing on the ends of three brass rods passing longi- 

 tudinally through the whole length of the blocks. The 

 distance apart of the wires was determined by means of a 

 micrometer microscope reading direct to 0*001 cm. For 

 this purpose, the wires could be viewed through the aper- 

 tures M, which were normally closed by plugs whose ends 

 cou formed accurately with the inner contour of the flow 

 tube. The tube was inserted in the flow system by means 

 of spigot unions as shown. The wires were disposed in a 

 horizontal plane in the flow tube, which was arranged hori- 

 zontally. The distance apart of the wires having been 

 ascertained, the current in the wires was adjusted by means 

 of a rheostat, so that for the respective distances the wires 

 were, in the absence of any impressed stream, maintained at 

 a constant temperature, and subsequently the current in the 

 flow experiments was adjusted to such appropriate value and 

 maintained constant to within 0*001 amp. The experiments 

 were directed mainly to ascertaining the change in resistance 

 produced in the second wire of the pair by the stream of air, 

 and for this purpose the ends of this wire were connected to 

 a Rayleigh potentiometer, constituted of two P. 0. boxes, in 

 which the total resistance was throughout adjusted to 

 10,000 ohms. The working current in the potentiometer 

 was derived from a Leclanche cell, and the readings were 

 reduced to volts by calibration from time to time against a 

 Weston cell certified by the N. P. Laboratory. The flow of 

 dry air was established as detailed in previous papers, and 

 the mean equivalent velocities of flow were throughout 

 reduced by calculation to 0°C. and 760 mm. pressure. 



Results and Discussion. 



Internal Diameter of Flow Tube 2*011 cm. 



External Diameter of Flow Tube 2*212 cm. 



Mean Diameter of Platinum Wires 0*101 mm. 



Temperature Coefficient of Platinum 



Wires 0*003750 



P of 1st Wire of Pair 0'2468 ohm. 



R of 2nd Wire of Pair ....... 0*2420 ohm. 



Resistance of 2nd Wire, heated in absence 



of impressed Stream 0*6931 ± *0011 ohm. 



Mean temperature of 2nd Wire, heated 



in absence of impressed Stream 531° C. 



