

. 



Current of Air flowing past heated Platinum Wires. 243 



Results and Discussion. 



Diameter of flow-tube 2*0534 cm. 



Diameter of platinum wires O'lOl mm. 



Temperature coefficient of resistance of platinum wires.. 0*003335 



Mean distance between successive wires 1*3 mm. 



Number of platinum wires employed 11 



Heating current 1*1 amp. 



Fixed ratio arm in Wbeatstone bridge 1000 obms. 



The respective resistances of the several wires at 0° C. v 

 and the temperatures to which they are raised by the heating- 

 current o£ l'l amp. subsequently employed, are set out here- 

 with in Table I, the wires being numbered consecutively in 

 the direction in which the flow was established in the tube i — 



Table I. 



Wire No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11, 



O ol C a ?onm) at } °' 2454 °' 2479 °' 2507 °' 2498 °' 2470 °' 247 ° °' 2470 ° -2498 °' 2545 °' 2500 °' 253 ' 

 Initial tern- \ 

 perature to 

 which wires 



were raised - 6() o 674 o 69Q o 68g o 695 o 69Q o 7Q5 o 66 go 672 o 635& 526 

 by current / 

 of 1*1 amp. ' 

 in absence 

 of flow (°C.), 



The mean value of the resistances at 0° C. of the several 

 wires was found to be 0'2484 ohm. The individual resist- 

 ances, except Nos. 1, 9, and 11, were all within 1 per cent, 

 of this mean value. The percentage variation from the 

 mean value in the case of the 1st, 9th, and 11th wires 

 respectively were 1*2 per cent., 2'4 per cent., and 2 per 

 cent. The distribution of temperatures amongst the several 

 wires for zero flow may be roughly described as a condition 

 in which the temperature increases considerably from the 

 respective outer wires of the series to the adjacent ones, the 

 remaining wires contained within the four outer wires being- 

 all raised to an approximately uniform temperature of 

 680° C. When the motion due to the slow current of air in 

 the flow-tube was impressed upon the free convection cur- 

 rents arising from the wires heated by a current of l'l amp., 

 it was found that, except in the case of the first wire of the 

 series, the resistance of the individual wires was increased. 

 An indication of the constancy of the air-flow was secured 

 by employing the leading pair of wires, in conjunction with 

 a Wheatstone bridge, as a hot-wire anemometer in the 

 manner already described. A comparison of the respective 

 thermal effects experienced by the several wires due to the 



R2 



