Air and JIi/ih'o</en h>i heating a Platinum IVire. 301) 



A vory conviMiIent method of keeping the wire tor a long 

 time at this liigh temperature was to ])lace the wire in series 

 with a few turns of thick wire wound on a '• choking '' coil 

 connected to the alternating town mains. To make visible 

 any condensation resulting from the expansion, the light 

 from a fish-tail ])urner placed edgeways was focussed on the 

 bulb, and by looking against a dark background in a direction 

 making a small angle with the light, one or two drops could 

 be easily detected. 



With the lirst platinum wire tried it was found that (in 

 tiir) heating the wire by means of a very small electric current 

 for two or three seconds produced no condensation nuclei, 

 for no condensation was obtained on expanding a few seconds 

 after the heating-current had been cut off. The value of the 

 i?xpansion used in this case was about 1*1. 



On gradually increasing the current through the wire, and 

 therefore also its temperature, a point was reached at 

 which condensation was obtained in the form of a small 

 shower on expanding, after the wire had been raised to this 

 temperature for one or two seconds. On increasing the 

 temperature still further, the density of the shower rapidly 

 increased, and soon dense fogs were obtained long before 

 the wire was sufficiently hot to be luminous. 



This result indicated that condensation nuclei large enough 

 to be detected with the expansion used w^ere only produced 

 when the temperature of the wire reached a certain value. 



In order to make quantitative measurements on this point, 

 an arrangement was adopted in which the experimental wire 

 formed one arm of a Wheatstone's bridge, the corresponding 

 arm being a spiral of thick german-silver wire (1*16 ohms 

 resistance) immersed in a bath of paraffin oil. The two other 

 -arms of the bridge were high resistances of the order of 

 3000 ohms, oot from a Post-Office Box. 



The arrangement is represented diagramatically in fig. 2. 

 In the battery-circuit a rheostat was inserted, by means of 

 which the current through the wire and so the temperature 

 of the wire could be varied. 



In the diagram ah is the experimental wire, and BD the 

 thick ware standard-resistance. Balance is obtained by 

 varying the resistance of the arm AC. 



By means of this arrangement measurements were made of 

 the resistance of the wire when condensation resulted on 

 expanding after the heating of the w^'re. To reduce the 

 determinations of resistance to centigrade temperatures, the 

 resistance of the wire was determined first in melting ice, 

 then in steam, and lastly when a very small grain of potassium 



