a Conductor the Measure of the Current passing. 363 



glass vessel than in the imperfectly exhausted brass one. The 

 second point is that the extension of the curve to the x axis 

 appears to indicate a temperature-coefficient of resistance for 

 the metal of which the slip was composed, which was believed 

 to be pure platinum, of about *375, which is higher than the 

 accepted figure; so that the exact position of this curve, 

 though it was very carefully taken, could not be relied on 

 without confirmation. 



16. The figures of the following table are different sets of 

 measurements of the resistance of the slip of fig. 8, at various 

 currents and after different treatment. 



Current in 

 milli amperes. 





Resistance 



in ohms. 





A. 



B. 



C - 



D. 



•3 



•32 



•319 



•318 



•3155 



•35 



•368 



•368 



•363 



•361 



•4 



•421 



•420 



•416 



•413 



•45 



•470 



•479 



•474 



•471 



•5 



•531 



•534 



•5295 



•529 



The figures A were taken as the bulb was received from 

 the maker ; B were taken ten days afterwards, a good many 

 experiments with currents not exceeding *5 ampere, alternating 

 and direct, having been made in that time ; G were taken 

 shortly after B, the slip having been heated previously for 

 one hour to a bright red by a current of *9 ampere; D were 

 taken after a further heating for j hour of the same current, 

 *9 ampere. The resistance of the slip when cold was unchanged 

 throughout. It had been raised to a bright-red heat before 

 originally sealing into the glass bulb. It seems clear that 

 the explanation of the changes between A, B, C, D must be 

 looked for in the small quantities of gas occluded by the 

 platinum before sealing in, and given off when subsequently 

 heated in vacuo ; and this effect may also explain the changes 

 in the resistance of a carbon filament noticed in § 8 above. 

 If a conductor of this kind is to be used as a gauge for the 

 measurement of current, either, as Prof. Callendar has 

 suggested to me, the exhaustion of the vessel must be poor, 

 so that the absorption or occlusion of gas by the platinum 

 does not make any appreciable change in the amount in the 

 vessel, giving the case of the results of § 13 ; or else the 

 exhaustion and sealing must be performed when the platinum 

 is at a bright-red heat, so that no occluded gases are left. 

 Either course seems to promise a satisfactory result. 



