354 Mr. W. A. Price on the Electrical Resistance of 



without any difficulty, the widening of the galvanometer-line 

 by the alternating current producing no appreciable incon- 

 venience in reading. With the workshop supply readings 

 were impossible, as the line was given a violent jerk every 

 time a motor was started or stopped. 



3. It was assumed that the temperature of R, a thin con- 

 ductor, will be the same for a direct current as for an alter- 

 nating current of the same root mean square ; so that if R 

 can be constructed to measure direct currents, it can be cali- 

 brated with those, and used to measure alternating currents. 

 All the subsequent experiments were accordingly made to 

 ascertain the conditions for its use with direct currents. 



4. Silver was chosen for trial. Being a pure metal, its 

 resistance changes rapidly with temperature, and its molecular 

 condition is unlikely to change with, repeated heating and 

 cooling as might be the case with an alloy. Being a good 

 conductor, it is heated rapidly by a low potential-difference. 

 It was available as foil *5 mil thick, in which form it has a 

 large radiating surface for a small heat capacity. In the 

 end we found this foil gave irregular results, probably from 

 being too fragile, and platinum-foil of 1 mil in thickness 

 was used instead. A few preliminary experiments showed 

 the results to be exceedingly sensitive to draughts ; and a 

 great many different arrangements were tried to protect con- 

 ductors from irregular cooling by currents of air. Conductors 

 of thin foil were enclosed in large vessels and in small, 

 packed in mica slips and in slag-wool. The containing vessels 

 were protected in various ways, or immersed in water or 

 paraffin-oil. The net results were clear. If any considerable 

 free body of air is near to the conductor, currents of air and 

 irregular cooling are produced when the conductor is heated, 

 and no consistent measurements are obtained. If, on the 

 other hand, the containing vessel or any solid packing is near 

 to the conductor, its heat capacity affects the temperature, 

 which only slowly reaches a permanent value. These results 

 indicate that the conductor must be contained in a relatively 

 large vessel, exhausted of air, and suggest that an incandescent 

 lamp may be suitable for small currents and large pressures, 

 and a slip of metal foil for large currents and small potential- 

 differences . 



5. Experiments on the above lines were started on — 



A. Carbon-filament incandescent lamps. 



B. A platinum ligament in a metal box, exhausted by a 



Fleuss pump. 



C. A platinum ligament sealed into an exhausted glass 



bulb. 



