354 Messrs. Trowbridge and Eichards on the Temperature 



mirror driven very rapidly by means of a small electric 

 motor. In a few cases zinc terminals were used, with no 

 appreciable difference in the results (Bighi *). The terminals 

 were re-pointed from time to time, and were always kept at 

 a distance of 1*3 mm. apart. With this apparatus the 

 photographs of perhaps 500 sparks were taken, and the 

 results are recorded in the following table. As a general 

 rule the spark containing the highest number of oscillations 

 upon any plate was taken as the representative one. 



The first column below records the resistance through 

 which the discharge had to pass before reaching the spark- 

 gap, while the second, third, and fourth record the number of 

 half-oscillations observed upon the photographs. 



Resistance Standards. 



Resistance. 

 Ohms. 



Capacity = 6000. 



Capacity =12,000. 



Capacity =18,000. 



No. of half- 



No. of half- 



Ho. of half- 



oscillations. 



oscillations. 



oscillations. 









37 



32 



1 





21(f) 



21 



2 



160 



14 



13 



3 



12-0 



11 



10 



4 



9-5 



8-5 



7 



5 



8-0 



7 



6 



6 



7-5 



6 



5 



7 



65 



5 



4 



10 



5-0 



4 



3 



15 



30 



3 





20 



2-0 



2 



1-7 



30 



10 



1 



1 



These figures correspond in general tendency with the less 

 precise determinations made by Feddersen f ; they show, as his 

 determinations did, that the larger the capacity the fewer the 

 number of oscillations. This tendency is especially noticeable 

 between two and ten ohms, the part of each curve which is 

 most capable of accurate determination. While noc perfectly 

 regular, these curves manifestly furnish the means of mea- 

 suring approximately any small resistance through which a 

 spark, followed by as much as one-half of an oscillation, is 

 able to pass. 



Having now our scale of measurement, we substituted for 

 our known resistances a Plucker tube attached to an admirable 

 automatic Toepler air-pump (of Kiss, Budapest), as well as to 

 receivers containing pure hydrogen and nitrogen. These 

 o-ases could be delivered individually into the tube at any 



* Nuovo Cimento (2) xvi. p. 97. 



f Pogg. Ann. cxiii. p. 437. 



