RECENT PROGRESS IN PHYSICS. 



367 



Itaten that the quantity of -|- E which returns on the discharge between 

 the knobs to the metal plate, shall be less, which is attained bj bring- 

 ing the knobs of the spark micrometer closer together. 



SECTION FOURTH. 



ELECTRICAL SPARK AND BRUSH. 



§ 79. Faraday's researches on the spark and brush. — Without going 

 into the theoretical disquisition, mentioned in another place,* which 

 iFaraday has given upon the spark and brush, T will present here only 

 the most important facts which he has obtained in his experiments 

 Jupon these phenomena of light. — (Pog. Ann., XL VII and XL VIII.) 

 I In order to compare the resistance which different gases presented 

 ito the passage of sparks, with the corresponding resistance of the air, 

 'Faraday used an apparatus, a sketch 

 "of which is represented in fig. 72 Fig. 72. 



Two small knobs, s and S, connected 

 with the conductor of an electrical 

 ■machine, were placed opposite to two 

 larger knobs, I and L, in conducting 

 -connexion with the ground. The 

 diameter of the balls was as follows: 



iBall s 0.93 of an inch. 



Ball^ 0.96 " 



Ball 1 2.02 " 



iBallZ 1.95 " 



; 



c 



( 



V 



j9 



^ 



I The constant interval v between s 

 and I was 0.62 of an inch ; the inter- 

 (val w between S and L was variable. 

 I It would have been better if the 

 itwo small balls s and IS had been 

 perfectly equal in size, and I and L 

 ,al80 equal; much more reliable con- 

 clusions could then have been drawn 

 from these experiments. 



The two balls 5 and I were placed 

 in a receiver, which could be exhaust- 

 jUdand then filled with dilferent gases. 



' The receiver being filled with air under the pressure of the atmos- 

 ;phere, the sparks passed alternately at u and v, when the intervals at 

 ■u were between 0.6 and 0.79 inches. When the interval at u was less 

 jthan 0.6 the sparks always passed here, but if it was greater than 

 ;0.79 the sparks then always passed at v. 



See § 24 in the Report for 1856. 



