308 



Dr. A. Macfarlane on the Disruptive 



the record of observations in full ; both for the sake of the 

 question considered, and also to give a sample of the observa- 

 tions made by the method. 





Deflection. 



Zero. 



Difference 

 of potential. 



Mean dif- 

 ference of 

 potential. 



/ 



300 

 295 

 290 

 293 

 296 

 297 

 296 



298 

 300 

 296 

 300 

 296 



315 

 295 

 310 

 290 

 290 

 305 

 300 



500 



496 

 490 

 486 

 505 



5> 

 J> 



505 



510 

 >> 



200 

 201 

 200 

 193 

 209 

 208 

 209 



207 

 205 

 209 

 205 

 209 



195 

 215 

 200 

 220 

 220 

 205 

 210 



► 203 



t 



> 207 



. 209 



Couple of small jars.-< 

 Without any jar ...-{ 



If the last three readings for the large jar are, as is very 

 probable, more correct than their predecessors, then the change 

 of capacity has no effect upon the readings, provided the index 

 of the electrometer is at rest at the moment of the discharge. 



Effect of Heating the Electrodes upon the Passage of the Spark, 

 When both disks were heated before a fire before being 

 screwed on, and sparks were taken immediately afterwards 

 for varying distance, we obtained a series of readings which 

 give a curve less bent at the beginning. It satisfies the pa- 

 rabolic equation 



V=87-04s-19-56s 2 . 



The diminution of the differences of potential for the smaller 

 sparks must have been due directly to the higher temperature 

 of the disks, or else to an alteration of the state of their sur- 

 faces brought about by the heating. A similar result was 

 obtained for hydrogen. 



To investigate this effect more minutely, I constructed a 



modification of an arrangement suggested to me by Clerk 



Maxwell for the purpose, and which was similar to that 



adopted by Sir W. R. Grove*. Two pieces of stout platinum 



* Roy. Inst. Proc. 1854. 



