486 Dr. Norman Campbell on the "Break" 



break lias caused such intense local heating that portions of 

 the terminals h:ive been raised to the temperature at which 

 notable thermionic emission begins, so that a discharge 

 between them can be maintained by a potential difference 

 quite inadequate to start a spark. 



With the object of testing this explanation measurements 

 were made on the rise of potential across the break as the 

 terminals separated. The circuit broken was the primary 

 of a magneto with a condenser of 1 mf. across the break ; 

 one terminal consisted of a ^ in. steel ball attracted with a 

 force of about 50 grams weight to the end of the magnet 

 which formed the other terminal. The limiting current was 

 1*2 amp. ; it was found that, so long as the current was 

 less than this, the potential at any given moment after 

 break was very nearly proportional to the current broken. 

 Accordingly the rise of potential was first determined with a 

 current less than the limiting value (1*0 amp.) ; if we 

 multiply all the measured values of the potential by 1*5 we 

 get a curve A in fig. 1, which represents the rise of potential 



Fiff. 1. 

















\ 



1 / 

 / 



1 



1 



l/= sees 



cf of pea, 



r in cm./s 



PC. 













B 







B' 













' V=2SO 



"N 





/^V=73 



\ 







































20 

 tfsec.x/0- s J 



which would follow when a current of 1*5 amp. is broken,, 

 if there were no arc formed. Curves B and B' give the 

 measured rise of potential when a current of 1*5 amp. is 

 broken, and the speed of the separation of the terminals 

 is 250 cm./sec. and 73 cm. /sec. in B and B' respectively. 

 It will be seen that from the very first instant the rate of 

 rise is much less rapid in B and B / than in A; that a stage 



