491 On the " Break " of a Magneto or Induction- Coil. 



on the time for which the current has passed *., but a definite 

 relation does not seem to have been established. It should 

 be mentioned that as the pressure between the terminals is 

 increased, V in general decreases and i 2 increases ; it is 

 difficult to determine how ^ changes. But no simple 

 relation could be found between the pressure and the values 

 of V and i. 2 . 



An attempt was made to establish a connexion between 

 the form of the full line curve and the least current which 

 would show arcing when the contact was broken. It was 

 thought, for example, that i 2 might represent this least 

 current. But no simple relation could be found. 



Summary. 



1. The notes deal with the question what conditions are 

 necessary to prevent sparking at the break of the primary 

 circuit of an induction-coil or magneto ; and, especially, 

 how far the speed of separation of the contacts is 

 important. 



2. Experiments show that over a wide range of speeds, 

 the greatest current which can be broken without sparking 

 is independent of the speed of separation of the contacts. 



3. On the other hand, the nature of the contacts separated 

 influences greatly the limiting current. 



4. The observations indicate that the " spark " at the 

 break is really an arc : this conclusion is confirmed by 

 measurements of the potential difference across the contact 

 as it opens. If the discharge is an arc, the independence of 

 the limiting circuit of the speed can be explained. 



5. If arcing can be prevented, by preventing heating of 

 the electrodes, the resulting break is perfect. 



6. Conditions can be found in which the " spark " at the 

 break is a true spark and not an arc ; the limiting current 

 is then dependent of the speed of separation of the contact. 

 But these conditions are not likely to occur in any magneto 

 or induction-coil. 



7. Some observations on the resistance of contacts are 

 recorded. 



March 1919. 



* In the observations of fig. 2 it is supposed that, before taking a 

 measurement of V, i is kept constant for a time so long that V reaches 

 a steady value ; this time may amount to more than a minute. 



