[ 481 J 



XLIV. Notes on the "Break" of a Magneto or Induction- 

 Coil. By Norman Campbell, Sc.D* 



Note — The work described in these notes was carried 

 out at the National Physical Laboratory under the direction 

 o£ the Advisory Committee for Aeronautics. The results 

 have been communicated in a confidential report to the 

 Internal Combustion Engine Sub-Committee of that Com- 

 mittee, who have now given their consent to the publication 

 of any portions which appear of pure scientific interest. 



(1) Introduction. 



IT is well known that if the greatest possible efficiency is 

 to be obtained from a magneto or induction-coil, that 

 is, the greatest possible ratio of the maximum secondary 

 potential to the primary current broken, it is necessary to 

 avoid sparking between the terminals at which the primary 

 current is broken. No elaborate theory is necessary to 

 explain the loss of efficiency due to sparking. In the first 

 place the spark involves the dissipation of some of the 

 electromagnetic energy originally present in the primary 

 current which would otherwise have been available for 

 conversion into electrostatic energy of the secondary. In 

 the second place the passage of the spark, even if it involved 

 no loss of energy, would prolong the time which elapses 

 between the first decrease of the primary current, as the 

 contact opens, and its total cessation ; very general consider- 

 ations will show that, if this time is so prolonged as to 

 become an appreciable fraction of the period of the oscil- 

 lations excited, a loss of efficiency will usually follow. 



It is always possible to suppress the spark at the primary 

 break by inserting a condenser of sufficient capacity in 

 parallel with the separating terminals. Until an adequate 

 theory of the induction- coil was developed, chiefly by the 

 work of Prof. Taylor Jones and his collaborators |, it seems 

 to have been believed that the suppression of the spark was 

 the only useful function of the primary condenser; and that 

 if the suppression could be achieved by any other means, 

 such as an increase in the speed of separation of the terminals, 

 the addition of capacity to the primary circuit would be 

 unnecessary. This view was, of course, largely based on 

 the classical experiments of Lord Rayleigh J, who showed 



* Communicated bv the Author. 



| Phil. Mag. Jan. 1909, p. 28; Nov. 1911, p. 706 ; April 1914, p. 580 ; 

 Jan. 1915, p. 1 ; Aug. 1915, p. 224 ; April 3917, p. 322. 

 t Phil.' Mag. ii. p. 581 (1901). 



PJiil. Mag. S. 6. Vol. 37. No. 221 . May 1919. 2 L 



