of a Magneto or Induction- Coil. 483 



sparking because it decreases the rate of rise o£ the potential 

 difference between the terminals. It is to be expected that 

 sparking would also be stopped by an increase in the rate of 

 rise of the spark potential, which would accompany an 

 increase in the rapidity with which the terminals separate. 



However, Lord Rayleigh's experiments seemed to indicate 

 that any practical application of the second alternative is 

 out of the question, because the necessary rate of separation 

 is so great. He found that no advantage was gained by 

 increasing the speed of separation up to that of a pistol 

 bullet, and that it was only when a rifle bullet was employed 

 to cat the primary circuit that any increase in the secondary 

 .potential was obtained. The critical velocity would doubtless 

 depend on the constants of the circuits and on the primary 

 current broken (for these determine the rate of rise of 

 potential), but in the case investigated it was so much 

 greater than anything which could be obtained with a 

 mechanical break that research in this direction has not 

 seemed promising. 



It may be noted that, if we knew the rate of rise of 

 potential and the relation between the spark potential and 

 the distance between the contacts, the determination of the 

 critical velocity would permit an experimental test of the 

 theory. The rate of rise of potential, can be determined 

 with any arrangement for measuring the wave-form, but 

 the relation between the spark potential and the distance 

 between the contacts is less easy to determine satisfactorily 

 for the very short distances which are here involved. It is 

 known that, as the distance is reduced, the spark potential 

 diminishes to the "minimum spark potential"" of about 

 350 volts at a distance somewhat less than O'l mm., that it 

 subsequently increases and then falls again very rapidly 

 when the distance becomes about 0*001 mm. In the interval 

 between '001 mm. and 0*1 mm. the relation is so com- 

 plicated that it is difficult to determine with the necessary 

 accuracy. 



(2) Experiments. — Some observations which have been 

 made in the course of the work described in a recent paper * 

 seem to throw some new light on the question ; and although 

 they are not sufficient to answer all the questions that can 

 be raised, they seem to point to conclusions of sufficient 

 practical importance to justify their record. 



Observations that seem to indicate that the theory in its 

 present form is not adequate to explain all the facts can be 

 * Phil. Mag. vol. xxxvii. ser. 6, p. 284, March 1919. 

 2 L 2 



