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Video clip demonstration of contact breaker arcing

On this page, we'd like to demonstrate, with the aid of some video clips, the effect of the capacitance of the condenser on the amount of arcing at the contact breaker. In each case, a Lucas K1F magneto was run up from a standstill to 3400 rpm (6800 rpm at the crankshaft for a four-stroke engine) and then back to a standstill. The contact breaker was set with a 0.35 mm (14 thou) gap. The points open at just before 9 o'clock, as can be clearly seen in the first few clips. A three-point spark gap was used, set to 5.5 mm and sparking in fresh air. You can view any two clips for different capacitances simultaneously in the upper and lower screens to compare them.

 
Capacitor Click to show video
clip in ...
Comment
No capacitor

Upper screen

Lower screen

Catherine wheel effect throughout the speed range.
24 nF

Upper screen

Lower screen

Catherine wheel effect at higher speeds
33 nF

Upper screen

Lower screen

47 nF

Upper screen

Lower screen

'Larger-than-pinpoint' sparks at most speeds
68 nF

Upper screen

Lower screen

100 nF

Upper screen

Lower screen

Slightly 'larger-than-pinpoint' sparks at some speeds
120 nF

Upper screen

Lower screen

150 nF

Upper screen

Lower screen

Occasional pinpoint sparks at the points
220 nF

Upper screen

Lower screen

300 nF

Upper screen

Lower screen

440 nF

Upper screen

Lower screen

 
 

The conclusion we have reached is that a capacitance of less than 100 nF will produce unacceptable arcing at, and erosion of, the contact breaker points. A capacitance of 150 nF and above is perfectly acceptable from this point of view. Between those two ranges, the contact breaker points may require more maintenance and have a shorter life than is desirable.