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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical explanation of magneto operation regarding current, E.M.F., and spark intensity at different speeds and timings.

Identifier  ExFiles\Box 4\5\  05-page078
Date  13th June 1920
  
-9- Contd.

progressively by advancing the contact breaker. On the other
hand, if the speed of the magneto be considerably increased,
say to 3000 r.p.m. the peak of current in both cases advance
and retard is now considerably later than before, and although
the peak value is still less for retarded position than for
advanced, the current at break in the advanced position is
smaller than for the retard position, because break for the
advanced position now occurs earlier relatively to the peak.
For the retard position it comes at or about the peak. Hence
in a magneto running at a high speed the stronger spark is
obtained with the lever fully retarded.
It will be instructive to further consider
how, given the points of make and break, the primary current at
the instant of break, and therefore secondary spark intensity,
depends upon the speed of the magneto. We have seen that in
order to maintain the necessary current in the primary winding,
which current maintains the existing core flux, an E.M.F. must
be produced in the winding by the slipping back of the core
flux. This E.M.F. is proportional to the rate of change
or slipping back of the core flux and can be maintained with
a smaller loss of flux if the time is shorter. When the
magneto speed is dead slow there is no appreciable E.M.F.
or current produced in the primary winding and the core flux
slips back or readjusts itself to the distribution it would
have if the armature were stationary, as fast as the armature

Contd.
  
  


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