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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).
Page discussing the principles of core flux, electromotive force, and primary current in an ignition system.

Identifier  ExFiles\Box 4\5\  05-page076
Date  1st November 1930
  
-7- Contd.

would be less, but the core flux has to be maintained with the
rapidly decreasing assistance of the permanent magnet, and
after a certain point against the E.M.F. of the permanent
magnet, and for this reason thee current must be greater,
also the rate of slipping back of the flux proportionately
greater. On the whole, the primary current will in general
increase right up to the point of break.
Another way of viewing the matter is to
consider that the open circuit E.M.F. wave still exists though
the circuit be closed, and starting from a zero value at the
instant of maximum flux, rises rapidly as it does on open
circuit, to a peak value, after which it falls. This E.M.F.
operating against the resistance and inductance of the primary
winding, sets up a current in the primary winding which also
would increase up to a peak value and then fall, unless cut
short by the opening of the contact breaker.
At any given speed, we will say for the sake
of argument a moderate speed such as 500 r.p.m., the peak
value of the current will become smaller as the point of
make is made to be later, the fully advanced point of make,
as referred to above, being coincident with the zero on the
curve of open circuit voltage. This is, as would be
expected, owing to the loss of the early portion of the open
circuit E.M.F. curve. The peak value will also occur very
slightly later for the later make. Generally, the point of
Contd.
  
  


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