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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).
Explanation of oscillatory current within a magneto's primary circuit, including the function of the condenser and safety gap.

Identifier  ExFiles\Box 4\5\  05-page083
Date  13th June 1920 guessed
  
-14- Contd.

condenser connected across the primary points. Such an
oscillatory current, damped by the effect of primary
resistance, therefore takes place. The frequency of this
oscillatory current can be estimated if the inductance of
the primary winding corresponding to the armature position
at break, the capacity of the condenser, and the resistance
of the primary winding, be known. The time of one oscilla-
tion is comparable with, though a good deal smaller than,
the timeinterval between break and the next make, so that the
regularity of the dying away oscillations would be disturbed
by the E.M.F. produced in the primary winding by the continued
rotation of the armature in the average field of flux due to
the permanent magnets. We say average because there is an
alternating flux due to the oscillation superposed upon the
flux due to the permanent magnets. Actually, however, this
state of affairs is not allowed to occur except at slow speeds,
as every magneto is provided with a safety gap over which the
secondary discharge can take place, so that there is always
one path or another for this discharge. Immediately after
break an initial portion, not more than one quarter period,
of this oscillation, would occur. At break the primary
current is not brought to a dead stop as in the case of ideal
primary break without condenser, but is reduced to zero, more
slowly at first, and more quickly afterwards, as the condenser
becomes charged up due to the influx of this current. In the
absence of the possibility of a secondary discharge the primary

CODE H-W-N-H-R.{Sir Henry Royce} NOV. M-U. PROG. M-R-H
Contd.
  
  


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