The electromagnet in Minox shutter is a solenoid -- a LR circuit with inductance L and resistance R.

The current i passing throught he circuit is governed by first order differential equation

E=iR+ L*di/dt,

or in Heaviside Operational calculus, be expressed as

i= E/(R+pL)

in which p is the differential operator.

If a timing circuit applues a voltage in the shape of Rect(t) (rectangular function), ie straigh up from zero to E than stays for duration T then drop to zero suddenly, then the current at the beginnign phase can be expresssed by Helmhotz'law

i=E*(1-exp(-R*t/L)/R

and at the closing end by

i=(E/R)*exp((-R/L)*t).

The quantity L/R is also known as LR circuit time constant. Hence when the voltage to the eletromagnet is cut off, the magnet does not release immediately, but delayed a little bit. Following this, at which point the magnet actually release the shutter depends on the the air gap between the magnet and the iron plate, any dust beteen them, the strength of the restore spring etc--- not as clean cut as mechanical shutter, for which the flash sync closing point is 'close shutter' trigger point.

-- Anonymous, March 26, 2000