Hal Davis's explanation of the benefits of high-voltage solenoids (MMD
971009) make me feel a little uneasy.

All electrical engineers learn, at some stage, that inductance is rather
like mechanical mass.  The more of it you have the more difficult it is to
shift it.  In the case of a solenoid "it" is current through the coil and
you shift "it" with voltage.  What does the useful work [i.e., creates
the force which moves the armature] in a solenoid is the ampere-turns.
That is the current multiplied by the total number of turns.  Thus it
seems that a high voltage coil (more turns, more inductance) is going to
be harder to get moving.  By harder I mean more voltage.

An example is the scanning coils in a TV set where the line scan coils,
which must work many times faster than the field scan, will have a much
lower inductance to save the need for ludicrously high voltages.
Incidentally the peak power in the line scan coils for a 1970s 25-inch
colour CRT (US: kinescope) is around 3 kVA!

What all this ignores is the _resistance_ of the coil.  A high voltage
solenoid will also have much greater resistance.  If, in a particular
solenoid, the resistive component is dominant ('low Q', to use the radio
term) then you will not need to apply high voltage peaks to change the
current in a hurry.  But you will dissipate more heat.  The high R, low Q
solenoid will be a tame, if hot, beast.

The conclusion from this rambling explanation (the maths would be easier
but difficult to put in an email) is that it does not matter how many
turns the solenoid has provided that the drive electronics can supply
the necessary voltages and currents at the correct times.  Obviously some
combinations of voltage and current are easier than others, especially
before the era of modern electronics.

I now leave it to Robbie et al to pick my argument to pieces and comment
on the capabilities of early (pre-electronic) solenoid drivers.

Jeffrey Borinsky