Hi, my first attempt at an Internet post.
The main reason for the external resistor is CURRENT limitting. When the
voltage is initially applied to the resistor/coil combination the entire
voltage drop is accross the coil. This is due to the nature of an Inductive
load. As time (a few milliseconds) passes, the voltage across the coil
decreases and the voltage across the resistor goes up. If there is no
resistor in the circuit, the coil will eventually (after a few more milli-
seconds) act like a long wire. The current in the coil, with no resistor,
will be limitted by the resistance of the wire. Those coils which are
designed to work without an external resistor must have some kind of internal
resistor or other means of limitting steady state current. If they did not
then points would fry much faster than they do.
Hope that this helps.
Ken Scott
Ken@holycross.edu
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