Ignition light mystery

It comes on when she fires up, giving me worries that I'll only make it half way to work, then goes out after a few miles.

Any suggestions as to why, and how far can you get if the battery isn't charging?

What actually causes this light to come on?

Many thanks for your experience and opinions......

The ignition light indicates that exitation current is being fed to the alty by the battery, (the alty needs a bit of a kick to start it generating as the rotor isnt permanently magnetic, the light shows this).
It comes on when the ignition is on and the output of the alty is less than the battery, and will be on if the rpm of the alty isnt high enough for it to start generating, if its on while driving first thing to check is the fan belt tension, then the battery/alty terminals, and in particular the solenoid terminals then check the battery with a hydrometer, if none of that shows anything up then you could have dodgy wiring or alty (usually the regulator pack).
Some good car accessory shops have a machine that will comprehensively test your alty, worth a tenner to know if its ok or not.
Without the battery charging you'll get about 20 miles with a good battery, maybe a bit more, with the lights radio and heater on you'll only last a few miles.