External flash with the Rebel XT

[MustISO]

The flash works fine on a 35MM EOS 650. The Rebel XT doesn't open its flash so I assume it can sense the external flash. For whatever reason the flash doesn't fire.
Do I need to do anything to get an external flash to work?
Is there a list of compatible flashes?

 

[virtuamike]

Try manual?

 

[MustISO]

Yep, the manual has 3 lines on using a non-Canon flash. Mostly they're just warnings not to use a high-voltage flash.