Since the current through the LED is determined by the voltage across it, it seems that connecting the motor in parallel is reducing the voltage. The "constant potential source" isn't quite constant.
Mike W.
One of the reasons that the battery voltage is reduced is that all batteries have an 'internal restance'. For a 9 Volt battery drawing 1 Amp the actual terminal voltage will drop a half a Volt or so. This voltage drop will dim the LED.
LeeH
(published on 08/05/2013)
