If it was being hooked up to a power source with a constant voltage, that sounds correct.

Dynamos, on the other hand, are relatively complex. They can be modeled as a voltage source that is proportional to the dynamo's rotational speed. This voltage source is in series with a large inductance (which provides an impedance that is also proportional to the speed) and a fixed resistance.

Combine this with the possible use of a switching power supply in the light, which in the case of a buck converter, effectively has a negative resistance (i.e. when the voltage goes down, the converter will try to draw more current).

Figuring out how these all interact requires some circuit analysis. Calculations were much simpler back in the days of incandescent bulbs... but they were so incredibly dim by comparison!

Steve in Peoria