Ω
i
Ω
i
1. Geometrical condition : dynamo feedback
2. Energetic condition
: induction > ohmic dissipation
Instability
conditions
Example of BullardÕs homopolar dynamo
j
B
moving
conductor
(velocity u)
Lorentz
+
Induction
electrical
currents
magnetic
field
threshold
:
:Magnetic
Reynolds number