  We consider the classical problem of kinematic dynamo action in simple steady
flows. Due to the adjointness of the induction operator, we show that the
growth rate of the dynamo will be exactly the same for two types of magnetic
boundary conditions: the magnetic field can be normal (infinite magnetic
permeability, also called pseudo-vacuum) or tangent (perfect electrical
conductor) to the boundaries of the domain. These boundary conditions
correspond to well-defined physical limits often used in numerical models and
relevant to laboratory experiments. The only constraint is for the velocity
field u to be reversible, meaning there exists a transformation changing u into
-u. We illustrate this surprising property using S2T2 type of flows in
spherical geometry inspired by Dudley and James (1989). Using both types of
boundary conditions, it is shown that the growth rates of the dynamos are
identical, although the corresponding magnetic eigenmodes are drastically
different.
