We study a modified Bullard dynamo and show that this system is equivalent to
a nonlinear oscillator subject to a multiplicative noise. The stability
analysis of this oscillator is performed. Two bifurcations are identified,
first towards an `` intermittent\rq\rq state where the absorbing (non-dynamo)
state is no more stable but the most probable value of the amplitude of the
oscillator is still zero and secondly towards a `` turbulent\rq\rq (dynamo)
state where it is possible to define unambiguously a (non-zero) most probable
value around which the amplitude of the oscillator fluctuates. The bifurcation
diagram of this system exhibits three regions which are analytically
characterized
