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...nt examples, in which the initial instability grows to a level where it interacts with the driving mechanism. In this paper we consider configurations with helical symmetry, as originally formulated by [1] and [2]. The Ponomarenko [3] kinematic dynamo can be regarded as a special case of helical flow. Here, a fully nonlinear laminar dynamo driven merely by a steady pressure gradient along a pipe is described. The pipe has a helical shape with a rectangular cross-section, as shown in figure 1. The work has relevance to the construction of laboratory dynamos [4]. Further details can be found in [6]. HELICALLY SYMMETRIC PIPE FLOW Helical symmetry is a natural generalisation of two-dimensionality (ε = 0) and axisymmetry (ε → ∞.) In terms of cylindrical polar coordinates (r, θ, z), a scalar function is helically symmetric if it depends only on r and φ = θ + εz, where ε is a constant. The symmetry direction is designated by the Beltrami vector field, H, which is related to the unit coordinate vectors eθ and ez by H = (ez − εreθ)/h 2 where h = (1 + ε2r2)1/2 and ∇∧H = − 2ε h2 H. The Navier-Stokes equations and the magnetic induction equation are invariant with respect to this symmetry, and th...