<p>A vortex amplifier (1) having a vortex chamber is included in a fluid flow line (2). A sensor (4) in the fluid flow line (2) upstream of the vortex chamber and operable in response to changes in the fluid flow controls a valve (5) in a further flow line (3) for introducing a control fluid into the vortex chamber. The arrangement results in an automatic control in the fluid flow line (2).</p>
