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A new concept of solenoid actuator is described which enables its armature to achieve response times of faster than 1 ms irrespective of mass or force to be overcome. This is a short-stroke device and designs to date have been limited to 1 mm although larger strokes are possible. With conventional solenoids the larger and more powerful they are made the slower their response time. Hence only tiny solenoid actuators with very low forces have been possible when operating times below 1 ms were required. This is because its armature mass increases faster than the force generated in its working face area. In the helical solenoid actuator its construction is such that its armature mass increases directly in proportion to the force required for a given stroke, thus an increase in its size or power does not affect its response time provided that input power is increased in the same proportion to its size. This is achieved by using a two-start helix with its winding wound along the root of one thread and returning along the root of the adjacent thread, the armature consisting of a thin tube with an internal two-start mating thread.