This invention demonstrates an improved method of constructing an artificial lift device for deep wells, in which the deadload of the pump mechanism is supported by a continuously-controlled gas spring. The reciprocating motion is produced by a very unusual form of linear motor using a plurality of radially disposed conducting vanes and having no wire coils. The machine is highly efficient and is able to deduce many of the conditions obtaining at the pump by computations from the measured power demand at each point of the pumping cycle. The device is self-protecting and self-optimising.