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Old ideas can sometimes gain a new lease on life. Ideas that have once been tried and then discarded as impractical can be re-introduced when changed circumstances or new materials make it possible to do things that were impossible in earlier years. A good example of this re-application of earlier technology is the linear motor and the work of Eric Laithwaite (1921-1997), who applied these devices to modern transportation systems, including a possible future application in space. Originally, the idea was clearly described in patents from the 1840s, and at least one piece of a linear motor made by Charles Wheatstone survives from that time. Such machines could not have been exploited in the mid-19th Century. The necessary current was not available in the days when electrical engineers were dependent for their supplies on batteries or massive permanent magnet generators. Towards the end of the century, practical generators with wound fields became available, and these were capable of producing large currents, but at the same time, it was also found that the same machines could operate in reverse, as motors, and so it was not necessary to design electric motors as separate machines.