From Eric Roberts
Reading about micro-engines reminded me of an old idea for conversion of direct heat to rotary motion which was apparently too inefficient for use at the macro scale, but should be more efficient at the micro scale (19 June, p 26).
The principle is best illustrated by a simple example: consider a disc of soft magnetic material which is free to rotate between the poles of a magnet whose field is applied across its diameter. If a small area of the disc at its circumference, not quite in line with the magnetic poles, is heated above its Curie point, then the disc will rotate. This is because of the way the field lines are diverted in the heated area, which cools as it rotates away, while a new area is heated. Obviously, the magnetic material will heat and cool much quicker if the disc is very thin, which should make the motor faster and more efficient at the micro scale.
This motor deserves serious investigation, not least because of its simplicity.
Bristol, UK
