Heating of nonmagnetic electric conductors by magnetic induction — Longitudinal flux

HEAT can be generated in a piece of metal or other conducting material by wrapping a coil around the piece, such as the cylinder shown in Figure 1, and causing a current of suitable magnitude and frequency to flow into the coil. This is called “induction heating.” The great advantage of this type of heating is that the heating can be accomplished without contacting the piece to be heated. This allows the heating to take place while the piece is moving or enclosed in a protective atmosphere. Also, the heat usually can be concentrated just where it is needed, and a piece usually can be heated faster by induction than by any other method. The desirable frequency to use for induction heating depends on the size and the electrical properties of the piece to be heated and may vary from 25 or 60 cycles per second for heating joints in iron pipes to a few million cycles per second for heating very small nonmagnetic pieces for soldering. Frequencies up to 10,000 cycles per second are obtained usually from rotating machines; for higher frequencies one must resort to spark or vacuum-tube oscillators.