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Published online by Cambridge University Press: 26 February 2010
Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable; the other is a cheaper thing, but the moths get in.
Stephen LeacockA high-frequency induction coil can be used to heat, levitate and stir liquid metal. This has given rise to a number of metallurgical processes, some old (such as induction furnaces) and some new. In this chapter, we shall discuss five.
(i) Induction furnaces. These have remained virtually unchanged for the best part of a century, yet we are still unable to calculate reliably the stirring velocity within a furnace!
(ii) Cold crucible melting. This is an ingenious process which combines the functions of an induction melter and a continuous caster, all in one device.
(iii) Levitation melting. This is now routinely used in the laboratory to melt small specimens of highly reactive metals. Unfortunately, if the levitated drop becomes too large, it tends to drip.
(iv) The electromagnetic valve. This provides a non-contact means of modulating and shaping a liquid-metal jet. It is a sort of levitation melter in which the metal is allowed to leak out of the bottom.
(v) Electromagnetic casting. Some aluminium producers have replaced the casting mould in a continuous caster by a high-frequency induction coil. Thus, the melt pool is supported by magnetic pressure rather than by mechanical means. It is extraordinary that large ingots, which may be a metre wide and ten metres long, can be formed by pouring the liquid metal into free space and soaking it with water jets!
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