Iron is rendered as soft as possible, by being heated to redness, and then gradually cooled.

Elec- tro-magnets have been made sufficiently powerful to keep sus- pended, a weight of at least 2,000 Ibs The keeper must bear some proportion to the size of the magnet, or it will not pro- duce a maximum effect.

15 The wire of the helix must be covered with a non-con- ducting substance, such as cotton, worsted, or silk ; otherwise the electricity will pass directly across from one part of the coil to another, and not along the wire.

16 ELECTRO-MAGNETISM, AS A MOVING POWER.

The very great force exerted by electro-magnets, soon suggested the idea of adopting electro-magnetism as a moving power.