16. _Circuit Breakers_ (Fig. 46).--Electro-magnets are used to open switches and thus protect dynamos and other machines against a larger electric current than they are able to carry. The switch is held closed by a spring which, by an adjusting device, may be tightened or loosened. A dynamo which we examined had its circuit breaker adjusted so that it would remain closed if any current under 1500 amperes passed, but if a greater current than that passed it would strengthen the magnet sufficiently to open the switch and thus break the circuit. 17. _Separating Iron from Ore._--In 1897 Edison first proposed to use an electro-magnet to separate iron from crushed earth. Fig. 47 represents the process. _E_ is an electro-magnet. _S_ is the stream of crushed ore containing iron. Gravity would cause all the material to fall into bin _A_, but the electro-magnet _E_ pulls that portion of the material which is magnetic to one side so that it falls into the bin _B_. [Illustration: Fig. 47] [Illustration: Fig. 48] 18. _Lifting Magnets._--Electro-magnets are made for use with hoisting apparatus to save the trouble of manipulating grappling hooks, etc. They may lift barrels and boxes of iron, the wood of the barrel or box being transparent, we say, to the magnetic influence. That is, the magnet will attract iron through the wood just as light will shine through glass. Such magnets are used to pick up from the bottom of the sea cases of hardware from wrecked ships. (See the accompanying illustration, Fig. 48.) In such cases the electric conductors which lead to and encircle the magnets must be well insulated from the water of the sea, otherwise the electric current would take the shorter path from one line wire through the sea water, which is a fairly good conductor, and back by the other line wire, rather than go the path of greater resistance around the magnet. Electro-magnets are coming into use in foundries, etc., for lifting heavy iron castings. [Illustration: Fig. 49]