94 PHYSIOLOGICAL PHYSICS. [Chap. ix. 



a rhomb made of fine steel, long in proportion to its 

 breadth. Its north pole is usually coloured blue ; near 

 its centre is a little depression by which it can be 

 balanced on a point of support. 



Care of a magnet. Magnets should never be 

 left without their keeper, or they will lose strength. 

 The keepers should not be knocked off, but slowly 

 moved off by a turning movement from north to 

 south. Magnets should not be let fall, nor suddenly 

 struck, nor rubbed with sand-paper, as the magnetism 

 may by these means be greatly diminished. They 

 should be kept from rust by the use of fine sperm oil. 



Paramagnetic and diamagnetic. Bodies 

 that are attracted by either pole of a magnet are 

 called paramagnetic. Among them are iron, nickel, 

 cobalt, and platinum. When placed between the poles 

 of a horse-shoe magnet, they turn their long axis so 

 as to be in line with the poles. 



Bodies that are repelled by either pole of a magnet 

 are said to be diamagnetic. Among them are bismuth, 

 antimony, lead, tin, copper, gold, and silver. Water, 

 sugar, starch, alcohol, muscle, and blood, are also dia- 

 magnetic. When placed between the poles of a 

 magnet they tend to set their length across the poles. 



Action of electric currents on magnets. 

 In 1819 Oersted of Copenhagen showed that a needle 

 suspended in the magnetic meridian was influenced 

 by a current of electricity passed along a wire 

 parallel to it. The experiment is performed by 

 placing a wire above a magnetic needle, and parallel 

 to it ; and another wire below the needle, and parallel 

 to it. The poles of an element may then be attached 

 to the extremities of either wire, and a simple key 

 interposed in the circuit thus formed. On closing 

 the key the current passes along the wire. When 

 a sufficiently strong current traverses either of 

 the wires, the needle is deflected nearly to a right 



