84 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



when it is subjected -to mechanical shocks or to changes of tem- 

 perature. After the residual magnetism has been reduced in this 

 way, a remainder is left which decreases but little with repeated 

 mechanical shocks and changes of temperature, and the magnet 

 is said to be aged. Permanent magnets for use in electrical 

 measuring instruments are always subjected to an aging process 

 which consists, usually, in placing the magnet repeatedly in hot 

 and then in cold water, and in subjecting it to a series of slight 

 mechanical shocks. 



Demagnetization. When iron is heated to bright redness it 

 loses its magnetic properties. Thus, red hot iron is not attracted 

 by a magnet. When a magnetized bar of steel is heated to 

 bright redness its magnetization disappears and the bar, upon 

 cooling, is found to be completely demagnetized. 



Any piece of iron or steel may be completely demagnetized by 

 the following operation : Place the piece of iron or steel in a coil 

 of wire through which a strong electric current is flowing. Re- 

 verse the current repeatedly and at the same time slowly reduce 

 its value to zero. This operation is called demagnetization by 

 reversals. A watch which has been disturbed by a strong mag- 

 netic field is usually demagnetized by this process. 



47. Intensity of magnetization. Magnetic saturation. Let m 

 be the strength of the magnetic pole at the end of an iron rod of 

 which the sectional area is s square centimeters. The ratio mjs 

 is called the intensity of magnetization of the rod. When an 

 iron rod is subjected to a stronger and stronger magnetizing field, 

 its magnetization becomes more and more intense and approaches 

 a definite limiting value beyond which it cannot be magnetized 

 however strong the magnetizing field may be. The iron rod is 

 said to approach magnetic saturation as it approaches this limit- 

 ing intensity of magnetization. The limiting value of mis is 

 about 1,730 units of pole per square centimeter of section for 

 wrought iron, about 1,600 for mild steel, about 1,310 for cobalt, 

 and about 540 for nickel. Permanent magnets of hardened steel 



