66 EXPLORATION GEOPHYSICS 



tivity, the effect of temperature on magnetic strength, and resistance to 

 corrosion or oxidation. 



In general, the Brinell hardness of a steel is somewhat proportional to 

 its magnetic strength for a given volume. Therefore the use of hard steel 

 would permit smaller magnetic systems to be made, which would be quite 

 advantageous. However in building a magnetic system, the magnetic parts, 

 as blades or in the form of a needle, for example, must be accurately shaped 

 and machined to close tolerances. This precludes the use of extremely 

 hard or brittle material. Also, resistance to corrosion in magnetic parts 

 is of importance. Magnetic instruments are often used in tropical climates 

 where moisture and fungus growths are extremely damaging to delicate 

 equipment. The steel which shows the least loss of magnetic strength with 

 increase in temperature (lowest magnetic coefificient of temperature) and 

 the least loss of magnetization with time is the most desirable. 



No artificial magnet, although it may be classed as permanent, will 

 retain all the magnetism that can be initially induced in it. When a piece 

 of steel is removed from its magnetizing field, a considerable portion of its 

 magnetization is lost immediately. After that, the loss in magnetism is pro- 

 gressively slower, and a fairly long time, up to several months or more, is 

 required for the magnet to acquire final magnetic stability. 



The time of stabilization of a magnet can be shortened considerably by 

 a series of treatments known as aging. This consists of subjecting a 

 magnet, when in its final form (as a system, or as a calibrating magnet, 

 for instance) to mild physical shock, vibration, and a number of cycles of 

 temperature change. The magnetic strength of a properly aged magnet 

 becomes very nearly constant. Although loss of magnetic strength with 

 time cannot be completely halted, a well-aged magnet or magnetic system 

 will not alter significantly under careful field use. 



Properties of Magnets 



The most simple type of magnet consists of a straight bar, and is known 

 as a bar magnet. A consideration of magnets of this geometrical form 

 conveniently sets up a quantitative basis for the analysis of the properties 

 of magnets, magnetic materials and magnetic fields. 



Magnetic Poles. — It has been found that there are small regions near 

 the ends of a bar magnet where its magnetic properties seem to be con- 

 centrated. (See points N and S, Figures 6 and 9.) These localities are 

 called poles, and they are situated at a distance of about 1/12 of the length 

 of the bar from its ends. Their existence and approximate position can 

 be demonstrated by the familiar experiment of dipping a bar magnet into 

 a box of iron filings. A great number of the iron particles will attach them- 

 selves to the bar magnet at and near the poles, and there will be practically 

 no attraction near the middle of the bar, as is illustrated in Figure 6. 



