Aviation Electronics Technician 1 - Historic Naval Ships Association

are known as angles of dip (fig. 4-20). At any givenpoint between the equator and the magnetic poles, therelationship of the angle between the earth’s surfaceand the magnetic lines of force is between 0° and 90°.This angle is determined by drawing an imaginaryline tangent to the earth’s surface and to the line offorce where it enters the earth’s surface. The anglethus formed is called the DIP ANGLE.If the same lines are traced only a short distance,300 feet for instance, their natural changes invariation and dip over such a short distance(short-trace) are almost impossible to measure.However, short-trace variation and dip in the area of alarge mass of ferrous material, though still extremelyminute, are measurable with a sensitive anomalydetector. This is shown in figure 4-21. The dashedlines represent lines of force in the earth’s magneticfield.Figure 4-20.-Dip angles.View A shows the angular direction at whichnatural lines of magnetic force enter and leave thesurface of the earth. Note that the angles of dip areconsiderably steeper in extreme northern andsouthern latitudes than they are near the equator.View B represents an area of undisturbed naturalFigure 4-21.-Simplified comparison of natural field density and submarine anomaly.4-16