=212. Inductive Effect of the Earth's Magnetic Field.=--The earth's magnetic lines of force are to be considered as filling the space above the earth, passing through all objects on the surface and into and through the earth's interior. The _direction_ of the earth's field is shown by the compass and the dipping needle. Magnetic lines of force tend to crowd into and follow iron and steel objects on account of their permeability. Therefore, iron or steel objects, such as posts, columns, etc., are permeated by the earth's lines of force, which in the United States enter at the top of these objects and leave at the bottom. The lines of force passing through these bodies arrange their molecules in line or magnetize the bodies. The _inductive effect_ of the earth's magnetism indicates how lodestones or natural magnets acquire their magnetized condition. So far as is known, magnetism produces no effect upon the human body. It can therefore be studied only by observing its effects upon magnets or bodies affected by it. Important Topics The earth's magnetic field, dip, declination, agonic line, induction by the earth's field. Exercises 1. How would a dipping needle be of assistance in locating the magnetic poles of the earth? 2. Will a dipping needle weigh more before or after it is magnetized? Explain. 3. It is said that _induction precedes attraction_. Using this idea, explain how a magnet attracts a piece of soft iron. 4. Devise an experiment to show that a piece of iron attracts a magnet just as a magnet attracts a piece of iron. 5. Give two methods for determining the poles of a magnet. 6. State three of the most important points in the theory of magnetism. What evidence supports each? 7. Why is a permanent magnet injured when it is dropped? 8. Name two important uses of the earth's magnetic field. 9. What magnetic pole would you find at the top of an iron post that has stood for some time in the ground? What pole at the bottom? How would you test this? CHAPTER X STATIC ELECTRICITY (1) ELECTRIFICATION AND ELECTRICAL CHARGES