. Farm machinery and farm motors. ts is deduced : Like poles repel and unlike poles force of this attraction is found to vary inverselyas the square of the distance, , increasing the distance 462 FARM MOTORS between the poles two times reduces the force actingbetween them 2X2^4 times. In other words, theforce is one-fourth as strong. 606. Magnetic materials.—Steel and iron are the onlycommon substances which show magnetic properties toany appreciable degree. STATIC ELECTRICITY 607. Static electricity.—If a hard rubber rod be rubbedwith flannel and then brought close to a suspen
. Farm machinery and farm motors. ts is deduced : Like poles repel and unlike poles force of this attraction is found to vary inverselyas the square of the distance, , increasing the distance 462 FARM MOTORS between the poles two times reduces the force actingbetween them 2X2^4 times. In other words, theforce is one-fourth as strong. 606. Magnetic materials.—Steel and iron are the onlycommon substances which show magnetic properties toany appreciable degree. STATIC ELECTRICITY 607. Static electricity.—If a hard rubber rod be rubbedwith flannel and then brought close to a suspended pithball the ball will jump toward the rod. By rubbing therod has been electrified and the action of the chargeis to attract the ball. This charge of electricity is notwithin the rod but is on the surface and is known asstationary or static electricity. Another example of thisis rubbing a glass rod with silk. 608. Laws of electrical attraction and repulsion.—If arubber and a glass rod be excited and suspended as shown I. Gloss FIG. 342 in Fig. 342 and brought close together it will be noticedthat they attract each other, but if two rubber rods besuspended in the same way and brought together, theywill repel each other. Hence the following law is ad-vanced : Electrical charges of a like kind repel each otherand those of an unlike kind attract. 6og. Density of charge varies with form of surface.— ELECTRICAL MACHINERY 463 Since all of the little particles of a charged substance,because of their mutual repulsion, tend to get as far awayfrom each other as possible, the density of a charge isvery much greater on the ends of an oblong body thanin the middle. If the ends be drawn to a point the chargewill become so intense that the point cannot hold it alland some of it will be given off to the air. 610. Lightning and lightning rod.—In 1752 Franklinwith his famous kite and key learned that there is elec-tricity in the clouds. He also showed that lightning isonly a huge el
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