488 MAGNETISM AND ELECTRICITY of the conductors. If the conductors are now separated without moving the rod, they will be found to have opposite charges. Charges are distributed uniformly only if the surface is smooth and flat or uniformly curved; on other surfaces the charges collect at the more pointed portions. Electrons concentrate on sharp points and cause the air to become ionized; that is, molecules of air become charged and thus conduct the electrons from the point. Sometimes this discharge at the tips of lightning rods, flag poles, or masts of ships produces a faint violent glow, called "St. Elmo's Fire," that can be seen at night. Similar discharges along highvoltage electrical transmission lines are called "corona" discharges. The ions repelled from the charged body create a wind which is capable of blowing a candle Fig. 231. The electric whirl. flame out of shape. The "electric whirl," which is based on the principle of the Fourth of July pinwheel, or rotary lawn-sprinkler, is also produced by a similar repulsion of ions from a charged body. In operating the Van de GraafT electrostatic generators, to be described later, the operators are housed within the charged spheres for protection against possible harm from the tremendous voltages produced, for charges cannot remain on the inner surface of the cage. Lightning Is a Static Phenomenon. As previously mentioned, lightning is the discharge between a cloud and the earth. The source of the charges is not thoroughly understood, although it is generally believed that it is produced by the intense friction and splitting of large raindrops brought about by the rapidly rising currents of air so characteristic of thunderstorms. The rising currents of air may possibly produce differences in charges due to the fact that the lower atmosphere is positively charged relative to the upper atmosphere. Very possibly the charge is also produced by the rapid condensation of moisture in such storms, the charges being concentrated on smaller total areas and thus producing higher potentials. Negative charges concentrate on the bottoms of clouds, leaving the tops positively charged. Discharges may take place between the top and bottom surfaces of adjoining clouds. A charged cloud may induce an opposite charge on the nearest portion of the earth. This induced charge will concentrate on trees, flag
STATIC ELECTRICITY 489 poles, or buildings which rise above the surface of the ground and thus act as pointed surfaces. Eventually this charge may become so great that the resistance of the air is overcome and a lightning flash occurs. To protect an object against lightning, metal rods with sharp points are used to discharge the induced charge. Such lightning rods must be well grounded in the earth, and it is generally recommended that they be connected with some object such as a copper boiler buried in the deeper damp earth. Static charges which cause sheets of Fig. 232. Three million volts of laboratory lightning striking a car in the Westinghouse high voltage laboratory. The bolt can be seen jumping over the left front wheel to reach the ground. (Courtesy of the Westinghouse Electric and Manufacturing Company.) Fig. 233. Lightning striking the Empire State Building, New York City. Although this building has been struck by lightning many times, no harm was done to the building because its steel framework acts as a Faraday cage. (Courtesy of the General Electric Company.) discharged by draping Christmas-tree tinsel over the machine. paper to stick together while using a mimeograph machine may be Gasoline trucks drag chains on the ground to prevent the accumulation of static electricity that might produce a spark and ignite the gasoline. The United States Census reports show that nearly two thousand people are killed or injured by lightning each year. When one is caught out of doors in a thunderstorm, he will be safe under a well-grounded iron bridge. In a violent storm he should sit down or lie down in a hollow spot away from trees or fences. In woods, seek shelter under the smaller trees. Do not go in swimming during a lightning storm.