(B) It is possible to light the gas by the above process. Have a friend turn on the gas just before you bring your finger to the jet, and be sure that the spark from your finger passes through the gas on its way to the conductor, the jet. (C) Bring your finger quickly near a small piece of tissue paper after you have scraped your feet along to charge your body. _=216. Static Electric Machines=_ are used to produce large quantities of static electricity. In the early forms, the electrification was produced by friction. Modern machines depend upon the principle of induction. The electrophorus (Exp. 68) is really a very simple form of induction machine. The potential of these machines is very great, as the spark may jump many inches. Thousands of Galvanic cells would be needed to make a spark an inch long. When the spark passes through the air it meets with an extremely high resistance, as air practically insulates ordinary electricity. This high resistance in the circuit reduces the strength of the current. While the potential is very high, the strength of the current is very low. (See Ohm's Law.) CHAPTER XIII. ATMOSPHERIC ELECTRICITY. _=217. Atmospheric Electricity.=_ The air is generally electrified, even in clear weather. Its charge is usually +. Clouds are sometimes +, and sometimes -. The cause of atmospheric electricity is not thoroughly understood. It is thought, by some, to be due to the friction of the particles of vapor upon each other. It is also thought that the evaporation of sea water, and the friction of winds, produce it. _=218. Lightning.=_ Benjamin Franklin, in 1752, proved by his famous kite experiment that atmospheric and frictional electricities were of the same nature. By means of a kite, the string being wet by the rain, he succeeded, during a thunder-storm, in drawing sparks, charging condensers, etc. Lightning may be produced by the passage of electricity between clouds, or between the cloud and the earth, which, with the intervening air, have the effect of a condenser. If one cloud is charged, it acts inductively upon another, producing in it the opposite kind of electrification. When the attraction between the two electrifications becomes great enough, it overcomes the resistance of the air, and lightning is produced. The flash is practically instantaneous. The direction taken seems to depend upon the conditions of the surrounding air. It has generally a zigzag motion, and is then called _chain lightning_. The air in the path of the electricity becomes intensely heated; it is this effect, and not the electricity which we see. In hot weather flashes are often seen which light up whole clouds, no thunder being heard. This is called _heat lightning_, and is generally considered to be due to distant discharges, the light of which is reflected by the clouds.