492 MAGNETISM AND ELECTRICITY phenomena produced in the laboratory. FrankHn devised an ingenious method of causing an approaching thunderstorm to announce its presence so that he could try out further kite experiments. He hung a small metal ball by a silk thread between two small bells. An approaching storm induced opposite charges on the two bells. The ball obtained a charge from one bell and was then repelled to the other bell, which neutralized the charge and gave the ball an opposite charge which repelled the ball again. The repeated attraction and repulsion caused the ball to ring the bells as it swung back and forth. It was Franklin's suggestion that lightning rods with sharp points be used for the protection of buildings. Franklin also made the first bifocal glasses, suggested watertight compartments for ships, and invented the stove still known by his name. Difference in Potential Is Essentially the Amount of Work Required to Carry a Unit Charge from One Body to Another. Tremendous differences of potential may be built up between the earth and thunderstorm clouds because so much power is required to pass an electric current through such long distances in air. The unit of potential difTerence is the volt, often called the statvoU when referring to static electricity. A potential difference of about 30,000 volts is required for each centimeter of distance in order to cause an electric discharge through air. There Are Several Important Applications of Static Electricity. Two important practical applications of static electricity are the Cottrell Process, described on p. 726, in which charged smoke particles are attracted to oppositely charged plates in smoke stacks, and a similar device, the precipitron, which removes a very high percentage of dust and other suspended matter from the air. The precipitron first induces a charge on the particles and then precipitates the particles by attracting them to charged surfaces. In 1926 the Norton Company discovered that the cutting property of sandpaper could be improved by 20 to 50 per cent by causing the sand particles to stand upright by passing them through an electrostatic field before the adhesive is dried. This company cooperated with the Arnold Print Works in developing a similar method to produce simulated embroidered cloth by "printing" an adhesive on cloth and then causing short cut fibers to cling to the adhesive in an upright position by means of electrostatic charges.
STATIC ELECTRICITY 495 STUDY QUESTIONS 1. Explain the origin of the term electricity. 2. What change took place (a) on the hard rubber, and (b) on the glass rod when it was electrified? 3. Discuss the theory of the production of lightning. 4. Explain the action of the electrical whirl. 5. Why is a pith ball first attracted to a charged piece of wax and then repelled? 6. Explain the action of the electroscope. 7. Give a short description of the static machine. 8. Name five conductors and five insulators. 9. What is the fundamental difference between an insulator and a conductor? 10. What is the value of a lightning rod? 11. List the requirements of a good lightning rod. 12. Describe two methods of producing static charges. 13. Why is static electricity most noticeable on dr>' days? 14. Explain the charges on bodies in terms of the electron theory. 15. Describe a simple condenser. For what is a condenser used? 16. How may it be shown that there are two kinds of electrification? 17. How may a positive charge be produced? Explain in terms of the electron theory. 18. State the facts of static electricity and explain each fact. 19. State the laws of static electricity. 20. Describe Franklin's kite experiment. What did it prove? 21. Distinguish between magnetic and electric forces. 22. How may a body be charged permanently by induction? 23. Explain why the whole electric charge is on the surface of a conductor. 24. Suggest a method of charging an electroscope negatively. 25. Why do gasoline trucks have a chain drag along the ground? 26. What is meant by difference of potential? In what terms is it measured? 27. There are at least three ways in which lightning could injure a person standing under a large tree during a thunderstorm. Can you suggest what they are? 28. Can you suggest a reason why we should not go swimming during a thunderstorm?