526 MAGNETISM AND ELECTRICITY rents are induced in the body; but as the frequency is decreased the amount of current which the body can stand hkewise decreases. Strong electric shocks may cause death; but, on the other hand, electric shocks are now being used to shock victims of anaesthesia back and to bring patients back to sanity from the living death of to life mental disease. STUDY QUESTIONS 1. Why are fuses used in all household lighting circuits? 2. What causes a fuse to blow out? 3. Explain the principle of arc lamps. 4. Why does a penny not serve as a good substitute for a fuse in a household lighting circuit? 5. Differentiate between science and invention. 6. W'hy are gases used in some filament lamps? 7. Explain the action of the photoflash lamp. 8. Explain the so-called "neon" lamps. 9. What is the thermoelectric effect? 10. How has the development of electric lighting affected civilization? 11. Our electricity bills are rated in terms of kilowatt-hours. What measurement is expressed in kilowatt-hours? 12. State Joule's law. 13. What is the relation between the resistance of a wire and its diameter? 14. What current will flow in a lamp whose resistance is 200 ohms when in a 110-volt circuit?
UNIT VII SECTION 7 THE GENERATION AND RECEPTION OF ELECTRO- MAGNETIC WAVES ARE THE FUNDAMENTAL PROBLEMS OF THE RADIO The real credit for everything that has been done in the field of wireless belongs, as far as such fundamental credit can be definitely assigned to anyone, to Professor Clerk Maxwell, who in 1864 carried out certain abstruse and remote calculations in the field of magnetism and electricity. — Abraham Flexner. Introduction. The majority of the great scientific developments of today represent the contributions of research men working in many different laboratories throughout the world. Each man adds his bit to the final result, but few discoveries are outstanding in themselves. There are usually a few pioneer discoveries of fundamentals upon which all future work is based. In this Section we shall study the basic principles of the transmission and reception of electrical energy in the form of electromagnetic waves, upon which the modern radio is founded. Maxwell Originated the Electromagnetic Theory of Light. In 1864 Clerk Maxwell, the great English mathematical physicist, reached the conclusion from his calculations that all forms of radiant energy are nothing more or less than electromagnetic waves. The "lines of force" studied in magnetism and in current electricity are thought to exist for every electron. These "lines of force" represent a convenient fiction to express the "field of influence." When the electrons are in a regular position, the field of influence becomes evident, as in the case of magnets, because of their ability to repel or attract bodies surrounded by similar fields of influence. When electrons are in no regular position with relation to each other, as is true in nonmagnetized matter, fields of influence surrounding the electron neutralize, rather than reinforce, each other. When electrons move in one direction, as in a current of electricity, there is a motion of these fields of influence. Now, what would happen to the field of influence if the electrons were to oscillate back and forth? According to Max- 527