. 508 MAGNETISM AND ELECTRICITY 7. What is tlu" principle of galvanometers and other electrical measuring instruments? 8. Should scientists be concerned about the practical importance of their researches? 9. What contributions did Lord Kelvin make to the field of electricity? 10. If you face the direction of flow of an electric current through a wire, will the direction of the magnetic lines of force be clockwise or counterclockwise? 1 1 Which hand should you place around a wire so that the finger will point in the direction of the magnetic field when the thumb points in the direction of electron flow? 12. Diagram the magnetic field surrounding a trolley wire. Have you ever observed the eff^ect of such a magnetic field on an automobile radio? Do power lines show similar magnetic fields? Why do telephone lines have much weaker magnetic fields? 13. Explain the action of the electric doorbell. 14. Who discovered the principle of electromagnetism? 15. Who invented the telegraph based on the use of electromagnets? 16. Describe a telegraph system based on the use of a compass instead of an electromagnet. 17. Upon what three factors does the strength of an electromagnet depend?
UNIT VII SECTION 5 ELECTROMAGNETIC INDUCTION MADE POSSIBLE THE PRACTICAL GENERATION, TRANSMISSION, AND UTILI- ZATION OF CURRENT ELECTRICITY The philosopher should be a man willing to listen to every suggestion, but determined to judge for himself. He should not be biased by appearance; have no favorite hypothesis; be of no school; and in doctrine have no master. He should not be a respecter of persons but of things. Truth should be his primary' object. If to these qualities be added industry, he may indeed hope to walk within the veil of the temple of nature. — Faraday. Introduction. Michael Faraday (1791-1867) was the first man to visualize and name the magnetic Hues of force. His studies led him to wonder why magnetism should not produce electricity, inasmuch as electricity Fig. 245. A diagram of Faraday's discovery. produces magnetism. The induction of electrical charges by other static charges of electricity suggested the possibility of inducing a current of electricity by use of the current from chemical cells. Faraday wound two coils of insulated wire on the same iron ring and passed a steady current through one coil, while he observed that no deflection was produced in a galvanometer attached to the other coil. He observed, however, that a slight deflection of the galvanometer occurred when the current was started or stopped. This observation was 509