UNIT VII SECTION 4 THE DISCOVERY OF ELECTROMAGNETISM LINKED MAGNETISM AND ELECTRICITY Behind all your practical applications, there is a region of intellectual action to which practical men have rarely contributed, but from which they draw all their supplies. Cut them ofif from this region, and they become eventually helpless. — John Tyndall. Introduction. Two discoveries of fundamental importance in putting electricity to work were made in the nineteenth century. The first discovery, made in 1820, that of H. C. Oersted, a Danish physicist, was that an electric current produces a magnetic field in its vicinity. The second discovery, in 1831, was made by Michael Faraday, who found that under certain conditions a magnetic field can be made to produce an electric current. These two discoveries laid the foundations for the harnessing of electrical energy. In this Section the general characteristics and applications of electromagnetism will be studied, reserving Faraday's discovery and its applications for the next Section. Oersted Discovered That an Electric Current Produces a Magnetic Field. Once the chemical cell had been invented, thus furnishing a convenient source of current electricity, investigators began to study the possible relationship between electricity and magnetism. Oersted had been trying to find out whether there was any effect when a wire carrying a current was held above a compass at right angles to it. No effect was noted. According to one story, he accidentally placed the wire parallel to the needle during the course of a lecture and was much surprised to see the needle turn aside. Oersted then tried placing the wire below the needle, and the needle was deflected in the opposite direction. A. M. Ampere, a French investigator, after whom the unit of electric current was named, worked out the principle of this relationship between the electric current and the magnetic field and suggested that 502
J ELECTROMAGNETISM 503 it could be used in communication. By sending an electric current through a wire, a compass needle can be deflected and can thus be made to transmit messages. The first electromagnetic telegraph was built in Gottingen in 1833 by Karl Friedrich Gauss and Wilhelm Weber. Another characteristic of Oersted's discovery was that no practical applications seemed likely at the time his work was published. Lord Kelvin said concerning this: Oersted would never have made his great discovery of the action of galvanic currents on magnets had he stopped in his researches to consider in which manner they could possibly be turned to practical account; and so we should not now be able to boast of the wonders done by the electric telegraphs. Indeed, no great law in Natural Philosophy has ever been discovered for its practical application, but the instances are innumerable of investigations apparently quite useless in this narrow sense of the word which have led to the most valuable results.^ At the same time, it should be pointed out that though Oersted was not seeking an immediately practical application, nevertheless he and his contemporaries knew that practical applications could be expected from the kind of knowledge he and others were gleaning. A Magnetic Field Encircles a Wire Which Is Carrying an Electric Current. It is now known that when an electric current flows through a straight wire a magnetic field is set up encircling the wire at all points. The direction of this field can be determined with a compass, which shows that the magnetic field is at right angles to the direction of flow of the current. When the direction of flow of the current is reversed, the direction of the magnetic field is reversed. The strength of the magnetic field surrounding a wire carrying a current is current being carried. The magnetic field proportional to the amount of surrounding a wire can ^ Direction of Electron Flow be nicely demonstrated by placing iron filings ^^" ' on a card through which a wire carrying a cur- . . . f °i''^c- . -T , 1 1 ri- tion oi Imes oi torce oi rent passes. Upon tappmg the card, the filmgs ^^e magnetic field enwill line themselves up in such a way as to circling a wire carrying show the direction of the lines of force. an electric current. ' Sir Richard Gregory, Discovery, the Spirit and Service of Science, The Macmillan Co., New York, 1929, p. 241. By permission.