Electrodynamics (1936)

our modern Industrial Age and many of the economic and social problems that accompany it have been due in part to man's discovery of how to set into motion vast numbers of electric charges and to distribute the electric current so generated far and wide like many other fund discoveries the discovery of electric current was a chance observation by a professor of anatomy Luigi galvani in the 18th century Galvan observed a mysterious twitching of a frog's leg that had been hung from a brass hook on an iron fence this was due to the passage of electric charges produced by the contact of the leg with the iron these materials actually constitute a battery in the brass and iron were two dissimilar metals and they were connected by a moist conductor through which the electric current could pass what do we mean by electric current let us make the hypothesis that an electric current consists merely of the motion of electric charges first we should investigate some of the properties of a current if we send an electric current upward through this Rod we discover that the influence of the current on these north magnetic poles causes the magnet to move clockwise around the rod on Breaking the current these forces acting on the magnet disappear on reversing the current the motion of the magnets is also reversed iron filings about a current carrying wire become magnetized and arrange themselves in circles thus tracing out the motion of the magnets previously seen regions where such forces act on magnets are magnetic fields we may imagine that so-called lines of magnetic force expand outward from the wire as the charges begin to move as we increase the strength of the current these lines increase in number and spread out when the current becomes constant the magnetic field is stationary when we shut off the current the accompan in magnetic field collapses back into the conducting wire suppose we wind the current carrying wire into a coil the magnetic forces of each Loop reinforce one another inside the coil and consequently the coil behaves like a magnet it possesses a north magnetic pole also a South magnetic pole if if we increase the number of charges flowing through the wire the magnetic field is intensified as we increase the number of turns in the coil the intensity of the magnetic field is increased I insert into a coil a bar of soft iron note that it does not attract these filings but a current sent through the coil magnetizes the iron this device is an electromagnet the small electromagnets commonly used in doorbells or telephones and the huge devices such as this illustrate the wide use of electromagnets in Industry electric currents invariably reveal their presence by magnetic fields since this is true let us now make a crucial test of our hypothesis that charges in motion constitute current we place negative charges on the metallic plates along the edge of this glass disc now as it whirls the needle is deflected by the magnetic field caused by the motion of these charges thus our hypothesis is confirmed since we have now proved that charges in motion create a magnetic field is it not fair to inquire how the magnetism of a piece of iron can be explained in terms of moving charges we will assume that the magnetism of the iron itself is due to the motion of electric charges within the atoms of the iron the magnetic field created by the coil orients these atoms so that their tiny magnetic fields are all lined up in the same direction this iron wire is attracted to a magnet when its temperature is sufficiently high the agitation of the atoms in The Wire becomes too great for them to maintain alignment the iron ceases to possess any marked magnetic property as the wire cools the magnetic property is recovered knowing that electric currents create a magnetic field and move magnets Faraday first asked the question can we not by moving magnets create electric fields and thus move charges thereby producing currents I insert a bar magnet into this coil which is connected to a galvanometer a current detecting instrument the instrument indicates electric current only when the magnet is moving as the lines of the magnetic field sweep across the loops they exert a force on the negative charges within the wire and cause these charges to circulate when the magnet is stationary these lines are at rest and no charges circulate in the coil as the magnet is withdrawn the charges move in the opposite direction Modern Electric Generators operate on this same principle fixed electromagnets create the magnetic field the coil is made to rotate the current produced in the coil changes Direction every half Revolution and if picked off as shown the Lan line will have an alternating current by a different pickoff system it is possible to deliver a current flowing in One Direction only we can induce current without mechanically moving Parts by utilizing the magnetic field of a current circulating in this smaller coil which we will call the primary to create a current in an adjacent secondary coil when the switch is closed a momentary current registers in the secondary coil when the switch is opened a current in the reverse direction is indicated as the magnetic lines of force originating in this primary coil at the left sweep across the loops of the secondary coil they cause its negative electricity to circulate while the current in the primary coil is held constant the line of force are stationary and have no effect but when we break the current in the primary coil the magnetic lines of force collapse and sweep back across the loops in the secondary coil producing a current in the opposite direction the amount of current produced in the secondary coil depends upon the total number of magnetic lines cutting its Loops in a given time by starting and stopping the primary current more quickly the same number of lines will cut the loops of the secondary coil in less time consequently the current is increased furthermore when we increase the number of Loops in the secondary coil each line of magnetic force cuts across a greater number of loops and thus causes more charges to circulate in addition the insertion of an iron core produces a greater number of magnetic lines in the primary these sweeping the secondary turns at the same rate as before cause a greater secondary current thus bringing us the picture of the modern industrial Transformer since the alternating currents from generators automatically reverse their Direction the magnetic field builds up collapses and reverses in the primary thus an alternating current is produced in the secondary the voltages in the two coils vary directly as the number of their Loops thus high voltages that are economical to transmit are reduced to low voltages that are safer and more useful in factories and homes we have seen that the story of electric current is marked by a number of epical Events first galvan's accidental discovery of electric current followed by Ed's discovery of a magnetic field about a current carrying wire then Faraday suggesting that there should be a complimentary effect was led to the discovery of electromagnetic induction it was knowledge of these fundamental principles of electrodynamics that has made possible the production and distribution of the enormous quantities of electric current that we employ in this Modern Age of electricity