‰  NOW 40 WPM  ‰  TEXT IS FROM NOVEMBER 2012 QST  PAGE 70‰
PAST THE COIL.  THERE ARE A NUMBER OF GREAT YOUTUBE VIDEOS THAT DEMONSTRATE
LENZS LAW IN WHICH THE INDUCED CURRENT CREATES AN OPPOSING MAGNETIC FIELD.
YOU MAY HAVE SEEN SOMEONE DROP A MAGNET INTO A COPPER PIPE THROUGH WHICH IT
THEN FALLS VERY SLOWLY.  THE INDUCED CURRENTS ARE CALLED EDDY CURRENTS AND
THEY SET UP A MAGNETIC FIELD THAT IS ALMOST, BUT NOT QUITE, STRONG ENOUGH
TO STOP THE MAGNET.  IF THE MAGNET DID STOP, THERE WOULD BE NO EDDY CURRENT
AND OPPOSING FIELD SO, AS A RESULT, THE 

FIELD LINKING THE TWO COILS TO CHANGE.  THE MOST OBVIOUS WAY IS TO SIMPLY
TURN THE CURRENT GENERATING THE FIELD ON AND OFF WITH A SWITCH.  ANOTHER
WAY IS TO MOVE ONE OF THE COILS SO IT ENCOUNTERS MORE OR LESS OF THE
MAGNETIC FIELD.  THIS IS THE PRINCIPAL BEHIND AN ELECTRICAL AC GENERATOR.
AN ARMATURE COIL IS ROTATED INSIDE A CURRENT CARRYING STATIONARY COIL OR
STATOR CAUSING THE ARMATURE VOLTAGE TO VARY AS A SINE WAVE.  SIMILARLY,
CHANGING THE CURRENT IN THE STATOR CAUSES THE ARMATURE TO TURN, CREATING A
MOTOR.  IN THIS MONTHS COLUMN WE WILL FOLLOW IN THE STEPS OF PIONEERS ANDRI
MARIE AMPHRE, HANS CHRISTIAN ORSTED AND FARADAY BY PERFORMING SOME SIMPLE
EXPERIMENTS DEMONSTRATING THE VARIOUS EFFECTS THEY DESCRIBED.  IS THIS
PURELY A HISTORICAL EXERCISE?  NOT AT ALL.  THESE PHENOMENA ARE AT THE
HEART OF RADIO WITHOUT THEM WE WOULD BE WIRELESS LESS.  EXPERIMENT 1
ORSTEDS OBSERVATION DURING A LECTURE, WITH EXPERIMENTAL APPARATUS SCATTERED
ACROSS A TABLE, ORSTED NOTICED THAT A COMPASS NEEDLE DEFLECTED AWAY FROM
NORTH WHEN HE SWITCHED CURRENT ON AND OFF IN A NEARBY CIRCUIT.  THIS WAS
THE FIRST OBSERVATION LINKING ELECTRICITY AND MAGNETISM AND IT WAS THE

HANDS ON RADIO H. WARD SILVER, NXAX, N0AX ARRL. ORG  EXPERIMENT 118 THE
LAWS AT WORK LAST  MONTHS COLUMN ENDED IN THE YEAR 1831 AS MICHAEL FARADAY
DISCOVERED ELECTROMAGNETIC INDUCTION.  IN FACT, THE DAY ON WHICH I FINISHED
THIS COLUMN WAS THE 181ST ANNIVERSARY OF THAT DISCOVERY AUGUST 29TH.  AS A
REVIEW, FARADAY SHOWED HOW CHANGING CURRENTS IN ONE CIRCUIT COULD INDUCE
SIMILARLY CHANGING CURRENTS IN ANOTHER CIRCUIT WITHOUT ANY DIRECT
CONNECTION BETWEEN THEM.  THIS OCCURS BY FIRST CONVERTING THE FIRST
CIRCUITS ELECTRICAL ENERGY INTO MAGNETIC ENERGY IN THE SURROUNDING FIELD
AND THEN BACK INTO ELECTRICAL ENERGY IN THE SECOND CIRCUIT.
ELECTROMAGNETIC INDUCTION IS DESCRIBED BY FARADAYS LAW AS EXPLAINED IN LAST
MONTHS COLUMNR.  THE FASTER THE MAGNETIC FLUX CHANGES BECAUSE OF MOTION OR
CHANGES IN CURRENT, THE LARGER THE VOLTAGE THAT IS INDUCED IN THE OTHER
CIRCUIT.  THE MINUS SIGN IN FARADAYS LAW GIVES RISE TO LENZS LAW THE
ELECTROMOTIVE FORCE EMF INDUCED BY THE CHANGING MAGNETIC FIELD CAUSES
CURRENT TO FLOW IN THE DIRECTION THAT CREATES A MAGNETIC FIELD OPPOSING THE
ORIGINAL CHANGE IN MAGNETIC FLUX.  LENZS LAW DESCRIBES THE BACK EMF WE
OBSERVE IN MOTORS AND THE KICKBACK VOLTAGE IN A RELAY COIL WHEN IT IS
DEENERGIZED.  LAST MONTHS FIGURE 3 SHOWS THE RELATIVE DIRECTION OF THE
CURRENTS IN BOTH CIRCUITS.  THERE ARE SEVERAL WAYS TO CAUSE THE MAGNETIC
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ