Electromagnetic Deflection

Early experimenters with electricity and magnetism noticed that an electric current produces a magnetic field. When a magnetic compass is placed near a wire carrying a direct electric current, the compass doesn’t point toward magnetic north. The needle is displaced. The extent of the displacement depends on how close the compass is brought to the wire, and also on how much current the wire is carrying.

When this effect was first observed, scientists tried different arrangements to see how much the compass needle could be displaced, and how small a current could be detected. An attempt was made to obtain the greatest possible current-detecting sensitivity. Wrapping the wire in a coil around the compass resulted in a device that could indicate a tiny electric current (following Figure). This effect is known as galvanism, and the meter so devised was called a galvanometer. Once this device was made, the scientists saw that the extent of the needle displacement increased with increasing current. Then, the only challenge was to calibrate the galvanometer somehow, and to find a standard so a universal meter could be engineered.

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You can make your own galvanometer. Buy a cheap compass, about 2 feet of insulated bell wire, and a 6-volt lantern battery. Set it up as shown in above figure. Wrap the wire around the compass four or five times, and align the compass so that the needle points along the wire turns while the wire is disconnected from the battery. Connect one end of the wire to the negative (−) terminal of the battery. Touch the other end to the positive (+) terminal for a second or two, and watch the compass needle. Donft leave the wire connected to the battery for any length of time unless you want to drain the battery in a hurry.

You can buy a resistor and a potentiometer at a place like RadioShack, and set up an experiment that shows how galvanometers measure current. For a 6-V lantern battery, the fixed resistor should have a value of at least 330 Ω and should be rated for at least 1⁄ 4W. The potentiometer should have a maximum value of 10 kΩ. Connect the resistor and potentiometer in series between one end of the bell wire and one terminal of the battery, as shown in following figure. The center contact of the potentiometer should be short-circuited to one of the end contacts, and the resulting two terminals used in the circuit.

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A circuit for demonstrating how a galvanometer indicates relative current.

When you adjust the potentiometer, the compass needle should deflect more or less, depending on the current through the wire. Early experimenters calibrated their meters by referring to the degrees scale around the perimeter of the compass.