In an experiment two bars (one levered and one stationary) are about 0.5 cm away

Question

In an experiment two bars (one levered and one stationary) are about 0.5 cm away from each other when no mass is placed on the top bar. When mass is placed on the top bar it moves down toward the stationary bar because of gravity. A current is the allowed to go through each bar in opposite direction thus causing a repulsive force between the bars. By adding mass to the top bar, the bars were returned to their original position of a 0.5 cm separation (b/c gravitational force cancels out magnetic repulsion). We had to perform this experiment with twice with the opposing currents switched in direction. Using the data we had to calculate the magnetic field constant for both the reverse and forward opposing currents. I need to know why averaging the two magnetic field constants derived from the forward current and reverse current helped compensate for the Earth's magnetic field!!!! Thanks

Explanation / Answer

*** Magnetic detectors such as magnetometers are used to measure magnetic fields or changes in magnetic fields. In the detection of a magnetic object, the magnetic signal of interest may be relatively small compared to the ambient field of the earth and accordingly to obtain higher sensitivity many magnetometers are of the type which provide an output signal indicative of only a change in the magnetic field at its location as opposed to the absolute value of it. To accomplish this, the relatively steady earth's magnetic field must be subtracted from the signal containing the magnetic field change of interest. *** In one type of compensation system in which the earth's magnetic field is subtracted from the measurement, a fixed voltage representing the earth's magnetic field is subtracted from the magnetometer output so that only changes to the field are observed. *** In another widely used compensation arrangement, the magnetometer is placed within a solenoid which is connected to a current source. The current through the solenoid is adjusted until it induces a field equal and opposite to the earth's magnetic field such that the magnetometer effectively sees a zero field. The solenoid field is then held at this constant value such that the output signal of the magnetometer is indicative of only change in magnetic field due to the magnetic disturbance. *** In the methods mentioned above, the currents must flow in opposite directions to accomplish the required task.

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