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Two identical looking iron bars \(A\)  and \(B\)  are given, one of which is definitely known to be magnetised. \((\) We do not know which one. \()\) How would one ascertain whether or not both are magnetised? If only one is magnetised, how does one ascertain which one? \([\) Use nothing else but the bars \(A\)  and \(B.]\)


When there is a repulsive force between the two bars, then both are magnetised. If it is always attractive, then one of them is not magnetised. The strength of the magnetic field is the strongest at the two ends \((\) poles \()\) and weakest at the middle region. This fact may be used to find out  whether \(A\)  or \(B\)  is the magnet. To see which one of the two bars is a magnet, take first \((\) bar-A \()\)  and lower one of its ends; first on one of the ends of the other \((\) bar-B \()\) , and then on the middle of \(B\) . If you observe that in the middle of \(B\) , \(A\)  experiences no force, then \(B\)  is magnetised. If you do not find any change from the end to the middle of \(B\) , then \(A\)  is magnetised.
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