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How can chemical effects of current in p...

How can chemical effects of current in potato be used to identify the positive terminal of a cell or battery concealed in a box?

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When the free ends of a conduction tester are dipped in potato a greenish blue spot is seen around the wire attached to the positive terminal of the battery. This helps in identifying the positive terminal of the cell or battery. The change in the colour is observed due to chemical effects of current in the potato.
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