While playing with some 9 volt batteries in a series circuit, I noticed that the voltage of the battery as measured by my voltmeter was increasing at a constant rate. After placing a bulb in my circuit, the voltmeter when placed from terminal to terminal read around 8.8 or 8.9 volts. Then after replacing the bulb with a resistor my voltmeter read 9.0 volts, then, soon after, it read 9.02 volts, then 9.04, 9.06, 9.08, and so on. I was able to reproduce these results multiple times. Why do the batteries seem to "gain voltage" at all?
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