Because they are malleable.
Gold is not the only metal used in electroscope. Copper is also used in electroscope.
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A reason why these metals are used is that they are malleable more than any other metal. To use the electroscope, you need to put a thin trey into it.
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Another reason, which is not the best and is not quiet sure to me, is that Gold is less reactive. If you put a highly reactive metal as trey’s substance, then that metal will react with the electrons that pass through it and will leave the observer a hard chance of having a good view.
