What are/were they meant to represent? When were they introduced? I can’t find any consistent reliable sources. I’d really appreciate references if anyone has any!
In: 27
The “I” represents a closed circuit, meaning electricity is reaching the thing it is supposed to be powering, therefore it is ON. The “O” represents an open circuit, meaning the electricity is not reaching the thing it is supposed to be powering, so it is OFF.
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