Hallway light switch makes bedroom light brighter

Question

When I turn my bedroom light switch on, the ceiling light turns on and seems fine. (Older North American home.)
When I then turn the switch for the hallway light on, the bedroom light becomes brighter.
Is the bedroom light somehow now getting 240v instead of 120v? And what would cause this?

Chris Cudmore · Answer

I'm willing to bet that there's a split circuit (Multi wire branch circuit) improperly wired here.
So when both switches are on, you're getting 240v between the two phases rather than the expected 120v between a single phase and neutral.

Hallway light switch makes bedroom light brighter

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