when the glass bulb is weighed by a brass weight in air both of them displaces air, but the glass bulb being of much larger volume displaces larger volume of the air. when the system is covered by a bell jar. which is then evacuated the upthrust due to displaced air will be absent and the glass bulb will weigh more. thus, pen with the bulb goes down.
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The bulb goes down
Explanation
when the glass bulb is weighed by a brass weight in air both of them displaces air, but the glass bulb being of much larger volume displaces larger volume of the air. when the system is covered by a bell jar. which is then evacuated the upthrust due to displaced air will be absent and the glass bulb will weigh more. thus, pen with the bulb goes down.