A glass bulb is balanced by a brass weight in a sensitive beam balance. Now the balance is covered by a bell-jar which is then evacuated; then:

A glass bulb is balanced by a brass weight in a sensitive beam balance. Now the balance is covered by a glass-jar which is then evacuated.After evacuating, the glass container would have no air in it. In order to maximise the mass of the glass bulb, since it contains air within the glass bulb. Then the pan that holds the bulb is going to go down. When the glass bulb is weighted in the air by a brass weight, both of them displace air, but the much larger volume of the glass bulb displaces greater air volume. When a bell jar surrounds the device, it is then drained. There will be no upthrust due to displaced air and the glass bulb will weigh more. So, the pan goes down.