Hauskbee's barometric light involved ionizing gas inside which instrument?

Hauskbee's barometric light involved ionizing gas inside which instrument?

• A. Mercury barometer
• B. Thermometer
• C. Gas discharge tube
• D. Pressure gauge

Answer: (A)

Barometric light comes from ionizing the tiny amount of residual gas in a very low‑pressure space. In a mercury barometer, the glass tube is sealed and inverted into a pool of mercury, creating a near vacuum above the mercury column. When an electric field ionizes that residual gas, it glows. This environment—an evacuated space above the mercury—is unique to the mercury barometer, which is why it’s the instrument involved. A gas discharge tube is a separate device designed to sustain discharge in gas, and a thermometer or pressure gauge doesn’t provide that evacuated space needed for the glow.

Barometric light comes from ionizing the tiny amount of residual gas in a very low‑pressure space. In a mercury barometer, the glass tube is sealed and inverted into a pool of mercury, creating a near vacuum above the mercury column. When an electric field ionizes that residual gas, it glows. This environment—an evacuated space above the mercury—is unique to the mercury barometer, which is why it’s the instrument involved. A gas discharge tube is a separate device designed to sustain discharge in gas, and a thermometer or pressure gauge doesn’t provide that evacuated space needed for the glow.