The correct answer is: D. Ionization gauge.
A Bourdon tube gauge is a mechanical pressure gauge that uses a Bourdon tube to measure pressure. The Bourdon tube is a curved tube that is filled with a fluid. When pressure is applied to the tube, it causes the tube to straighten. The amount of straightening is proportional to the pressure. A pointer is attached to the tube, and it moves along a scale to indicate the pressure.
A Pirani gauge is a thermal conductivity gauge that is used to measure low pressures. It consists of a filament that is heated by an electric current. The filament is placed in a chamber that is evacuated. When the pressure in the chamber is low, the filament cools down. The amount of cooling is proportional to the pressure. A circuit measures the current that is required to keep the filament at a constant temperature, and this current is used to indicate the pressure.
A micromanometer is a type of manometer that is used to measure very small pressures. It consists of a U-shaped tube that is filled with a liquid. The pressure difference between the two arms of the tube causes the liquid to rise or fall in the tube. The height of the liquid is proportional to the pressure difference.
An ionization gauge is a type of vacuum gauge that is used to measure very low pressures. It works by ionizing the gas molecules in the vacuum chamber. The amount of ionization is proportional to the pressure. A circuit measures the current that is produced by the ionization, and this current is used to indicate the pressure.
The ionization gauge is the most sensitive of the four types of gauges, and it is the only one that can be used to measure pressures of the order of 10”’ torr.