In this video, I will show you the working principle of McLeod gauge, so here is the diagram of McLeod gauge which consists of measuring capillary, bulb, side glass, piston and cylinder arrangement which containing mercury. Initially mercury is released up to cut off point and unknown pressure is allow to accommodate bulb and capillary. In second step, mercury by piston and cylinder arrangement is raised to such a pressure that it reaches up to reference point in side glass at same time gas is compressed in capillary which is measured by height h. this gauge is used to measure very low pressure. its works on principle of boyle's law.