the siphon barometer

The siphon barometer , used domestically . it consists of a bent tube containing mercury and having a vacuum at the closed end . an iron weight is attached to the ends of a light cord passing round a grooved wheel , and rests on the mercury at the open end ; a second and smaller weight is attached to the other end of the cord . when the air pressure decreases , the mercury rises in the open part of the tube , the iron weight is pushed up , and the wheel turns ; a pointer attached to the axle of the wheel then rotates over a scale previously calibrated in inches of mercury . when the air pressure increases , the mercury and the iron weight in the open part move downwards , and the pointer rotates in the opposite direction .





The source :- principles of physics .