<P> A mercury barometer has a glass tube closed at one end with an open mercury - filled reservoir at the base . The weight of the mercury creates a vacuum in the top of the tube known as Torricellian vacuum . Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir . High atmospheric pressure places more force on the reservoir , forcing mercury higher in the column . Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir . Since higher temperature levels around the instrument will reduce the density of the mercury , the scale for reading the height of the mercury is adjusted to compensate for this effect . The tube has to be at least as long as the amount dipping in the mercury + head space + the maximum length of the column . </P>
<P> A mercury barometer has a glass tube closed at one end with an open mercury - filled reservoir at the base . The weight of the mercury creates a vacuum in the top of the tube known as Torricellian vacuum . Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir . High atmospheric pressure places more force on the reservoir , forcing mercury higher in the column . Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir . Since higher temperature levels around the instrument will reduce the density of the mercury , the scale for reading the height of the mercury is adjusted to compensate for this effect . The tube has to be at least as long as the amount dipping in the mercury + head space + the maximum length of the column . </P>
what keeps mercury inside the tube in a mercury barometer
what keeps mercury inside the tube in a mercury barometer
weight of the mercury column balances the atmospheric force exerted on the reservoir
weight of the mercury column balances the atmospheric force exerted on the reservoir