mercury cell (electrical battery). A type of primary electrical cell (a cell that cannot be recharged). A mercury cell uses zinc for its anode (positive terminal), mercuric oxide for the cathode (negative terminal), and potassium hydroxide for the electrolyte.
A mercury cell has a longer life than either an alkaline cell or a carbon-zinc cell, and its low internal resistance allows it to hold a constant output voltage until it is almost completely discharged.
mercury switch. A form of electrical switch using mercury in a glass tube as a conductor. Two wires are brought into the glass tube in such a way that they are near each other, but do not touch. When the tube is tilted in such a way that mercury sealed in the tube covers the ends of the two wires, the switch is closed and current flows through the mercury. When the tube is tilted in the opposite direction, the mercury moves away from the wires, the switch is open, and no current flows.
mercury thermometer. A form of liquid thermometer. A glass tube having an extremely small inside diameter is attached to a small reservoir completely filled with mercury. The volume of mercury in the reservoir is determined by its temperature. As the mercury absorbs heat, it expands and rises inside the tube. A temperature scale is marked alongside the tube, and, since the height of the mercury in the tube relates to the temperature of the mercury in the reservoir, the level of the top of the column of mercury can be marked to indicate the temperature.
mercury vapor lamp. A form of electric-arc lamp. An arc is formed inside a transparent tube between two electrodes in an atmosphere of mercury vapor. Mercury-vapor lamps produce a bluish-green light containing a large amount of ultraviolet radiation.
mercury vapor rectifier (electrical component). A type of electrical component, used to change alternating current into direct current. Drops of liquid mercury inside an evacuated glass tube are changed into mercury vapor by heat from a built-in filament. Mercury vapor can be ionized by a low voltage of one polarity, but it must have an extremely high voltage of the opposite polarity to ionize it. This makes it a good rectifier. Mercury rectifiers have a small voltage drop across them when they are conducting, and they are efficient conductors of large amounts of current in high-voltage circuits.
meridian (navigation). An imaginary great circle on the surface of the earth that passes through the North and South geographic poles. The reference, or prime, meridian passes through the Royal Observatory in Greenwich, England, and the meridians are numbered in degrees east and west to the 180° meridian, which is the international date line.
mesosphere. The portion of the earth’s atmosphere extending from about 45 to 55 kilometers (28 to 34 miles) up to about 80 to 95 kilometers (50 to 59 miles). The mesosphere begins at the top of the stratosphere, and in the mesosphere, the temperature generally decreases with altitude.