=Battery, Gravity.= A battery in which the separation of fluids is obtained through their difference in specific gravity--for example, the bluestone cell. The sulphate of copper solution, being the more dense, goes to the bottom, while the zinc solution stays at the top. In its action the acid at the top corrodes the zinc, while at the bottom the solution is decomposed and deposits metallic copper on the thin copper plates. =Battery, Leclanché.= An open circuit battery consisting of a jar, a porous cup, and the carbon and zinc elements, the electrolyte of which is a solution of ammonium chloride (sal-ammoniac). The carbon plate is placed in the porous cup, and packed in with a mixture of powdered manganese binoxide and graphite, to serve as a depolarizer. A half-saturated solution of sal-ammoniac is placed in the outer jar, and a rod of zinc suspended in it. Another form of the battery is to omit the porous cup and use twice the bulk of carbon, both elements being suspended in the one solution of sal-ammoniac; this form of battery is used for open-circuit work only, such as bells, buzzers, and annunciators. It is not adapted for lights, power, or plating purposes. =Battery Mud.= A deposit of mud-like character which forms at the bottom of gravity batteries, and which consists of metallic copper precipitated by the zinc. It only occurs where wasteful action has taken place. =Battery of Dynamos.= A term used in speaking of a number of dynamos coupled to supply the same circuit. They may be coupled in series or parallel. =Battery, Plunge.= A battery in a cabinet or frame, so arranged that the active plates can be removed or raised out of the solutions. This is usually accomplished by having the plates attached to a movable frame which, by means of a ratchet-shaft and chains, can be raised or lowered. Its object is to prevent the corrosion of the plates when not in use. =Battery, Primary.= A voltaic cell or battery generating electric energy by direct consumption of material. The ordinary voltaic cell, or galvanic battery, is a primary battery. =Battery, Secondary.= A storage-battery, an accumulator. =Battery Solution.= The active excitant liquid, or electrolyte, placed within a cell to corrode the positive element. Also called Electropoion. =Battery, Storage.= A secondary battery; an accumulator; a battery which accumulates electricity generated by primary cells or a dynamo. =Battery-gauge.= A galvanometer used for testing batteries and connections. It is usually small in size, and may be carried in a pocket. =Battery-jar.= A glass, earthen, or lead vessel which contains the fluids and elements of each separate cell of a battery. =Baumé Hydrometer.= (_See_ Hydrometer, Baumé.)