[Illustration: FIG. 37.--Volta’s “Crown of Cups.” The metallic elements C and Z each consisted of two metals, the plate C being of copper and the plate Z of zinc. They were placed, as shown, in the glass vessels, which contained salt water and ordinary water or lye. Into each vessel, except the two end ones, the copper end of one arc and the zinc end of the next were introduced, the series, however long, ending with copper dipping into the terminal vessel at one end and zinc into that at the other. The arrangement is almost exactly that of a modern one-fluid primary battery.] When the elements are connected at their terminals by a wire or conductor a chemical action takes place, producing a current which flows from the copper to the zinc. This device is called a _cell_, and the combination of two or more of them connected so as to form a unit is known as a _battery_. _The word battery is frequently used incorrectly for a single cell._ That terminal of the element from which the current flows is called the _plus_ or _positive pole_, and the terminal of the other element the _negative pole_. Cells are said to be _primary_ or _secondary_ according as they generate a current of themselves, or first require to be charged from an external source, storing up a current supply which is afterwards yielded in the reverse direction to that of the charging current. An electric current is generated mechanically by a _dynamo_. In either case _no electricity is produced, but part of the supply already existing is simply set in motion by creating an electric pressure_. An electric current, according to the third method, is generated directly from heat energy, as will be later explained; the current thus obtained is very feeble. [Illustration: FIG. 38.--Hydrostatic analogy of fall of potential in an electrical circuit.] [Illustration: FIG. 39.--Showing method of connecting voltmeter to find potential difference between any two points as _m_ and _n_ on an electrical circuit.] =Strength of Current.=--It is important that the reader have a clear conception of this term, which is so often used. The exact definition of the strength of a current is as follows: _The strength of a current is the quantity of electricity which flows past any point of the circuit in one second._ _Example._--If, during 10 seconds, 25 coulombs of electricity flow through a circuit, then the average strength of the current during that time is 2-1/2 coulombs per second, or 2-1/2 amperes. =Voltage Drop in an Electric Circuit.=--A difference of potential exists between any two points on a conductor through which a current is flowing on account of the resistance offered to the current by the conductor.