Primary batteries are used to operate call-bells, table pushes, buzzers, night latches and various other forms of electric alarms besides which they are used in gas lighters, thermostat motors and for many special forms, all of which form an important part in the affairs of everyday life. Primary battery cells for household use are made to be used in the wet and dry form, but the dry cell is now more extensively used than any other kind and for most purposes has supplanted the wet form. Formerly all primary cells were made of zinc and copper plates placed in a solution called an electrolite, that dissolved the zinc and thus generated electricity, the electrolite acting as a conductor of the electricity to the opposite plate. In later electric cells the copper was replaced by plates of carbon and from the zinc and carbon cell was finally evolved the present-day _dry cell_. When the use of electric cells reached a point where portable batteries were required, a form was demanded from which the solution could not be lost accidentally. The first electric cells in which the electrolite was not fluid was, therefore, called a dry cell. These cells are not completely dry. The electrolite is made in the form of a paste that acts in the same manner as the fluid electrolite and is only dry in that it is not fluid. [Illustration: FIG. 254.--Electric dry cell.] [Illustration: FIG. 255.--Details of electric dry cell.] In construction the dry cell is shown in Figs. 254 and 255, the former showing its exterior and the latter exposing its internal construction. The container is a zinc can which is lined with porous paper to prevent the filler from coming into contact with the zinc. The zinc further is the active electrode, the chemical destruction of which generates the electricity. The parts enclosed in the container are: a carbon rod, which acts as the positive pole; and the filler, composed of finely divided carbon mixed with manganese dioxide and wet with a solution of salammoniac. The composition plug, made of coal-tar products and rosin, is intended to keep the contents of the can in place and prevent the evaporation of the moisture. Binding posts attached to the carbon rod and soldered to the can furnish the + and-poles.