Capacity, Unit of.
The unit of capacity is the capacity of a surface which a unit quantity
will raise to a unit potential. The practical unit is the surface which
a coulomb will raise to one volt, and is called the farad, q. v.
Capacity, Storage.
In secondary batteries the quantity of electrical current which they can
supply when charged, without undue exhaustion. It is expressed in
ampere-hours. The potential varies so little during the discharge that
it is assumed to be constant.
Capillarity.
The reaction between liquid surfaces of different kinds or between
liquid and solid surfaces due to surface tension. Its phenomena are
greatly modified by electric charging, which alters the surface tension.
Capillarity is the cause of solutions "creeping," as it is termed. Thus
in gravity batteries a crust of zinc sulphate often formed over the edge
of the jar due to the solution creeping and evaporating. As a liquid
withdraws from a surface which it does not wet, creeping as above is
prevented by coating the edge with paraffin wax, something which water
does not moisten. It also causes the liquids of a battery cell to reach
the connections and injure them by oxidation. The solutions creep up in
the pores of the carbons of a battery and oxidize the clamps. To give
good connections a disc of platinum or of lead is used for the contact
as not being attacked. Another way is to dip the upper ends of the dry
and warm carbons into melted paraffin wax, or to apply the wax to the
hot carbons at the top, and melt it in with a hot iron.