Elenco Electronic Component Kit - Manual (Page 9)

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DIELECTRIC CONSTANT, What is it?
The dielectric (rubber diaphragm in the water pipe
analogy) in a capacitor is the material that can
withstand electrical pressure (Voltage) without
appreciable conduction (Current). When a voltage is
applied to a capacitor, energy in the form of an
electric charge is held by the dielectric. In the
rubber diaphragm analogy the rubber would stretch
out and hold the water back. The energy was stored
in the rubber. When the plunger is released the
rubber would release this energy and push the
plunger back toward its original position. If there
was no energy lost in the rubber diaphragm, all the
energy would be recovered and the plunger would
return to its original position. The only perfect
dielectric for a capacitor in which no conduction
occurs and from which all the stored energy may be
recovered is a perfect vacuum. The
DIELECTRIC
CONSTANT (K) is the ratio by which the
capacitance is increased when another dielectric
replaces a vacuum between two plates. Table 3
shows the Dielectric Constant of various materials.
To make a variable capacitor, one set of stationary
aluminum plates are mounted to a frame with a
small space between each plate. Another set of
plates are mounted to a movable shaft and designed
to fit into the space of the fixed plates without
touching them. The insulator or dielectric in this type
of variable capacitor is air. When the movable plates
are completely inside the fixed plates, the device is
at minimum capacitance. The shape of the plates
can be designed to achieve the proper amount of
capacitance versus rotation for different
applications. An additional screw is added to
squeeze two insulated metal plates together
(Trimmer) and thus set the minimum amount of
capacitance.
CAPACITORS
Air, at normal pressure 1 Mica 7.5
Alcohol, ethyl (grain) 25 Paper, manila 1.5
Beeswax 1.86 Paraffin wax 2.25
Castor Oil 4.67 Porcelain 4.4
Glass flint density 4.5 10 Quartz 2
Glycerine 56 Water, distilled 81
Table 3
THE VARIABLE CAPACITOR
Figure 13
Frame
Movable
Plates
Shaft
Fixed Plates
Trimmer