The term Capacitance refers to an electrical phenomenon whereupon an electric charge is stored.  In the following figure a Capacitor is shown comprised of two metal plates separated by a small distance. When the switch is closed, the battery will cause plate number 1 to be positively charged. Conversely plate number 2 to be negatively charged because of electrons being repelled from the negative battery terminal to plate number 2.

In the above circuit,  an electric field is established between the plates, even though there is an air gap.

 In the construction of a Capacitor, the air gap is usually replaced or combined with other solid or thin-filmed dielectrics. The dielectric is actually the insulation material that is used between the plates.   The dielectric material selected is a function of the circuit application.  In general the name of a capacitor describes the dielectric being used. There are capacitors (electrolytic types) which filter ac ripple in a DC Power Supply.  Electrolytic and tantalum capacitors are always labeled and are polarized.  The Ceramic, mylar, plastic film, and air capacitors are not polarized and therefore their leads need not be marked.

Capacitance Units

 Capacitance is measured in units of farads, however most practical values of capacitors are in microfarads, abbreviated as uF.

 The uF is equal to one-millionth or 1/1,000,000 of a farad or E-6 farads  in E Notation or E-6F

A capacitor 0.47 uF can also be written as 0.47/1,000,000 or 0.47E-6 in E Notation.  Capacitance can also be specified in pico farads or 1 millionth of a microfarad.  In other words 1 pico farad = E-12 farads or E-12F.

Model Train Applications

Capacitors are not just used in DC power supplies but  are an integral part of many electronic circuits. Many electronic schematic ciruit diagrams and associated parts lists are available to the model train do-it yourself  hobbyist  in magazines and on the internet to perform special functions like crossing gate comtrol,  etc.

If you wish to acquire a better understanding of Electronics Theory, I suggest you go to  the following  link: Electronics

In general, the folowing vendors are recommended for any of the above electronic components:

Radio Shack – Local Store or on-line at              

Futurelec at