This type of variable capacitor is designed to be set when a circuit is made and then left without further adjustment.
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.
Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the page on logic gates. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).
A rheostat has 2 contacts and is usually used to control current. Uses include controlling lamp brightness or motor speed and changing the rate of flow of charge into a capacitor in a timing circuit.