Electric Resistance and Ohm's Law.

The flow of charge in a material is determined by a property, known as resistance. The unit of resistance is ohm. The resistance varies in different materials. For example, gold, silver, and copper have low resistance, which means the flow of current in these materials easy. The materials like Glass, plastics, and wood have very high resistance, which means it is very difficult for the current to pass trough these materials.
 
A German scientist Georg Simon Ohm experimented with circuits and found out the relationships between current, voltage, and resistance. It became known as Ohm's law and can be written in an equation V=IR, where V is voltage, I is current, and R is resistance.

Resistance of a piece of wire (or any other conductor) is proportional to its length  and is inversely proportional to its thickness or sectional area.
R = k * (L / A)
where:

A potentiometer uses this characteristic. It can change its resistance by controlling its length. This apparature is used in T.V. volume, light controller, and everything else dealing with changing its electricity gradually.

Symbol for potentiometer is and its inside looks like this.

The gray bar inside is connected to the black wire of its around, and it rotates (see above). Let's say an electrical line is connected to the left and the middle. If the gray bar is at the left, then the circuit will have small resistance because the length of the black wire outside is shorter. When the gray bar trun to the right, then it will have larger resistance.

Voltage Law and Current Law

Voltage Law

"In any circuit, the sum of decrease in voltage equals the sum of its increase."
For example, imagine a circuit with a battery and a bulb. If the battery generates 3 V, then the bulb must consume 3 V, because the sum of decrease in voltage (-3 V consumed by the bulb) must be equal to the sum of increase in voltage (+3 V generated by the battery).

Current Law

"At any node (or junction point) in an electric circuit, the total current entering the junction or node is equal to the total current leaving the junction or node”. In other words the algebraic sum of all currents at node (or junction point) is zero.
 

A junction is any point where wires are splits into two or more.

In the following diagram, incoming current (2 A) is being split into half at a junction point (1 A), and then is combined back to the original current (2 A).