RESISTOR CIRCUIT SYMBOLS

Resistors ( R ), are the most fundamental and commonly used of all the electronic components, to the point where they are almost taken for granted. The principal job of a resistor within an electrical or electronic circuit is to "resist" (hence the name resistor), regulate or to set the flow of electrons (current) through them by using the type of conductive material from which they are composed. Resistors can also be connected together in various series and parallel combinations to form resistor networks which can act as voltage droppers, voltage dividers or current limiters within a circuit.

Resistors are "Passive Devices", that is they contain no source of power or amplification but only attenuate or reduce the voltage or current signal passing through them. This attenuation results in electrical energy being lost in the form of heat as the resistor resists the flow of electrons through it.

In all Electrical and Electronic circuit diagrams and schematics, the most commonly used symbol for a fixed value resistor is that of a "zig-zag" type line with the value of its resistance given in Ohms, Ω.

The symbol used in schematic and electrical drawings for a Resistor can either be a "zig-zag" type line or a rectangular box. 

The following figures are the electronic symbol for resistor used in different circuits.

Fixed value resistor symbols

Variable Resistor - Rheostat Symbols

Variable Resistor - Potentiometer Symbols

Variable Resistor - Preset Symbols

Reference:

http://www.electronics-tutorials.ws/resistor/res_1.html