Composition of Resistors

• Resistors are usually made out of metallic alloys (wirewound resistors) or carbon composites (composition resistors). Usually they are surface mounted or integrated into the circuit. Resistors are the simplest passive element of the circuit (passive meaning the element is not capable of generating energy).
  
  

Variable or Fixed

• Resistors can either have variable or fixed resistance. Most are fixed, meaning the resistance never changes. A resistor with fixed resistance is a linear resistor. Variable resistors -- sometimes called potentiometers -- have adjustable resistances. These resistors are called nonlinear resistors. Both fixed and variable resistors can be either wirewound or composition, with composition being used for large resistances.
  
  

Resistance

• Resistance is the ability of a circuit to resist current and is measured in ohms (more on Georg Simon Ohm's law later). Based on resistance, there are two types of circuits: open and short. In an open circuit (such as a circuit with a broken wire), the resistance measures at infinity. In a short circuit, the resistance measures at zero.
  
  

Conductance

• Conductance is resistance's reciprocal. If resistance is infinity (as in an open circuit), conductance is zero, and if resistance is zero (as in a closed circuit), then conductance is infinity.
  
  

Ohm's Law

• Ohm's law is named for Georg Simon Ohm, a German physicist. Ohm's law says that the voltage across a resistance is directly proportional to the current flowing through it. Therefore, without a resistor, the circuit would have infinite current. This law applies to linear resistors.