Ohm's Law

• If you use a voltmeter to measure the potential difference between two points along a good conductor, such as a wire, you will obtain readings of zero volts. In many places in an electronic circuit, parts such as transistors need specific voltages to produce and amplify signals. A resistor can provide this voltage. The rule called Ohm's Law determines the potential difference across a resistor. To find it, simply multiply the current flowing in the resistor in amps by the resistance in ohms.
  
  

Polarity

• Any time a resistor has a potential difference across it, one side will have a positive polarity and the other will have a negative polarity. By touching a voltmeter's probes across the resistor, you can see the polarity. If the meter reads a positive voltage, the red probe is touching the positive side. If the meter reads negative, then the black probe is on the positive side.
  
  

High Voltage

• Up to a few hundred volts, the size of a resistor does not matter much. At higher voltages, the electricity wants to jump over a small resistor. Electricians call this arcing and they generally prevent it wherever possible. When a circuit has arcing, it loses control of electrical energy. It can damage components and cause injury. For higher voltages, electronic designers use longer resistors made of materials that prevent arcing.
  
  

Multiple Resistors

• You can find the potential difference across a single resistor if you know the current and resistance. For multiple resistors, the same principles apply, but you must pay attention to how they are connected. For resistors in series, one connected to another in a daisy chain, the current is the same through all of them. You apply Ohm's Law to find the potential difference across each one. Your work is simpler if the resistors have a parallel connection, like the rungs of a ladder. Find the voltage for any one resistor using Ohm's Law. All resistors in a parallel circuit have the same potential difference.