Voltages above maximum applied voltage may cause an arc between the resistor terminals. At high voltages, RP can also become significant, allowing current to leak around the internal resistance. High-voltage resistors must be kept clean. Fingerprints, oil, dirt, and dust all create unwanted current paths, lowering RP and increasing leakage or even arcing. This is why resistors for use in high-voltage circuits are long and thin with their terminals far apart — to minimize leakage and maximize the ability to withstand high voltage. 


 
After resistor's value, maximum applied voltage, power dissipation is the next most important characteristic of a resistor. An overloaded resistor often changes in value over time and can often get hot enough to burn itself and surrounding components. Every circuit designer learns the smell of burnt resistor sooner or later!


 
The common rule of thumb is to calculate how much power the resistor will have to dissipate and then use the next largest size or a factor of two higher dissipation rating, whichever is larger. The power rating is based on unobstructed air circulation around the resistor. For resistors dissipating more than a watt, arrange nearby components so that air can circulate freely. If possible, mount power resistors horizontally so that convection cools all parts of the resistor equally.