For a practical illustration of resistors’ usefulness, examine the photograph below. It is a
picture of a printed circuit board, or PCB: an assembly made of sandwiched layers of insulating
phenolic fiber-board and conductive copper strips, into which components may be inserted and
secured by a low-temperature welding process called ”soldering.” The various components on
this circuit board are identified by printed labels. Resistors are denoted by any label beginning
with the letter ”R”.