The electrical design of the common hair dryer is based almost entirely on relatively simple principles learned in introductory physics classes. Just as biology students dissect a frog to see the principles of anatomy in action, physics students can "dissect" a hair dryer to see how principles of electricity are used in a real system. They can discover how engineers solve problems such as how to vary between low and high heat and fan speed by simply moving the position of a single switch. Principles of alternating versus direct current, series and parallel circuits, electrical safety, voltage dividing, ac rectification, power, and measurement of resistance and continuity all come in to play.