to arcing. The contact area of the switch is constructed of materials that resist corrosion and arcing. Many higher current switch designs rely on the separation arc to assist in dispersing contact corrosion. A switch designed for high-current high-voltage use may become unreliable if operated at very low currents and low voltages, because a non-conductive oxide layer builds up without an arc to disperse it. Two kinds of sparks may occur during switch operation. On closure, a few sparks, like those from a flint-and-steel, may appear as a tiny bit of metal is heated by friction to incandescence, melted, and thrown off. On opening, a bluish arc may occur, with a detectable "electrical" (ozone) odor. Subsequently, the contacts may be seen to be darkened and pitted. Damaged contacts have higher resistance, rendering them more vulnerable to further damage and causing a cycle in which the contacts soon may fail completely. To make a switch safe, durable, and reliable, it must be designed so that the contacts are held firmly together under positive force when the switch is closed. It should be designed so that, regardless of how the operator manipulates the actuator, the contacts always close or open quickly. In the construction of many small switches, the spring that stores the mechanical energy necessary for the snap action of the switch mechanism is made of a beryllium copper alloy that is hardened to form a spring as part of the fabrication of the contact. The same part often also forms the body of the contact itself, and is thus the current path. Abusing the switch mechanism to hold the contacts in a transition state, or severely overloading the switch, will heat and thus anneal the spring, reducing or eliminating the "snap action" of the switch, leading to slower transitions, more energy dissipated in the switch, and progressive failure. Variations on design Push button The push-button light switch has two buttons: one that closes the contacts and one that opens the contacts. Pushing the raised button opens or closes the contacts and pops out the previously depressed button so the process can be reversed. In the U.S., the buttons were commonly black; the "on" button typically had a white mother-of-pearl (real or simulated) inlay to indicate its function. By convention, the switch was customarily installed with the "on" button on top. Push-button switch reproductions are available on the market today for