Which of the following is NOT one of the three kinds of bi-metal elements used in appliance controls?

The correct answer is that the piston is not one of the three types of bi-metal elements used in appliance controls. Bi-metal elements typically consist of two different metals that expand at different rates when heated. This property is utilized in various control mechanisms to respond to temperature changes.

The three common types of bi-metal elements are helix, coil, and snapdisk. Helix and coil designs take advantage of the differential expansion of the metals to create a mechanical movement that can be used to operate switches or valves in appliances. The snapdisk also functions similarly, utilizing a disk that "snaps" from one position to another at a specific temperature, providing a simple and reliable mechanism for temperature control.

The piston, on the other hand, is more commonly associated with systems that involve fluid movement or compression and is not part of the bi-metal element category used in temperature-based appliance control systems.