Bi-metal Bi-metal refers to an object in which two separate metals are joined together. Unlike alloys, the metals are not mixed. Instead, they comprise two layers: a high-expansion metal (originally brass but now usually a high-expansion stainless steel) bonded to a low-expansion steel (now usually a very low-expansion iron–nickel alloy trademarked as Invar). On heating, the high-expansion side expands more than the low-expansion side, setting up shear stresses between the two layers and causing the sandwich to bend in a curve that straightens out again and reverses on cooling. Horologist and Clocktime creator Dr John C. Taylor OBE created the bi-metal thermostatic control that automatically switches off your electric kettle every time you make a cup of tea.