How Does a Piece of Metal Know the Temperature? Two Metals That Disagree

Every metal expands when heated and the amount differs between metals, with brass expanding roughly twice as much as steel over the same temperature rise. Bonding a strip of one to a strip of the other along their whole length means that on heating, one side tries to become longer than the other while being firmly attached to it, and the only way to accommodate that difference is for the whole assembly to curve, with the faster-expanding metal on the outside of the curve. Cooling reverses it, and cooling below the temperature at which it was made curves it the other way. The movement is entirely repeatable and depends only on the temperature.

The deflection of a flat strip is small, so several arrangements amplify it:

• •Making the strip long and thin, since deflection grows with length squared
• •Coiling it into a spiral, which converts bending into rotation of the free end
• •Coiling it into a helix, which does the same over a longer effective length
• •Snap action mechanisms that hold position and then jump, avoiding slow contact
• •A magnet that pulls the contacts together decisively as they approach
• •Adjustment screws that set the temperature at which the movement operates

The device governed an extraordinary range of equipment for most of the twentieth century. Domestic thermostats used a coiled version turning a mercury switch or a pair of contacts, which is the familiar dial on a wall. Ovens, irons, kettles, water heaters and refrigerators all used one to hold a temperature. Circuit breakers used one to trip when current heated it, which protects against sustained overload and still does. Flashing indicator lights in vehicles used one that heated, bent, broke the circuit, cooled and remade it, which is the origin of the clicking sound. Dial thermometers use a helix turning a needle directly. Fire alarms and safety cutouts used them everywhere.

Differential expansion is a general fact and it turns up wherever two materials are joined and the temperature changes. Bridges and rails need expansion joints for exactly this reason, and rails buckling in heat is the failure that follows from not having them. Filled teeth crack when a metal filling and the tooth expand differently. Glass breaks when one part heats faster than the rest and tries to expand while the cold part restrains it, which is why laboratory glass is made from a formulation that barely expands at all. Old sealed windows fail as the frame and the glass move against each other over years. Electronic components crack away from circuit boards after enough heating cycles, which is a leading cause of equipment failure.

Electronic sensing has replaced the device in most applications, because a semiconductor sensor is smaller, cheaper, more accurate, and produces a signal a controller can act on rather than a mechanical movement. That allows programmable behaviour, remote control and logging, none of which a bending strip permits. Against that, the mechanical device needs no power supply, cannot fail in a way that leaves a heater running, works in environments that destroy electronics, and costs almost nothing. It therefore survives in safety cutouts, in simple appliances, in circuit protection and in dial thermometers, which is a fair summary of where mechanical reliability still beats electronic capability.

Two bonded metals expanding at different rates force the assembly to curve when heated, with the faster-expanding one on the outside, and the movement is repeatable and depends only on temperature. Coiling the strip converts bending into rotation and amplifies the movement enough to operate a switch. The device controlled thermostats, appliances, circuit breakers and indicator lights, and survives where mechanical reliability matters more than capability.