An unfired cone (right) with others at various stages of bending. It can take 20 or 30 degrees to go from straight until bent as the first one. But the more a cone bends the faster it goes down (between the next two may only be 5 degrees). If the tip touches (as has happened with the front one) then it no longer indicates temperature change accurately. It is wise to have a cone in all glaze firings to verify the electronic readings.
Devices that melt and bend in a ceramic kiln at specific temperatures when subjected to specific up ramps. Today, cones are used to calibrate controllers.