A recording thermometer is a type of thermometer that records temperature changes over a period of time. The recording thermometer uses a strip that coils and uncoils as the temperature changes. One end of the strip is attached to a long, light metal lever that holds a special pen. Tiny movement of the bimetallic strip causes much larger movement of the free end of the lever and the pen. The pen traces a rising and falling line on a strip of paper attached to a slowly turning drum. The drum usually makes one turn every seven or so days, so afterwards each strip of paper contains a complete and accurate record of temperature changes for a whole week. There are normally two types of metal on the bimetallic strip. Mostly one is steel and one is copper. Because these metals expand and contract