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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 at different rates.When one of these metals expand it curls tighter, when one contracts it uncurl slightly. When it curls or uncurls, the data is converted into electric signals, which record the temperature change.