A digital recording thermometer is often called a temperature data logger.
A type of chart recorder, one end of a bi-metallic strip is attached to a long, light metal lever that holds a special 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 afterward each strip of paper contains a complete and accurate record of temperature changes for a whole week.
When it curls or uncurls, the data is converted into electric signals, which record the temperature change.