A student designs an experimental setup to investigate how the electrical resistance of a negative temperature coefficient (NTC) thermistor varies with temperature. They construct a circuit consisting of a variable DC voltage source, an ammeter, a voltmeter, a switch, and connecting leads, with the NTC thermistor placed inside a beaker of water on a heatproof mat.
State the name of one additional piece of measuring apparatus the student would require to successfully carry out this investigation.