An acoustics lab technician is setting up a test circuit to measure how the sound pressure level of a miniature signal transducer (speaker) changes with voltage.
The laboratory's DC power supply only provides discrete, fixed voltage outputs of 1.5 V, 3.0 V, 4.5 V, and 6.0 V.
Describe how the technician could modify the circuit by adding a single standard component to enable them to continuously vary the potential difference across the transducer.