An apparatus comprises a signal generator; first and second electrodes, the second electrode being connected to a first terminal of the signal generator; a processor; and a first switch configured to connect the first electrode to the processor, the first switch having a first position and a second position. The processor is configured to determine one or more values of a first electrical parameter based on a signal received from the first electrode via the first switch in the first position, and to determine one or more values of a second electrical parameter based on a signal received from the first electrode via the first switch in the second position.