Trust is a crucial part of human-robot interactions, and its accurate measurement is a challenging task. We introduce Trusty, a handheld continuous trust level measurement device and investigate its validity by analysing the correlation between its measurements and self-reported trust scores. In a study with 29 participants, we evaluated the effectiveness of the device with an autonomous wheelchair in a mobile navigation task. The participants collaborated with an autonomous wheelchair to deliver packages to predefined checkpoints in an unstructured environment, and the performance of the wheelchair was manipulated to be either under a good-performing condition or a bad-performing condition. Our first finding reveals a notable influence of wheelchair performance on self-reported trust. Participants interacting with a good-performing wheelchair exhibited increased trust levels, as evidenced by higher scores on post-experiment trust questionnaires and verbal self-reported trust measures. Additionally, our study proposes Trusty as a continuous measurement tool for assessing trust during HRI, demonstrating its equivalence to self-report measures and traditional questionnaire scores.