When in its ordinary metal and plastic form, it has buttons and delicate runes engraved all over it. The Sensor is part of the Shadowhunters' basic tools, as it alerts them to the presence of demons by vibrating, heating, or producing a clicking sound. Additionally, because of the runes on the Sensor, it can also be used for emergency combat with demons, as Clary Fray once managed to kill a Ravener demon by jamming the device into its mouth, making it choke on the device's runes.
However, the device cannot fully register the presence of Forsaken. The Sensor's response will also be stronger or more faint depending on the power or distance of the demon frequency it picks up. It can also overlook the presence of demons when their ethereal forms are in different dimensions, like when the Greater Demon Abbadon hid behind the entrance to the Portal in Madame Dorothea's apartment.
By 1878, Henry Branwell was already working on the Sensor and has even mentioned several times that he almost has it working. Eventually, Henry did perfect it, and Shadowhunters, at one point, began using it as a day-to-day tool for hunting demons.
- Clockwork Angel
- Clockwork Princess
- City of Bones (first appearance) (chapter 3)
- City of Ashes
- City of Fallen Angels