The Corenthal/Adam Roberts Notes appear to refer to a device being created to detect the presence of Sigma Radiation and an unusual collection of carbon, as found in the black cloth (which was also mentioned in the Corenthal Letters dated 7-22-1995 and 7-23-1995) that is described in those notes. It is speculated that this is that device, and that it is designed to detect Slenderman. The device could be potentially used as a warning mechanism or perhaps a locator for offensive purposes.
In the video Sigma, the device is speculated by Evan (following information given to him by Jeff) to measure sigma radiation, emitted from Slenderman. He tests two blood samples, one from him, one from Alex. According to the device sigma radiation is present in Evan's blood, but not Alex's. On August 11th Jeff tweeted that he, Evan, Vin, and Steph had all tested positive for Sigma radiation, according to this device.