The transformer's other terminal connects to one of three terminals on the signaling device. In 1900 Gray worked on an underwater signaling device. Bus movements were governed by a dispatcher using an electric signaling device. A few years later he was recognized as the inventor of the underwater signaling device. Strobes are used by scuba divers as an emergency signaling device. These are canister-type grenades used as a ground-to-ground or ground-to-air signaling device. A ringer box is a telephone signaling device, similar to a bell box. One such signaling device was tested on December 31, 1900. A doorbell is a signaling device typically placed near a door to a building's entrance. The Signalscope included a glow-in-the-dark signaling device, whistle, magnifying glass, and Sky King's private code.