The object Scanadu is working on is an oval disk about two inches wide and a half-inch thick. Held to the forehead, it uses light to measure oxygen intake, an accelerometer to figure out how far the chest extends in breathing, and a small electrical plate under the thumb to measure heart rate. Other sensors, some still in development, will measure temperature, blood pressure and other body functions. In some cases the sensors are taking direct measurements, including some from a mild circuit that is created by the loop of the arm to the forehead and through the body, which the device reads and sends to a smartphone app. Still others are planned to deduce more information, using software that draws from among the different measurements.