The radio pill relies on a temperature-sensitive quartz crystal oscillator whose vibration frequencies are well known for temperatures ranging from 60 °C to 150 °C. For instance, the crystal oscillates at 262.25 kilohertz at the normal body temperature of 37 °C. The electronic components calculate the temperature and transmit the data as a digital signal. Power comes from a silver oxide hearing aid battery that holds enough energy for nine days of temperature readings. The capsule remains in the body for only 24 to 36 hours before it is eliminated.
The temperature readings are transmitted wirelessly to a handheld receiverdata recorder. As the digital signal induces a voltage on the pill's communication coils, this voltage creates a quasistatic magnetic field with a radius of about a meter. When a doctor holds the receiver to the small of a patient's back, a magnetic coupling between the pill and the receiver induces a voltage in the handheld device's antenna, which is then demodulated to retrieve the original temperature data.
Because magnetic communication does not generate a propagating wave and there is strong attenuation of the signal with distance, the data are hard to intercept and virtually free from interference—even if there are dozens of other patients walking around him with radio pills in their guts. Creating such a magnetic communication bubble also requires very little power, which allowed the radio pill's designers to use the tiniest of commercial batteries.
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