Several years ago headlines were made by a small radio transmitter capsule which could be swallowed by the patient and which would then radio internal pressure data to external receivers. This original capsule contained a battery and a transistor oscillator and was about 1 cm. in diameter. Battery life limited the use of this ``pill'' to about 8 to 30 hours maximum.

A refinement of this technique has been described by Drs. Zworykin and Farrar and Mr. Berkely of the Medical Electronics Center of the Rockefeller Institute. In this novel arrangement the ``pill'' is much smaller and contains only a resonant circuit in which the capacitor is formed by a pressure sensing transducer. As shown in Fig. 4, an external antenna is placed over or around the patient and excited 3000 times a second with short 400 -- kc. bursts. The energy received by the ``pill'' causes the resonant circuit to ``ring'' on after the burst and this ``ringing'' takes place at the resonant frequency of the ``pill.'' These frequencies are amplified and detected by the FM receiver after each burst of transmitted energy and, after the ``pill'' has been calibrated, precise internal pressure indications can be obtained.

One of the advantages of this method is that the ``pill'' can remain in the patient for several days, permitting observation under natural conditions. Applications to organs other than the gastrointestinal tract are planned for future experiments.