The Cathode Ray Oscilloscope

The cathode ray oscilloscope (CRO) is used to measure the electrical events in living tissue. In the CRO, electrons emitted from a cathode are directed into a focused beam that strikes the face of the glass tube in which the cathode is located . The face is coated with one of a number of substances (phosphors) that emit light when struck by electrons. A vertical metal plate is placed on either side of the electron beam. When a voltage is applied across these plates, the negatively charged electrons are drawn toward the positively charged plate and repelled by the negatively charged plate. If the voltage applied to the vertical plates (X plates) is increased slowly and then reduced suddenly and increased again, the beam moves steadily toward the positive plate, snaps back to its former position, and moves toward the positive plate again. Application of a "saw-tooth voltage" of this type thus causes the beam to sweep across the face of the tube, and the speed of the sweep is proportionate to the rate of rise of the applied voltage.

Another set of plates (Y plates) is arranged horizontally, with one plate above and one below the beam. Voltages applied to these plates deflect the beam up and down as it sweeps across the face of the tube, and the magnitude of the vertical deflection is proportionate to the potential difference between the horizontal plates. When these plates are connected to electrodes on a nerve, any changes in potential occurring in the nerve are recorded as vertical deflections of the beam as it moves across the tube.