40 Applied Biophysics 



opposite plates. In the gas-focused type of tube, the combined 

 action of a small amount of gas within the tube, and of the nega- 

 tive grid potential, causes the beam to be focused to a fine spot. 

 A modern high-vacuum type incorporates several refinements. 

 Instead of a simple plate for the anode, two or more cylinders 

 are used ; focusing is brought about by electrical optical means. 

 The pair of deflecting plates in the vertical plane are called the 

 Y plates, and those in the horizontal plane are called the X 

 plates. The deflectional sensitivity of the cathode-ray tube is 

 insufficient to produce a record when the heart potentials are 

 applied directly to it. A high-gain amplifier is therefore neces- 

 sary to magnify these potentials sufficiently to give a trace on 

 the screen of the tube. The output of this amplifier is connected 

 to the pair of Y plates, and thus gives a vertical trace. If re- 

 quired, the vertical movements can be photographically recorded 

 on moving film. If it is desired to view the wave form of the 

 electrical variations of the heart on the screen of the cathode-ray 

 tube, it is necessary for the beam to move slowly across the 

 whole of the screen of the cathode-ray oscilloscope in the hori- 

 zontal, or X axis, from left to right. This movement is given 

 by a time-base circuit. For the direct visual observation of the 

 electrocardiogram, the fluorescent-screen material used in the 

 tube is chosen to have a very long afterglow, so that the trace 

 of the spot, when seen in a darkened enclosure, is visible for 

 several seconds after the spot has gone by. 



Rijlant,'*^ Schmitz ^"^ and Matthews ^^ were among the first 

 who adapted the cathode-ray tube to electrocardiography. They 

 used the cathode-ray tube merely as a recording device, and 

 not as an oscilloscope. Robertson ^"^ introduced a new electro- 

 cardiograph employing the cathode-ray tube as an oscilloscope 

 and fitted with a screen having a long afterglow, which permitted 

 direct visual observation of the electrocardiogram. Brokes- 

 Smith ® devised a similar apparatus, but without any device 

 to obviate origin distortion. Asher and Hoecker "^ mention in 

 their paper that Wilson has adapted the afterglow cathode-ray 

 oscilloscope to electrocardiography. 



The cathode-ray tube has been adapted to vectorcardiography 



