Applications of Electronics in Medicine 39 



a specially prepared surface. The resultant electrocardio- 

 gram is -FrT of standard size, and must be viewed through a 



microscope for direct visual observation. If a permanent 

 standard-record-size electrocardiogram is desired, the original 

 record must be sent to the agents for enlarging. 



The ink-writing electrocardiograph uses a valve amplifier and 

 an ink-writing oscillograph. The record is made on inexpensive 

 paper tape. It is immediately visible, and requires no process 

 of developing or fixing. The upper-frequency response of the 

 instnmient is limited, due mainly to the friction between the 

 writing pen and the recording paper. 



For exact reproduction of the wave shape of the electrocardio- 

 gram, it is essential to use an oscillographic recording element 

 which will respond to the highest-frequency components. Such 

 a device is the cathode-ray oscillograph. The cathode-ray tube 

 is essentially an oscillographic indicator characterized by two 

 striking and valual^le properties : first, the almost complete 

 absence of inertia in the recorder, and, secondly, the two-dimen- 

 sional recording field. The tube, itself, is essentially a compli- 

 cated thermionic valve. It contains, at one end, an electrode 

 structure, called the "electron gun," and, at the other end, the 

 fluorescent screen. The "electron gun" possesses a filament, a 

 cathode, a grid, and an anode. The electrons emitted by the 

 heated cathode are accelerated by the high positive potential 

 of the anode, and are caused to pass down the length of the 

 tube in the form of a narrow beam. These high- velocity electrons 

 impinge on a fluorescent screen, and there give rise to a spot 

 of light. The direction of motion of the electrons, forming the 

 electron beam, is affected by electric or magnetic fields. At any 

 point between the accelerating system (or "electron gun") and 

 the screen, the beam may be deflected by the electric or magnetic 

 field ; the resulting displacement of the spot is a measure of the 

 strength of that field. In the most usual arrangement, the 

 cathode-ray tube is fitted with two pairs of deflecting plates 

 mutually at right angles, and the deflection of the spot along 

 an axis is closely proportional to the voltage difference between 



