2 BELL SYSTEM TECHNICAL JOURNAL 



ray, down the length of the tube. At the end of the tube is a screen 

 of fluorescent materials, which shines brightly at the point where the 

 ray strikes it. We can therefore see where the ray ends on the screen. 

 Another pair of electrodes in the form of two plates P and Pi is intro- 

 duced so that the cathode ray passes between them (Fig. 2). Now, 



D^s^p 



M'l'H 

 — H|.|i|i|i[- 



Fig. 1 — -Schematic of cathode ray tube. 



by a battery or otherwise, we apply a voltage between the plates, so 

 that one is positive with respect to the other. The electrons of the 

 ray, being negative charges, are during their passage between the plates 

 drawn toward the positive plate and emerge in a different direction 

 because of the applied voltage. Similarly, a magnetic field applied by 

 the magnet N-S, across the path of the ray and in the plane of the 

 paper, as in Fig. 3, makes the ray emerge in a direction out from the 



Fig. 2 — Tube with electrostatic deflection. 



page. The amount of the deflection is a measure of the strength of 

 the applied magnetic or electric field. We have, then, in this cathode 

 ray a pointer which tells the magnitude of the field that deflects it. It 

 is, furthermore, a pointer that is almost without mass and sluggish- 

 ness ; it is almost not a material pointer. It can therefore follow varia- 

 tions in the applied field that are very rapid, as we shall see presently. 

 Because of this property the instrument has been used extensively for 

 studying the electrical phenomena of such a range of subjects as power 

 machinery, telephone apparatus, radio transmission and electric surges. 



