184 ELECTRICAL MEASUREMENTS 



which one end is coated with a phosphor that glows under the impact 

 of a beam of electrons. At the other end of the tube an electron "gun" 

 beams electrons toward the face of the tube. Along the electron path 

 two pairs of charged plates deflect the electron beam in the vertical or 

 horizontal direction. A potential diff^erence between the vertical control 

 electrodes moves the electron beam upward or downward. A similar pair 

 of electrodes controls movement in the horizontal direction. In the 

 laboratory oscilloscope the horizontal control electrodes are set to make 

 the electron beam sweep across the face of the tube, return, and sweep 

 again. The sweep frequency can be a few hundred to many thousands of 

 cycles per second. The signal to be observed controls the vertical move- 

 ment of the electron beam. The phosphor on the tube face glows for a 

 short time after the passage of the electron beam, so that in effect the face 

 of the tube shows a continuous curve, horizontally across the face, vary- 

 ing in height as the signal varies. 



The cathode ray oscilloscope is the standard test instrument in the 

 electronics laboratory and can be used for a variety of other purposes. 



Power supplies 



The electricity delivered to a building is almost always alternating 

 current. In the United States it is 60-cycle alternating current, usually 

 110 to 130 volts or some multiple thereof. Except in special circum- 

 stances, instruments must operate with this source of power. Vacuum 

 tube amplifiers require 100 to 300 volts d-c between the cathode and 

 the anode; means of providing these voltages must be available. 



A diode vacuum tube can function as a rectifier, or for some applica- 

 tions a semiconducting device like a selenium rectifier is preferred. In 

 either case, the resulting direct current pulsates with the same fre- 

 quency as the original a-c. Figure 13-7 shows a half-wave vacuum tube 

 rectifier and a full-wave rectifier which offers some improvement. Even 

 full-wave rectification results in a pulsating d-c voltage, however. A filter 

 composed of inductances and capacitances or of resistances and capaci- 

 tances is used to smooth out the fluctuations. The inductance or resist- 

 ance tends to oppose any increase in the voltage, but the capacitors tend 

 to oppose any decrease in voltage. The result is a direct current with some 

 "ripples" but a current considerably smoother than the original pulsating 

 d-c. 



If even better control is desired, an electronic regulating system can 



