Rose, Allen M.

KnowledgeBank at OSU

 The Knowledge Bank at The Ohio State University Ohio State Engineer   Title:  The Electron Pencil Creators:  Rose, Allen M. Issue Date:  Nov-1924 Publisher:  Ohio State University, College of Engineering Citation:  Ohio State Engineer, vol. 8, no. 1 (November, 1924), 12-13. URI:  http://hdl.handle.net/1811/33654  Appears in Collections: Ohio State Engineer: Volume 8, no. 1 (November, 1924)       12 T H E O H I O S T A T E E N G I N E E RTHE ELECTRON PENCILBy ALLEN M. ROSE, E. E. '26, of Radio Station WEAO.The principle of the cathode ray oscillograph was dis-covered by Braun 25 years ago and the tube has alwaysbeen known by his name. In Braun's tube, however, theelectrons were produced by a high voltage discharge,between the metal terminals through the remnants ofgas left in the tube. Some of the electrons went througha tiny hole in the terminal plate and struck the end ofthe tube making a spot of light where the stream oflight ended. If an electric voltage was applied betweenthe plates the stream would be deflected toward the posi-tive plate and the spot moved across the screen.The Braun tube had two major limitations. The airleft in it became gradually exhausted and had to berenewed, while the voltage required to operate it wasfrom 10,000 to 50,000 volts direct current. The ap-paratus necessary to generate a current of this voltageand its maintenance was expensive and it was also dan-gerous for the operator to handle. In addition the tubewas not stable. The electrical characteristics variedwith the amount of gas in the tube and the electronstream was falsely deflected by negative charges col-lected on the glass walls of the tube. These chargescollect on the walls of the tube because the pressure ofthe gas is not high enough to dispose of them. For thesereasons the Braun tube was never used as much as itdeserved when its advantages were considered.The development of the vacuum tube, however, openedup a way to obtain the stream of electrons more easilyby the use of the heated filament. In Fig. 1 F is thefilament which is heated by a six volt battery as in theordinary vacuum tube. Another battery built up ofsmall radio "B" cells provides 300 volts between thefilament and the other electrode A. This electrode isin the shape of a little tube. The voltage at which it ismaintained draws off the little electrons from the fila-ment. They pass through the hole in the plate S anddown through this tube. From there they shoot ondown between the plates P15 P., to the end of the vacuumtube, where they strike the chemical coating of the screen,making a bright dot. To simplify matters only onepair of plates is shown in the diagram. The other pairis at right angles to the first and is used to swerve thebeam at right angles to the motion produced by the firstpair of plates. The plate P2 has a lead which passesthrough the glass to a terminal and the other is connectedto the tubular terminal A and from there to a terminaloutside the tube. In this way when a voltage is putacross the two plates the stream of negative electronswill be swerved toward the positive plate.When it is desired to measure a current instead of avoltage, two small coils, Lt and L2, which consist ofa few turns of wire, are placed on opposite sides of thetube. The magnetic effect of the current deflects theelectron stream in a direction parallel to the plane ofthe coils and the luminous spot will be moved as before.In the cathode ray oscillograph recently developed byDr. J. B. Johnson the disadvantages of the Braun tubehave been overcome. The adoption of the heated fila-ment did away with the necessity of maintaining a highvoltage between the filament and the other electrode. Theelements were so designed that the electrical character-istics of the tube were maintained independent of thepressure of the gas in the tube. It was found in thedevelopment of this tube that when it was exhausted toa very high vacuum the individual electrons of thestream separated. This stream of electrons shootingout through the tubular electrode resembles very mucha stream of water from a hose with which a small boycan draw designs on a board fence. The electronsseparate, just as the globules of water do, making thestream wide where it strikes the end of the tube.Of course the boy when making his figures on thefence doesn't care whether the pencil he is drawing withis an inch in diameter or a foot. However, when youare going to calculate the action of an electric currentyou must use a sharp pointed pencil. So a way had tobe found to focus a stream of electrons down to a veryfine point.Consequently, a tube was constructed containing asmall amount of argon, an inert gas. First the tube wasexhausted of all the air possible even to baking the glasswhile it was being exhausted so as to release from theglass all the gas which could be freed by the heat gener-ated during operation. Then the argon gas was intro-duced. Now argon is made up of monatomic molecules,each of which has a nucleus positively charged withelectricity, and surrounded by two negatively chargedelectrons held to it by electric attraction. In the opera-tion of the tube the free electrons shoot down the tubeat a velocity of 6,000 miles per second and when one ofthem hits one of these molecules which is moving at therate of only one-fourth of a mile per second, the forceof the collision knocks off one or more electrons fromthe molecule. Formerly the positive charge of the nu-cleus was neutralized by its ring of negative electrons,but when some of the electrons are knocked off, the nu-clues, now positive, begins to attract free negative elec-trons. As these nuclei are heavy as compared to the fly-ing electrons, they are simply buffeted around by thelatter and they stay in the line of the electron streamwhere they are formed. Therefore, there is along thewhole length of the electron stream a line of positivenuclei which attract the free electrons and hold themin the straight and narrow path in spite of the repulsionbetween electrons which tempts them to spread out.Further, the dislodged electrons shooting off in all direc-tions soon fill the space outside the stream with negativecharges which repel the flying electrons keeping them intheir own path. In addition the pressure of the gas inthe tube is high enough to denude the glass walls of thetube of negative electric charges.In order to prevent the bombardment of the filamentby positive ions which destroy its oxide coating and thusrender it inactive, the filament is sealed in the glassmounting tube, and to prevent the ions from striking thefiloment when they come through the hole in the plateS, the filament is bent in a circle as shown in the photo-graph. The filament is made of ribbon instead of wire,which leaves just an edge exposed to these ions.The great advantage of the cathode ray oscillographlies in the fact that the stream of electrons forms a nearlyweightless pointer whose movement will accurately fol-low the changing conditions in the circuit to which it isconnected. By avoiding mechanical inertia, as it does,it is capable of recording frequencies up to millions ofcycles per second.This interesting strument can be used in many ways.When one set of deflector plates alone is used the elec-trical potential to be measured causes the luminous spotto become a bright line whose length is proportional toTHE OHIO STATE ENGINEER i3AUDIO AMPLITUDEloo >•AUDIO AMPLITUDEh* too HAUDIO AMPLITUDE< * 133 * •/0O_75-50_25-25-5o—IS—IOO—PERFECT MODULATIONAUNDER MODULATIONBOVER MODULATIONCF/6.2the amplitude of the voltage wave. If now another volt-age varying with time is applied to the other pair ofplates the beam will be swept across the field and itswave-form can be seen. Thus, by talking into a tele-phone transmitter one can "see himself talk."The familiar "hysteresis loop" can be shown mostvividly with this oscillograph. The magnetizing currentmoves the spot "from side to side" and the resultantmagnetism in the sample moves it "up and down." Sucha set-up can be made by winding the magnetizing coilon a fibre tube, into which is slipped a thin strip of thematerial. The tube is so placed that the end of thesample comes up to the oscillographic tube where itsmagnetism can swerve the flying electrons.By connecting each set of deflecting elements to alter-nating currents of different frequencies, the spot willtrace out Liccajous figures. If the frequencies are steadyand one is an exact multiple of the other the patternwill be stationary, otherwise it will change as the phaserelation of the current changes. For two currents dif-fering widely in frequency, the pattern may be too longfor its ends to appear on the coated end of the tube,but the fact that it is steady shows that one frequencyis an exact multiple of another. Thus the carrier waveof a broadcasting station may be amplified and impressedon one set of deflector plates, while a locally generatedfrequency impressed on the other set of plates is adjustedto a match with it. The local wave is then matchedwith another local frequency at say, 1/100 of its fre-quency and this in turn is "stepped down" to a fre-quency which can be measured easily.In radio broadcasting it is important that the voicefrequency voltage which modulates the radio frequencyoutput shall be large enough to fully load the trans-mitter, yet not large enough to overload it. Direct evi-dence of this is given by connecting the cathode rayoscillograph in such a way that while the radio outputmoves the spot up and down the audio input moves itsideways. According to the theory of modulation, theradio current should vary uniformly from zero to amaximum as the voice voltage moves from one extremeof its cycle to the other. Figue 2-A shows this condi-tion, or 100% modulation, Figure 2-B shows a loweramount; corresponding to a softer passage in the music.Figue 2-C shows over-modulation, which happens whensome ambitious operator "crowds" this transmitter in atry for some new distance record. Over-modulationmeans distortion and dissatisfaction from his regularnear-by audience.The chief value of this cathode ray oscillograph is toget quick visible indications of what is going on in anelectric circuit. It can therefore be used to explore aFie. Isituation and ascertain roughly what is going on as afirst step to devising measurements which will be moreaccurate. For example, after the apparatus is set uphysteresis loops can be taken very rapidly on one sampleafter another as against a half day each by the moreaccurate "point-by-point" method..Used in radio broadcasting work the cathode ray oscil-lograph provides a means of constantly observing thedegree of modulation obtained. Whether used in thelaboratory, the radio station, or the class room, the newelectron pencil will prove a valuable tool for practicalscientific workers.too— iH2 5 -25—50—7 5 -loo—RADIO AMPLITUDE75 —50—25—\25—50—75—00—too.

The Electron Pencil

https://kb.osu.edu/bitstream/1811/33654/3/OS_ENG_v08_i01_012.pdf

The Electron Pencil

Abstract

Similar works

Full text

Available Versions

KnowledgeBank at OSU