ApproTed :

Ili!:SIGN AND CONSTRUCTION

OF

A MECHANICAL OSCILLOSCOPE

BY

HAMPTON JENNINGS DAILY

A Theeis Submitted to the Graduate Committee For the Degree ot

MASTER OF SCIENCE

in

Electrical IDgineering

Head ot Department

Dean~ Siliieering

Chail"'DB.l1, Graduate CODIIlittee

Virginia Polytechnic Inetitute 1936

TABLE OF CONTmTS

pago I • IN'l'RO DUCTI ON. • • • • • • • • • • • • • • . • 1

I I • HISTORY. •. • • • • • • ·• ., • • •. • • • • • • • • • 2

III . REVI ... OF LITE TU.RE ." •.•• •• . S

IV- 'l'lf1; ~IQ TION ••. ..... ...• 6

The ob jec"l of t he lBTest1gat1on ••••• •• ••• ; •.••• 6

• t hod ot Procedure ••• • • • • • • • 6

Reaulte .••••• •.••••..•. .•...•••.• ·a

V. DICUS ION OF RltSUL'l'S •••••••• lO

VI . Appendix • - •.•••••••••.••••••• Ua

l. l!'I~ UENCYi A " LENG'm i~O OR ••••• l~

u . s RVE. • . • • • • • • • • • • • • • • 13

II . SI ~ C S 'I .ll ZERO AXI • 14

..... .... .. 15

V •. USEFUL CI RCUIT" • • • • • • • • • • • 16

l

I D OD CTI ~

C plet a. l ysiS t ny Of th COl pl probl ~ h1oh rape th

b ne;in re and oc 1 chine

ag ould b lmoet 1rn os 1bl i not Tory dit ie t ttho t met od ot

die ct phenCJilUCrm • 1eh tak plac •

y b reoolve into t o lemente

or di et1on , o or t h ps d t1 a d th oth r

p op rtio- 1 to th tion with res et

to · n s z ro . ''j1 co 1 ed into

on th sult 1o y b pt y 0 eros e ction or an ocean ve. Th h 18h.t to t he plit de and

t distance t~ crest to cr ot b 1 .. ' ... h oscillo ra t h i bur-

den of yei t o oill sa

t e o oUlo cop nabl s

n t o b ohe rv d d1r.ec lYe

Oac11 o 1a dcrtte to b o and to t h nd

ph r 1t n a • .:. h diffi culty thi actio in port bl d 1c ic n ded

lit l o or no a l; t h inc nttv t o buil ding - ch•

.f ie l o illo op •

ut or e t o Xp hi o sine re a prooiatio ot he id aad -

istanc g1v n by t h mb ~s ot t~ tn chi atatr 1 the De r1 nt of Phystes

Th 1r helpful eri tioi hao id gr eatly 1n his work.

J ( I

11 HISTORY

The recordi o by point driven by a mov•

ing chant~ wae probably t he first torm of th oscillogi ph. 8 I t

ua d this ethod of recording what took p ce in the cylinder o ie steam

engin in 1786 . Thi indicator s devised by ·•tt is sentially t indica•

tor card of today. The next

1ras · de by William homso

jor step in the development of the oscillograph l (Lord el in) in 1867 w en h · sip on cord r

s introduced in order t o make etticient us of th first aucceseful trans ..

atlantic cables . The first recorded a pt tion ot t

ehanical to th electrical wae de by illiam

oeoillo ra.ph tro 2 dd 1 • Th o ci l t n

1rror «US developed by Duddell waa easenti lly t he tr1ng osaill graph as

have it today. 'ihie de elop nt was in 1893 .

e

Blondel5 brought out in 1891 moving coil gelvan ter which he adapted

to drive a mirror in an oscilloscope with mediocre success . e l 1ting

tact or in Bl ondel' s €l lvanametor was th

hot W1 type of oscillograph wa broug t out i n 1 7 but nvr

wid ly ed b cause or t he tact it a practically useless at frequenc1

above five cycles per second .

r t he time William ddell intro uced hi oso1llat1 trror tb

dovelo_ nt of th oscillograph has b en one of con tant impro m nt and no

one peeitic change can b ointed out and c lle t he t eign1f1e nt d•

vane • The developments most pertinent in th 11 t of t his investigation

will be dtscu sed 1 detail tmd are not included in the hietory of this

subject.

lite rat on ny aubj ct ie

t ot t thod of pre tation ivee

nal 1 •

J th r of th oscillograp c his appli•

e tio:n ot g pre ation of rh t Look p1 c cylinders

sot co 1m ort ne only t o h s t •

L rd K lvtn. it hie i p o ~cor r , 1 oed oth r t pi ton in

t o t o ci og ph .

con .r1b tio of r!illi ~ dd ll ov r sbaao ll provo nt t hus tar

t oecillo in it ern t o •

hie d alt sub j cts but

t hi n t ork ll b d in ete:11.

pr b bly the r r t t o uec

s on tb

oaeill scope. •

so nt r que cy a

t out y Cplins: r

111 wit in t or

the troCJ:uezlC

s l rro .... 1z • T

0 th t th

a c rreepo di

1rty-two candle po

I'O 1 • The

• :'h1o 11 i ted by d

nc l~ bo to b ob

be to b S.noro ed about 1x t1 th

t b tll n t ion i ncrease" t ho product

irror th refo - rror iz · ll ..

e in e qui tnt n 1 t y ot th light sourc • A

obil h d light l sub titutcd for t

in th'1 o otllog ph tro a ste 1 etri

1 th t o ir or ount on o end nd support at the oth r .

4

polar oscillograph bas been built by 6 rr1eon and its outstanding tea•

ture is the method used tor dr1vine the oscillating irror. The mirror 1o

p1Yoted centrally alo one axis and a permanent gnet type of loud speak•

er was plac d so that its driving pin and t he tree edge or the · rror were

in line . flexible cou-pling between thU driving pin and the mirror allowed

1t to vibrate t"reely. The mirror was one-half inch in diameter, being rel•

atively larg for an oscillograph. The large size or the mirror de posst-

bl the use ot a f'laehl i ght lamp as the light source . h. photographic tilm

waa so plaCed and driven b-y clockwork or 'by motor that upon applying a tran-

sient or by manunl control , the osc1llog::raph, the instant a transient was

applied was pl•ced in operation automatically. This gave a circular or polar

to to t ho transient or recurrent phenomena t aking pl ace. illuminating

arra ment caused th tace ot a clock to be photographed at the instant t he

oscillograph was pl aced 1n operation thus 1v1 the time of the transient .

The apparatus used t the l ast Worlds Fair was described by . 111na7 and

differs from all the different types discussed . This oscilloscope ueed no

lens but t h vibrating mi rror w s pal"8bolic a the light source at one focal

point nd the screen at t he other. A small incandescent lamp was used as the

light source. rotating mirror ot many races waa used; ao spaced that the

light ltpot lea\rin the ecre n at one end when another appeared at t he

oth r end. The vibrator wtls of the ttleetro-dyna.mio type .

On of t he more 1nt etin type of osetllo rsph is the on built bp a Legs. v1ously only on pattern could be shown tvi t h one v ibrating mirror

but the oscillograph to be de.scribed here can show two waves simultaneous l y .

A sw:itch1 device shift s the vibrati mirror trom on source ot excitation

to anot her ilil the interval trom the disappearance of the spot of light tram

one end of the screen and its reappearance attbe other end. The mirror dr1V•

elei ent is of the string oscillograph type . ith this ewitching arrangement

several interesting connections a~ possible . The po er t otor in alternating

current circuits y b show by bavi . one pattern showing the voltage wave

and tbb oth r pat ern bowing th current drawn by an inductive device euch

as in induction otor. t mating current before and after reot1t1cat1on

may be sho v ry easily by tl'te correct cor.mection • An interestins :pose1-

b1l1ty ls circuit by which an oseill ting di charge ith a 60 cycle timing

v show1 s ultan ously y b obtained . 1 re I on t b plate with tour

diugrams diffe nt circuits shows s 1 connect ion tor show!

t curr&nt drawn by in induction tor. igure li giv s th connection for

sho 1n an alt rnati current

~i III illustrates th connections for e owing n oeeill tory d1scbar

timtn~ ou lY• h last fi sho t co ections tor

the 1 of the curren behind t h . voltage in an i n uotio ·tor or

t po 1· t ctor a tour only b usod y th

. d tion or s ;ply 0 mir.ror e will be

xp i.Md i n o d t 11. It the .souro of xci t i on 1 a i ·t d trom on

pl ce to noth r 1lc th ecreen 1 d · r t e to t e r-

ion, o v s c fr a di 1:rror spe

ight bav to inet· d to V· conti an •

The Ob ject ot Investigation

IV INVESTIGATION

The purpose of this ittY at1gat1on was to develop a portable or .semi port-

able oeeilloseope with a light ouree needing no adjusting and giving eutt1-

cie.nt illumination to enable the observer to .see clearly the chan s taking

place on the ecreen. The brilliancy of the il:sage must b so that the wave

pattern nay be anal ysed in deteil and also that waves ot different trequen-

eies y be caused to stand stationary on the screen. This oscilloscope must

includ an amplifier so that minute currents may be amplU''ied enough to ac-

tuate the 'fibre.ting cbaniem.

ot the investigat.ton was attacked first . The

light source must be so that it will require little or no adjusting and this

narrowed the ti ld down to an incandescent lamp with a concentrated filament.

-.n aut obile headlight ti nt 1e concentrated and gives the requ.ired bril•

Haney and a 21 candle- power lamp wa selected.

The optical system wee devised so t hat the lieht tr the lamp passed thru

a pin hole 1 diately in front ot t he l p and passed thru a long tub wh1oh

prevent d any extraneou light from tallin on thr screen and then struck the

vibrati 1rror which s pl ced just beyond the end ot the tube . t r leav•

ing th vibrating mirror t light :pass d thru a condensing lene which focused

ot t pi ol on the screen. h siz of the li · t spot on the screen

was approximEI.tel y qual to the retio of the di tance tro the screen to the

lena and from the p n hol to t 1 ns t 1 t he area of t e pin hole .

illumination obte.ill(ld on the screen is proportional to the product ot

the d1men$ions of th mirror when this op ical sy te is us d and this fact

, led to lts adaptation to this osctlloeco • The. light spot elSe ta indepen-.

dant or the mirror size and the· tntens1 ty ot the spot is dirootly propo~

t1onnl to the mirror size thus the only Umttl tno·tor in the · rror size

ts p rently the power n oeaee.ry to drive it.. dyna ic type ot loud speaker

Rs a-m1l8ble tor thts use . rror about one-half eentt ter was · de from

a. V&l'Y thin labratory cover es . he e1lver1 or this cover lass was done

by t -.e ohelle· r., lts thod . ( · e any handbook on o etry} Sev :rol s1z~s of

rro~ were tried to .a e which gave the beat results 'before tb size mentioned

we aopt d .

:'he dymm1c s k r had be n used to drive an eight inch cone 1n ir so

tho ow r avatl ble e eoneldoroble. The con e · ed ae s all tbe

c<:>na consisted of one tal r1 · bout nine inches in dim.·

et r . thin st.rip ot phosphor bronze apring obtained t aut . b1le

peed ~eter was tretob d cross the rae of th speak r an<i bout one lf

an inch trQil tl1e 4r1vin eo11. The mirror w ounted on this strip and. con•

n . eted to the driving coll by e s ot l.e'f'er arm a:rl"8Dg nt . Thue a~ ove-

nt of th · drl1'1 coil caus \ rror to vibrat bout an ule which

coincided ith t 1 nlal axle ot tho phosphor b nze etri •

Cons1d rabl 1fficulty as encount d in obt ini or de'V'i .ing a s tis-

cto:cy thod ot drivtn t e OflCilla.tt irror tho high t of flexi

quickly aaus d ta1lu of the phosphor bran· dri'f'ing strip first tried . Sev•

eral zp dt nts were tried with no suoce s tU l 1iher drive was tried.

'l'hta dri.\f1n thoa roo se or oi t · work eatiaf'actorily.

Considerable pl1tude of the spot on t so en a toun to be due to

tho direc current rippl in the etitied alternating current d in the

ape r fi ld. o 11 ound on the tiel t core and o connected

1n r1 s with tl~ drlvt coil s to duce this ros1d 1 plttude to zero.

a The pparatu required n audio-frequency amplitter which w s constructed

rith a standard amplltler ctroutt whtch 11 be found in anr text or n4l>OQ

d 11 with dio. A c pl t die tic sketch of the oscilloscope in•

Uti r 111 be toun4 l1et d 'Wlder table ot contents. Tbe con-

ctions to t coil e brought out to suit ble sw1 tch1'JI8 arre.nge-

nt so that other pheno na than those necessttattns ampliti.oatlon y be

IJh~ •

in ry test the final ro and placi of th TON

1ous rt was l 1nte ettog t ot • rc brought to

to light . The frequency of the v1bret1ng mirror and ot the moving coil eye-

t could b c nged a oat e.t will by 1nero st or d&c:reaatns the dauU>1ne

of use of frequency here is intended ae synonymous w1 th

rcso nt r qu ncy. 1ncre ~1ng t h d ptn tb rror et~e could be in-

Cl~&.sea . he t queney s low r d untU maxim · response was obtain d at a-

bout 200 cycle per eeond with no a ping p de. Th c parothely large sa

ot tho ovi coil and its uount1 o tbe detormini tactat tn t his low

IMX. . um t:requency roeponse .

,0 light struck the :rror when th . a. le or 1no1denoe pproaohed ~ero

and consequent y . 11 t · reflected ; theretoJ'Ie the light ource and

t · speaker · e pl. ecd so t t this condition e approao d as closely

s expedient. e e co dtt1on aros tn plac1 h rotating rror and

th e eondit1ona were· siJnulat d.

Th plac1 o:f the cond ,nei lens w

s r (ff t o evtls than of c. eo1ent11'1c ap

a tter of chooei the lee-

ta.rther fro the ligh . source a · d along tb.fl b

o s1bl po ition ne

ot li t and th1a

position ve a ll spot of light on the screen, was very intense but vc

9

.low xtmurn amplitude and very bad distortion on the peaks or the waves .

the other alternative gave a larger s ot ot light which was consequently

le .intens with uah gre ter amplitude and lees distortion on the wave

eak • This condition placed the condensing lens near the vibrating mir.

r or . The latter positi.on was chosen tor the pe nent location ot the lens.

Tb t tme axU could be shown S.n th ways ; ene by means of' a rota t 1ng mira

ror and t he other by a mirror vibl'i!!lting in a plane at right angles to the

ttr t 1rror. The latter thod could be used to advantage it a , raboltc

mirror could be used aa the ttret m11'l'O:r and the second could be placed

close to the first eo t hat a small a1ze could be used tor the second . It

the second could be driven ln eynchroniem with the ttrst , wlth a wave with

both Bides symmetrical to be shown this system could be worked out very nice.

ly. It an unsymmetrical wav were to be shown a device would have to be

introduced to stop the light durt th instant when the light spot was

going one way on the screen. Tb.e rotat1 type ot time axis mirror lends

1tselt very readily to showing waves of dif'terent frequencies and,a1nce

this type was available , wa selected tor this oscilloscope. It consist d

of tour rrora set at right ngl s and mounted on a vertical shaft so

mount d that the drtvt motor ran 40 times taster than the mirror. The

gear1 s of the orm type and gave an even control C"er the. mirror

spee by means of a potentiometer in the motor circuit .

peech sound vee sh01J the.ir complex formation on th1a osc1lloeeope

ith great brilliancy . 'l'he response is much greater at the low r tntquen-

c1e as was pointed out previously. Sixty cycle current or volta e waves

show the sine curwature with good detail. Th uses ot this device are le-

gion and no attempt ill be made to enumerate the all .

10

v DISOUt'C'l1() . OF SULTS

~ tar as 1t bas been able to ascerta in, this type oscilloscope has never

boon used to ascertain t he frequency of an o se rved wave a lthough large num- ·~

ber of articles have b en read on t hi s and rol t ed subjects. n a wave pat-

t rn 1a observed the mir ror otor sp d may be varied until the ave pattern

is stationary. • t chacotcr y be used tor determini the motor ape d . The

relation between otor and mirror sp e 1s a confltant , thereto the mirror

epee or rot tions per ninut arc known. The distance trom tho rot tins mirror

is fixed at ten inches and t he a ve patt rn may b me sured directly on t he

screen. fh1s gives enough data to determine the f requency ot tho obse rved .ave.

The follow! f ormula explains t procedure .

t : ?JI J: 10 X . P . r.~. 60x40 L

• 0 .02618 X R. P. • .. 1.

ere: r : cycles per second L : l ength of w ve 1n inches

: r otations per inut of motor

this relation that doubling the R.P. ; • would

double L i t the frequency r mains t he sam • This is exactly what happens

and provides a observations . chart s howing the relation

between otor peed and wave lengths w.itb line ot eo.netant frequency , be.-

tween limits of · pproximatel y 2000 eyolea per second fn.dzero cycles per

cond.

rany improv nt ha elf evident as wor has regressed and

some of the e wil l be tabulated .

mirror driving unit with a htgher frequency response would add greatly

to the utility and versat111 ty of this $8C:Ulosco;pc. Hi gher order ha:rmonioa

could be shown in more detail . nd pass , high and low trequenoy filters

could be applied to the input and t heir ef'tect shown visually.

11

rota't1 1:rror • tth la e n er ot t ce woul d eho the Mttel'rl8

'With granter brilliance as t he otor e,Pt~ted and oonee uentl y t he m1nor

peed could b reduo d . The pe·rtoda ot darkness ot the ecreon would be :redue d

and trequenciee oY r w1 ft\Dge could be ehown ae a continoue waTe a t much

slower irror spec • mbe nUI'ft er or irror facoe for t he rotating

· rror woUld be d11'eotly proport tonal to t he l e h ot th ecrcen nd to t l1e

dist nee ot th rotati rror tro t he seMen.

on of t h pr ry for t his oe~illoec e nd on h1ch t h uthor

bel t •ee te t o enabl ·. t h student of p ice t o bett r und ret nd

t l1 to and Mt\U"e ot ny ot the OOJ:lll) u;ot our everyday l i,fe ;pl'Obl e .

u1noere effort bae 'b expended to o this t nv eti t ton thorough and

co pate and to compile o. report whl oh would be a orcd1 t to t he author nd

to th ohool. • ell that tho tforts v bee-n mediocre prevails .

1

2

6

7

a

Vl

Appendix

omson, 1824- 1007, 8 one of the world: greatest and was rn1sed to the pee:m e for h1 bril l i ant

titl s Lord KelTtn.

William Boi Duddell, 1872-1907, Flectrical ~ngi er.

8 a brilliant British

Andr E. Blondel, F neh F.leotrleal Engineer and Professor t t h ation 1 .. oe nic 1 College in ris •

. . . . K. • Oplinger , 200.

eetrical in ri , Voltnne. 53 , 1934, ge

• A. rrison* g 368, 1929 .

11 Telephon Labore.tor1 s Journnl , 1olume 8

R. F. 1 llina, 11 . boratories l<ecord, Vol • 13 , o . l ept . 1934, page 17.

oseph • !.egg , 1 ctr1eal J'ournal , July 1927, Volume 24, No. 7 ge 41 .

l ~ •

11

SINE CUEVE

Thia picture was taken by allowing the light trom the oacilloacope

to tall directly on the tilm and the inegulari ty 1a probably due to

the ditticulty ot placing the tilm ao that the time axis mirror could

rotate tre•l7•

14

III. TWO SINE CURVES WITH ZERO AXIS

This uposure was made similar to number II. with the exception that

a time axis was gi-ren and a wa-re 180 degrees out ot phase with the

first wave. The curvature at one peak was round to be due to iron fil-

ings in the air gap of the dynamic apeaker.

l6

lV"' OOf."J? .. " ' D. &.0

•

v ..

IND. ~~~ 1/o~ N07i

osc. I II

L

=w osc. osc.

III JJl

in txt u e to

t1