US1877243A

US1877243A - ising

Info

Publication number: US1877243A
Authority: US
Publication date: 1932-09-13
Anticipated expiration: 1949-09-13

Description

2 sneet-sheet 1 Sept. 13, 1932. G. lslNG GALVANOMETER Filed oct. 1e. 1929 Sept. 13, 1932. G. lslNG 1,877,243

GALVANOMETER Filed tOct. 116, 1929 2 Sheets-Sheet 2 Patented Sept. 13, 1932 dUNE'ED STAT-ES PATENT ortica 11.

GUSTAF ISING, F STOCKS'UND, 'SWEDEN GALVANOMETER Application filed October 16, 1929, Serial N'o, 399,991, `and in Sweden October 19, 1928.

The present invention relatesto moving-l which the said conditions are all simultane-y ously complied with.

As is well known, there has existed a certain definite limit above which the sensitivity lof a galvanometer could not be increased. y Said limit was determined by the occurrence of such irregular. oscillations of the movable system about its position of equilibrium asl are caused either by shocks of the molecules of the surrounding medium or by small. ir-

:regular current fluctuations of molecular v origin in the closed circuit. From the equipartition principle, it follows that the mean amplitude of the resulting Brownian movement of the coil will be the same, whatever Y. be the cause of this motion.

According to the invention, the maximum theoretical limit of the sensitivityof a galvanometer is in eect reached, on the one hand, by increasing the optical accuracy to such 'extent as to make the said Brownian Cil fluctuations visible; and on the other hand, by

choosing a theoretically favorable value ofv the damping of the movable system; and by removing` the influence of external disturb# ances and concussions. The ideal conditions contemplated by the present invention are as Y follows: Y i (a) The magnification of the deflection produced by optical means should be so high,

i that the mean value of the Brownian fluctuations corresponds to a small fraction, for instance 1/15-1/10, of a division on the readingofi' scale, which scale is itself just discernible by the naked eye. (t) In order to reduce the apparent current fluctuations, which correspond to the Brownian vibrations of the coil, as nearly as, possible at yfixed values of the time required for the deflection and at fixed values ofthe resistance of the coil circuit, the electromag- Vnetical dampingshould not be made too small.

If the so-called critical damping (which corresponds to the aperodical limit) is chosen as unit, the said electro-magnetical damping should not be appreciably smaller than unity and should preferably considerably exceed the air damping. y'

(c) The instrument should be free from external disturbances to such extent that the temporary displacements of the Zero position, caused by such external influences are, on an average, smaller than the mean Value of the Y Brownian fluctuations.

In order to embody the condition (a) the coil is in one form provided with `a light pointer, consisting, e. g., of a fine quartz or glass wire, the end of which is observed underv a very high microscopic magnification. ln order to malte it possibleto use an observation microscope with high numerical aperture, the instrument is so devised that a powerful observation objective may be placed close to the end of the pointer. At very high microscopic magnification, the image of the pointermay be rather blurred, even with high numerical aperture. In order to increase the accuracy of the readingvstill further, means are provided by which it is made possible to make clearly visible the diffraction fringes accompanying the image of the pointer and to read off the deviation of the pointer by observing the displacement of these fringes on a scale. i y The condition b) may be deduced from the following reasoning VIf 15 is the deiiection of the coil system, and A the elastic control (i. e. the restoring moment for unit angular displacement) the mean value (t of the amplitudeof the irregular Brownian deflections, according to the equipartition principle, is determined by the expression of freedom of the system, said energy at room rent variation, corresponding to the value of determined'by will be 'found tobe in which the function f (a) gives the ratio Ver1.

in which expression R is the total resistance "of the galvanometer circuit, Gr is a quantity fdep'endent on the damping of the movable system, and t is the time of indication, i. e.l the time elapsing from the closing of cuit until the movable system, within a certain tolerance (in the present case, 1/100() of the final deflection), has reached its end position. lf a and al are the values of the total damping and the air damping respectively, measured in the critical damp-ing as unit, the factor G in (2) may be written I @zal t/T between the time of indication t and the period of oscillation'l (in undamped con-y dition).

As may be seen from (2), the obtainable current sensitivity, which is obviously inversely proportional to z', is proportional to On the other hand, the factor G is also of considerable importance for the result, and

sliould'be made as small as possible. The

curves in the accompanyingdiagram illustrate the relation between the factor G and the value of a at three different values of al. As may be seen from the curves, the smallest value of G fory small values of al corresponds to a value Vof a slightly less than unity, while, for any value of al, a .somewhat higher but still small value of G may be obtained by making a great and considerably greater4 than This is obviously just the condition (t) which should be satisfied at least when the external resistance of the coil circuit is small, i. e. when the instrument is short-circuited. As will be seen, the condition a1 a which is included in stands in acertaincontradiction to the condition (a), as the limitation of the air damping will obviously limit the pointer length, while, on the other hand, a great pointer length is of course favorable for Vreading small deflections. It will accordingly be necessary to compromise between the conditions (a) and (b) in regard to the pointer length. It will be explained below inV what way this may be done to the best advantage.

The condition (o), according to which the instrument should be substantially free from Vexternal disturbances, is Vcomplied with by making the coil exactly symmetrical with respect to the rotary axis and suspending it the cirwith such smalldimensions, and in lsuch ay shape, that it can be placed on the stage -of Aan `vordinary lmicroscope, the fordinary con- Vdenser of the microscope A.b eingthen preferably replaced by a weal microscope objective. Designing the instrument in said manlnerhas the vadvantage that the optical equipment I and the adjustment devices of an existing microscope may be used in connection with the use of the galvanometer.

YA further suitable arrangement which is useful, particularly in connection with very delicate coils, consists in providing they coil with a connecting piece in the form of a thin rodextencing in the direction of the axis of symmetry (rotary axis), which rod connects` Y two diametrically opposite points ofthe coil and serves as a stilfening spine of the coil. lThis spine facilitates an-exactattachment of the suspension wires (torsional wires). lt

.takesup the tensile stress of these wires so that said stress doesfnot deform the delicate windings ofthe coil. Furthermore, the said spine is adapted to beengaged by the damping means of the instrument when the coil is to be arrested.

The invention will be more scribed with reference to the accompanying drawings.` Figure lis a verticalv section on the line l-l in Figure 2 of a moving coil galvanometer designed according to the invention and adapted to be mounted'on vthe stageof an ordinary microscope. Figure 2 shows the same instrument partially in plan view and partially in section on the line 2-2 in Figure l. Figure 8 is a vertical section on the line 3-3 in Figure l. Figure 4 is a side elevation of the instrument as viewed In the embodiment according to Figures l `to 5 the moving coil system is disposed in a rectangular casing 1 the bottom side of which extends into the bearing plate 2 adapted to rest on the stage in an ordinary microscope and .provided with a circular aperture 3 the centre of which co-incides with the axisiof the microscope at the correctpositioning ofthe instrument on the microscope stage.

The moving system of the instrument consists of a circular coil 4L suspended between two fine torsional wires, stretched by means of springs 5 fixed at projections 6. The coil can turn in usual manner between the pole faces 7 of a permanent horse shoe magnet 8 enclosing the rest of the instrument which 3 pole faces project inwards through opposite sides of the casing. The coil14 surrounds an annular core 9 of soft iron supported by a collar 10 which projects into the casing from a plate 11 disposed inside the supporting plate 1 forming a side wall of the casing. In order to prevent deformations of the light coil and to facilitate tliearresting, two opposite pointsof the coil co-inciding with the axis of rotation are united by means of a stiffening connection 12, consisting of a rod or tube, for which axial bores'13 are disposed in 'the annular iron core 9.

In the inner cavity of the annular iron core 9 an arresting device for the coil is disposed and adapted to engage the rod 12.'

Said arresting device consists of two holding clamps 14, preferably provided with a coating of cork or the like and adapted to approach or to move away from the rod 12 symmetrically.

The holding clamps consistof angular pieces the feet 15 of which are slideably disposed.

in horizontal slots in theplate 11, and are, by

means of helical springs 16 (Fig. 5) brought to bear against a conical member 17 disposed on the arresting screw 18. Said screw extends somewhat eccentrically through a bushing 19 which is rotatably journalled'in a disk 20 secured to plate 1. The outer end of the bushing 19 is provided with a threading carrying a stop nut 21.

On the drawings the clamps are shown in cal member 17 ismoved outward by turning the screw 18 the clamps then ybeing pulled together and squeezing therod 12. i

On accountV of the delicacyA of the movable system it is necessary that the holding clamps engage therod parallelly and simultaneously. 'To adjust the clamps to be parallel with the rod 12 the circular plate 2O is adapted to turna small angle after the screws v2O `(Fig. 4) have been loosened.

The symmetrical adjustment of the4 holding clamps to'equal distances from the, rod 12 takes place by turning the bushing 19 after loosening the stop nut 21. After the adjustment ofthe arresting device has been carried out in this manner the coil may safely be engaged by said device without a previous slackening of the suspension string.

The `coil 4 carries at its lower portion a pointer 22 consisting e. `g. of a line Vquartzoi glass wire. Said pointer projects into a flat box or pocket 23 extending from the casing 1. In the bottom side of the pocket a glass window 24 is disposed to illuminate the pointer. The upper side of the box 23 consists of a detachable lid into which a cap 25 turned upside down istapped which cap at its bottom is provided with "a thin glass window 26 disposed closely above the endof the pointerI opposite the window 24. The cap 25 is intended toV accommodate the ,observation ob.-

jective. By this disposition the observation Objective may approach the poillferWhlG the' interiorbf the galvanometer remains closed so that no disturbing aircurre-nts may penetrateltheleto. 1 Y i `The samegend can alternatively alsobe attained without the .concave cap426, in case the microscope objective is introduced into the interior of the instrument through an aper- 'ture in the topof thebox, the objective being then enclosed by a soft; packing, byway of Y example a rubber,V packing, at the` point of llllTOJICBOII.V ,1 i Y' y g Y `*The lid ofthe projecting flat box 23'carries in-the, shownl embodiment of the `galvanometerV also ltwo insulatedsmall metal plates` 27, (Figa 1) which normally are not in use,`

and are thencoupled` to the instrument casing butwhich lin certain researches may serve -toxreduceithe directional force of the movable system by means ofyelectrostatic charging i. e. to astatize the instrument. The indicater 22' should then be electrically conducting. 1

Although of an instrument which jcan be mounted in an ordinary microscope brings certain important advantages it may belsuitable in order to .obtain the utmostpermanency of thezero point,tomount the microscope objective on the galvanometer proper as is'dia-grammati- `cally shown in Figure 6.` In contradistinction to the above described embodiment in Y separated position. Atthe'arresting the conithe above described arrangementwhich the optic axis is parallel with the axis Y of rotation of the coil system the optic axis is accordingto the embodiment in Figure 61 perpendicular to the axis of rotation and parallel with the `pointer 22.` The latter is ibo providedvwith an extension 28 bent at a right angle which projects downward `between an observation objective29 disposed in a light aperture inthe instrument casingrl and-an illumination objective -30 also disposed Y close its to the pointer.- The light from a source of y the casing, which aperture is disposed lright opposite the light aperture of the ,observationy objective. The arrangement according'M i to Figure, Gvbrings the advantage that variations in the temperature Vand therefrom resulting `thermicexpansions only have the smallest possibleinfluence'- upon the position of thefobjective or the objectives in relation ,l

to the pointer, lin such a way that-evenat a very high magniiication the zero position of the galvanometer and the accuracy oflthe` image are'substantially variable. Y

Said arrangement has furthermorethe advantage that a central diaphragm 'maj7 be constructively embodied with the observation 'i' objective iny such a manner that said `diaphragm may be disposed on or removed from the objective from the outsideV withoutopening the instrument.j Such an easily shiftable arrangement of a central diaphragm in an instrument of the present kindbrings' certain advantages asWill'be described inthe following. In'the case of a high microscopic magni- 'icationthe reading-oif of the deflection is.

y preferablyrcarriedout by means ofthe difistration in caseV fraction fringes accompanying lthe blurred image of the indicator. f At a given value of the microscopic: magnification .the lWidth of the fringes are smallest i.v e. the accuracy in the readings-off highest, ifonly certain edge portions of the objective cooperate to produce the image. YThis is attained-by lusing a ce-n- 4 tral diaphragm." Especially in the photo-`r Y graphic registration of small deiiections said lcentral diaphragm is`of importance to obtain `the utmost sharpness of the fringes The 'diffraction' fringes obtainable Without centraldiaphragm are not quite so sharp but adequate for subjective reading-off. They liancy, be of advantage in photographic regtinctness is not required.' Y

TheV dimensioning yof the movable system l 'y "inclusive of the pointer Will be of special importance if the object in .View is to obtain the highest possible degree of sensitivity inthe galvanometer;V It ispreviously known (see Phil'. Mag; vol-I, April 1926, page 827) that there isa ,certain limit system. In order to Abe able to reach said limit Without" evacuating the instrument, yv'vhich would involve great vrdn'liculties both forY the manufactureand the operation of the instrument, one must follow certain rules st atedbelow in dimensioning the instrument. The damping by air frictionacting .upon the lmovable system should be less than the electromagnetic damping at closedcircuit, and ,should preferably 'only deliver a smaller portion of the criticaldamping i. e. the damping required to renderthe motionv of the system only just aperiodici f To dimension the movable system lof the galyanometer in such a manner that said .lim-

it of sensitivity is reached the following dimensioning rules have to be followed.

The Aair "friction/al resistance `against anarrow cylinder being displaced .transversely .With a velocity u creates a 'force. counteract- `ing the displacement Which per unit length amounts to plu Where 201 is a certaincoe'licient Vrepresenting the; influence of the airvisccsi-v Aty. This results in that the frictional vino-V ment Q exerted u1 on a straight ointer ro- 1 .jecting perpendicularly Vto the axis of rota.-

.tion and havingtheylength a cm. can be calculatedwith a gooddegree-of approximation from the formula may also, on account of their higher brilv the highest attainable dis- Y for the sensitivity, `depending'upon the erratic Brownian fluc- 35 tuation's inthepcs-ition of the galvanometer in ulichrexpeeoft is the4 angular Velocity y( p=vthe angularvdeiiection in radians). ln similar manner the frictional moment against the coil can be approiiimatelyjcalculated.y If it is desired that the `quantity Q calculatedfrom4 should constitute'the fraction l/m (by Way of eX;

ample l) of the criticalvdamping' moment (relating to thev a-periodic limit) and the di-V rectional force is set equal to (in CGS units), the total moment of inertia equal to Kv and the period of oscillation' reduced to the undamped condition equal to T, lone has If the value 2.1014'ergs of the' molecular' energy -at roomtemperature is'inserted in Equation (l) the followingk expression ,is ob# tained, A I

, c i@ .Y ln order thatthese small fluctuations may be rendered visibleon the reading-olf scale,

a very high microscope-magni-icaticn n must y be chosen, preferably such that the magnified average Brownian deflection is of the order of 6' of a division on the scale. The numberr n that isl the number of divisions on the scale corresponding to a displacement of one millinieterl in the object space of course represents here the useful magnification.

power of the microscope that jof a scale di- In other Words7 the magnification shouldbe so" ychosen in relation to the optical resolving vision corresponds to a displacement which Y i Y n is yoptically accurately discernible. deflec-` tion of one division then corresponds to the turning angle of 1 a-lOn radians. Assuming that the mean Brownian deviation, when magnified, equals T15 of a scale division, one iinds and thus, considering (3),

(II) A=4l0`10n2a2 The Equations (I) and (II) determine now in combination the quantities a and A in relation to the chosen values of t, m and-a i. e. they serve as basis for the dimensioning of the galvanometer system in lorder to `attain the theoretical limit ofy sensitivity of these instruments.

' I claim:

l. In a galvanometer in combination, a

magnetic field system, al moving-coil cooperating therewith, a stiifening rod connecting two oppositev points of they coil, elastic sus- .i

, and allowing of small rot-ary movements of the coil about the axis of said rod, clamping means adapted to cooperate with said stifening rod, a pointer projecting from the coil, a light passage through the instrument extending perpendicular to the plane of movement of the pointer end, and means for applying microscopic instrumentalities to said light, I passage for observing the movements of the pointer. 3. In a galvanometer m combination, a magnetic iield system,'a moving-coil cooperating therewith, a stiifening rod connect` ing two opposite points of the coil, elastic suspending means connected with Asaid rod and allowing a rotary movement'of the coil about the axis of said rod, two holding clamps disposed on opposite sides of said rod, and

' means for adjusting said clamps to be parallel withand symmetrically positioned Vin relation to the rod so as to engage the rod simultaneously. i

4. In a galvanometer in combination,y a magnetic field system, a moving-coil cooperating therewith, elastic suspending means carrying said coil and allowing of small rotary movements thereof about' a certain position of equilibrium, a short pointer proj ecting from the coil, a light passage through the instrument extending perpendicular to the plane of movement of the pointer end, anV observation objective mounted on the galvanometer and applied toy said light passage, and means for adjusting said objective with vrelation to the pointer end.

5. VA galvanometer as claimed in claim 4 characterized in that the observation objective is provided withV` a central diaphragm which is constructively embodied with the objective in such a manner that it easily may be placed in positionon or removed from the objective without opening the instrument o changing the objective setting.

6. A coil galvanometer as claimed inclaim` 3 characterized in that the holding clamps are pressed by means of springs against at wedge member which is displaceable between the clamps by meansof a screw extending excentrically through a rotatable bushing.

7. In a galvanometer in combination', aV magnetic field system, a moving-coil operatingtherewith, a casing enclosing said moving-coil, elastic suspending means carrying said coil and allowing of small rotary movements thereof aboutk a certain Vposition of equilibrium, a pocketprojecting from the casing, a pointer projecting from the coil into said pocket, the bottom and lid of the pocket f being'provided with lightv apertures, and

' means for applying microscopic instrumentalities to said light apertures for observo ing themovements of the pointer.

8. In a galvanometer as claimed in claim 7, a concave adjustable cap screwed into the lid and forming a. recess for accommodating an observation objective, the lightiaperture being arranged at the bottom of said concave cap. 9. In a galvanometer in combination, a magnetic field system', a movin-g coil, said coil being symmetrical with respect to its rotary axis and co-operating'with said mag-V netic field, suspending torsional wires stretch.- ed in opposite directions from the zcoil and allowing of small rotary movement'sthereof about a certain position of equilibrium, a-

pointer projecting from the coil, 4and means for applyingy microscopic instrumentalities for reading of the deliectionof said pointer,

Vthe length-of the pointer bearing a relation to the elastic control and the moment of'inertia such that therBrownian uctuations: are

perceivable at the microscope magnification j used, and that the damping moment setl up by'theair friction is smaller than the electromagnetic 'damping-moment. In testimony whereof I affix my signature.

' GUSTAF ISING.