US2291713A

US2291713A - Galvanometer

Info

Publication number: US2291713A
Authority: US
Publication date: 1942-08-04
Anticipated expiration: 1959-08-04

Description

Aug- 4, 1942. y I G. Hl-:FLEY l '2,291,713I

GALvANoMETER Fild Feb. u14, 41940 i Patented Aug. 4, 1942 GALVANOMETER Gilford Heiley, San Antonio. Tex., assignor to Olive S. Petty, San Antonio, Tex.

Application February 14, 1940, Serial No. 318,926

'1 claims. (ol. 1v1-95) This invention relates to improvements in galvanometers, of the so-called string type, in which one or more strings are supported in a magnetic field, whereby on the passage through the string of a pulsating current, the string is caused to `vibrate transversely of the direction of the fleld. An optical system is usually associated with the string or strings, whereby the vibrations may be recorded on a sensitized sheet.

The invention is particularly concerned with string galvanometers of the type in which the string is substantially inextensible, a yielding support being provided to permit iiexure and lateral vibration of the string. For example, it is a common expedient to anchor one end of the string to a rigid support, and to connect the other end to a second rigid support through a coil spring, the latter being extended as the string is displaced laterally. While this type of suspension is reasonably satisfactory, it has certain In the first place, while a coil spring may bedescribed -as having a low rate, in that the restoring force exerted thereby increases very slowly with the increase in elongation of the spring, the rate is not sufficiently low to be ignored in sensitive instruments, for example, galvanometers such as are employed in the recording of seismic impulses in the eld of geophysical exploration. 'I'hus as the elongation of a coil spring associated with a galvanometer string increases due to lateral displacement of the string, the restoring force is also increased; andthe record of string displacement is therefore not directly proportional to the amplitude of the applied impulses so that distortion of the record results. Again, the change in restoring force exerted by the spring for deflections of the string of different extent results in variationof the natural frequency of the string. Furthermore, surging of the spring is likely to occur, and it occasionally happens that adjacent turns of the spring will directly engage during the reception of signals of high amplitude, causing serious distortion of the record.

A second objection to the use of a coil spring in a string galvanometer is based on the fairly high electrical resistance offered by the spring. This resistance not only induces a loss as ,the direct result of the resistance offered to the passage of electrical energy, often approximating a 50% loss in the case of copper strings having a resistance of ohms, but an indirect loss occurs as the generates a back E. M. F. which' is proportional to the number of lines of flux of induction .cut by the string in a unit time. This E. M. F. produces a current tending to damp the string, and

the larger the current so produced, the greater the damping for any given 'E. M. F. It c-an be shown that the damping current is proportional to l/R, where R is the total resistance of the string and spring in series with the external shunts. If, however, the combined liesistance of the string and spring is halved, the resistance of the shunt may also be halved, and the same ratio of energy dissipated in the string and in the shunt may be maintained. yet the total resistance in the damping circuit constituting the string and` shunt is halved. Hence, the damping will be doubled. By means of the present invention it is possible to reduce the resistance in the spring from -about 5 ohms to about .1 ohm; consequently, the damping characteristics of the system may be tremendously improved. l

A third objection to the use of the conventional l coil spring for suspending a galvanometer string lis based on the occurrence in coil springsV of objectionable transverse vibration, the natural frequency of vibration of the spring often falling within or close to the range of received impulses. These vibrations may apparently be classified as forced vibrations imposed on a Aresonant system; regardless of their nature, they are found to be highly objectionable in that they interfere with proper recording of the received signal, and it is therefore quite desirable to employ ajspring having a natural frequency which is more remote from the desired signal frequency than is afforded by the conventional coil spring.

It is the object of the instant invention to avoid these various objections to the use of a coil spring by the employment of yielding means of a character such that these difficulties are minimized or completely overcome.

More specifically,y it is an object of the invention to replace the usual coil spring in a string galvanometer with a spring whichmay be regarded as a form of leaf spring, but which is so shaped as to avoid o'r compensate for the objectionable high rate which is characteristic of leaf springs in general. In the preferred form of the invention, this is accomlished by the use of a nat or leaf spring which is'generally heart shaped and which is preferably disposed in the plane of lateral vibration of the spring. It is found Y that such' a spring affords in effect a lever arm 'or arms of which the length varies with ilexure result of low damping. Thus a string in motion S0 as nearly or completely to compensate for the increased restoringA force developed on flexure. Because cf the relatively short length and increased cross-sectionalarea of such a spring as compared with an equivalent coil spring, the electrical resistance may be greatly reduced'and the natural transverse frequency may be greatly in' is secured to the string 2l. The width of thel spring 30, measured transversely of the plane of Figure 2 is an end elevation of the structureshown in Figure l; y

Figure 3 is an enlarged view of certain of the elements shown in Figure 2, including the -preferred type of supporting spring;

Figure 4 is a plan view of the elements shown in Figure l which provide the magnetic field;

Figure 5 is a fragmentary sectional View taken substantially on the line 5-5 of Figure 4;

For convenience in describing the invention, reference is made herein to the accompanying drawing illustrating a preferred embodiment thereof, and specific language is employed to describe the same. It will nevertheless be understood that no limitation of the scope of the in vention is thereby intended, various modifications and alterationsof the illustrated structure being contemplated, such as fall within the spirit of the invention as outlined in the foregoing general description and recited 4in the appended claims. J

The invention has been illustrated as applied to a conventional type of multiple string galvanometer employing a magnet indicated generally at I0 including a core II, pole pieces I2 and one or more windings I3 surrounding the core, the

windings being energized from conductors I5 in .which is caused 'to flow a current for the -which is driven at a constant speed past the beam, vibrations of the strings thus producing traces on the sheet which are representative of such displacement. Pulsating current, representative of a series of separately generatedimpulses which it is desired to record, is applied .to each string, and the amplitude of vibration of each string in a plane parallel to the faces'of the -p'ole pieces is thus representative of the amplitude of the pulsating current.

The arrangement thus far described is entirely conventional, and no significance is attached tol the details thereof. It is contemplated that anyv .equivalent system may be employed in lieu of that shown, the novelty in the instant case residing solely in thel method of suspending each of the strings. which is about to be described.

'I'hus each string is secured at one end thereof,

' spring. In other words, we would have an equivfor example the lowerend, to a fixed support v 22. Secured toa second fixed support 2 2 isal plurality of springelements, one being secured to the adjacent end of each string. These spring elements are of the flat or leaf type and aregenerally heart shaped'as -shown more lmrizicularlyA in Figures 2'and 3, as viewed ina direction perpendicular to the plane of vibration 'of the strings. The convex apex '28 ofthe spring Il may be secured directly to the support 22, but isl preferably connected thereto by means of a wire orroaaawhichissecureatothempportand' to the spring, the rod having such rigidity that it does not partakeof the vibration, imparted to the string. The concave apex 33 of the spring vibration -of the string, and the thickness of the,

spring, as measured in the plane of vibration o! the string, are of course so selected as to apply to the string a restoring force which is suillcient for the pllrDOse of providing a satisfactory record of thereceived impulses, but it will be appreciated that these dimensions are in general such that the electrical resistance of the spring willbemuchlessthanthatofacoilspringof adequate restoring :force .and suilicient in other respects for the intended purpose.

Whenan electrical impulse is applied to a stringsoastoinduceinthestringlateralvi- .brations, the associated' spring 30 will be ilexed and will move toward the position shown in dotted unes m'Figurc a. In any leaf spring is can be shown. that while fiexure actually occurs over the entire length of the spring, it may be that the spring ilexes about a point intermediate .the ends thereof for the purpose of determining the leverage exerted by the` spring. While this point may be displaced slightly during ilexure,

'the displacement is not considerable. In the case of the spring disclosed herein, it may be assumed that iiexure occurs approximately at the point 35 on each arm of the spring, and the effective leverage exerted by the spring may therefore be measured by the perpendicular distance between this point and the axis of the aient construction if we provided an articulated system of ngid levers extending between the

points

28, 35 and 23, and yleldingly anguiar displacement oi these levers by the provis ion of torsion springsLor the like at one of the points 35 and 2l, increase with spring flexure. Consequently, although the restoring force also increases on exure, the increase in restoring force is approximately compensated by the increase in the lever arm, so that the tension spplied to the string tending to restore the same to its initial position is approximately constant regardless of the amplitude of displacement of the string'. Thus by the use of this 'type of spring, I have in effect materially reduced the spring rate, and correspondinglylessened the extent of distortion of the record. Furthermore, since the tension applied to the string does not materially vary, I may employ a spring having a relativen large restoring force when in repose.

- In the recording of seismic impulses, the natural frequency ofthe impulses is of a relatively low order. Consequently, it is diiiic'uit to design 'a coil spring which is otherwise satisfactory as regards restoring force and other characteristics, which is not at the same time characterized by a correspondingly low natural frequency of lateral vibration It will be appreciated that in general the leaf or flat spring has a much higher natural transverse frequency than an equivalent coil spring, and it canbe shown that in the case of the spring illustrated herein, a natural frequency of lateral vibration can be established, which is at least three times that of a coil spring, and the spring is accordingly substantially free from critical vibrational periods. Thus it is possible by use of the spring disclosed herein to avoid undesirable vibrations and consequent blurring of the record line. That the electrical vresistance of the present spring may be reduced a conducting string is supported for lateral diswell below that of an equivalent coil spring is quite apparent, and the advantages inherent in the use of a spring of low electrical resistance have been' heretofore outlined.

It will be appreciated from the foregoing that certain features and advantages of the instant invention may be achieved even though the shape of the spring varies to a considerable extent from the preferred shape illustratedherein, and that in its broader aspect the invention is not limited as regards the shape or size of the spring.` It will also be understood that changes in the transverse sectional contour of the spring may be effected without materially altering the result. For example, the spring may be formed of round rather than at stock, and the' spring is described herein as a fiat or leaf spring merely in order to distinguish over the conventional coil spring, the term being used in a broad sense.

Havingv thus described the invention, what is' claimed as new and desired to be secured by Letters Patent is:

1. In a string galvanometer of the type in which a conducting string is supported for lateral displacement in a magnetic neld, supporting means for the string including a spring having the ends thereof disposed substantially in the string axis and having an intermediate'portion extending laterally of said axis and beyond said ends as measured in the direction of said axis, said spring ends being secured respectively to the string and to a support so as to increase the spacing 'between the ends on increase of spring displacement, saidspring being so constructed and shaped that the force exerted thereby on said string acts through a lever arm which increases with string displacement.

2. In a string galvanometer of the type in which a conducting string is supported for lateral displacementin a magnetic eld, supporting means for the string including a generally heart shaped spring having the apices thereof disposed substantially in the spring axis and connected to the` string and a support respectively so as to increase the spacing of the apices on increase of string deflection, said spring being so constructed and shaped that the force exerted thereby on said string acts through a lever arm which in- Y creases with string displacement.

3. In a string galvanometer of the type in which a conducting string is supported for lateral displacement in a magnetic field, supporting means placement in a magnetic field, supporting means for the string including a flat spring having the ends thereof disposed in the string axis and connected to the string and a support respectively so as to increase the spacing of the spring ends on increase of the string deflection and having an.

intermediate bowed portion tensioned to urge said endstowardeach other, the bowed portion of 'said spring extending beyond said ends as measured in the direction of the string length, said spring being so constructed and shaped that the force exerted thereby on said string acts through a lever arm which increases with string displacement.

5. In a string galvanometer of the type in which a conducting string is supported for lateral displacement in a magnetic field, supporting means for the string including a flat spring having the ends thereof disposed in the string axis and con-l nected to the string and a support respectively so as to increase the spacing of the spring ends on increase of the string deflection and having intermediate, similarly shaped bowed portions extending on opposite sides of said axis and beyond said string ends, as measured in the direction of the string length, said spring being s constructed and shaped that the force exerted thereby on said string acts through a lever arm which increases with string displacement.

6. In a string galvanometer of the type in whichy a conducting string is supported for lateral displacement in a magnetic field, supporting means for the string. including a nat spring having the ends thereof disposed in the string axis and connected to the string and a support respectively so as to increase the spacing of the spring ends on increase of the string deflection and,having intermediate. similarly shaped bowed portions, .extending on opposite sides of said axis, said u a spring being so constructed and shaped that the leverage exerted byV said bowed portions increases with string displacement to compensate substanl tially forincreasing spring tension.

'I.` A spring for use in string galvanometersto apply slilllstantiallyk constant sp1-lng tenso varying string displacement, said spring being generally heart shaped and having the apices so connected respectively to the string and to a fixed support as to increase the spacing between said apices on increased string flexure, said spring being so constructed and shaped that the force exerted thereby on said string acts through a lever:v arm which" increases with string displacemen GIFFORD HEM-EY.