Tuning Fork Construction For Electronic Wrist Watches

Abstract

A tuning fork construction for electronic wrist watches including a substantially U-shaped tuning fork oscillator and a U-shaped magnetic circuit mounted at the end of each tine of said tuning fork, each of said magnetic circuits including a pair of mounting plates secured to the respective tine and carrying at least one substantially semicircular permanent magnet. A coil is positioned intermediate the mounting plates of both of said magnetic circuits in the path of the flux thereof.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to wrist watches having tuning forks, wherein the tuning fork is utilized as a time standard and the oscillation of the tuning fork is converted into the rotary motion of a ratchet wheel and transmitted to the index mechanism of the watch.

The conventional tuning fork construction for electronic wrist watches incorporates a magnetic circuit formed with a cylindrical permanent magnet disposed in a cup and a hollow cylindrical coil inserted in the space between said cup and magnet. The magnetic circuits of the two lines of the tuning fork are each closed in order that they may not affect each other, and cause fluctuation in frequency. However, the shape of such cups is substantially cylindrical, and such having fork constructions cannot be utilized for thin-type wrist watches. Further, such prior art constructions proved expensive to maintain and assemble. By providing the U-shaped magnetic circuits according to the invention, and by providing means for adjusting the frequency of the tuning fork, the foregoing deficiencies in the prior art have been avoided.

SUMMARY OF THE INVENTION

Generally speaking, in accordance with the invention, a tuning fork construction for electronic wrist watches is provided including a substantially U-shaped tuning fork type oscillator, a U-shaped magnetic circuit associated with each tine of said tuning fork, each including a pair of spaced overlapping mounting plates mounted on the end of each tine, and carrying at least one substantially semicircular permanent magnet, and a coil positioned in the space between said mounting plates in the path of the flux of said magnetic circuit, said permanent magnets being oriented to produce a flux which follows a path including the mounting plates and the end of the tine associated therewith.

A pair of semicircular permanent magnets may be mounted respectively on each mounting plate of each of said magnetic circuits in spaced relation, and said coil may consist of a single circular coil positioned in the space between both pairs of permanent magnets in overlapping relation therewith. The end portions of the tines of said tuning fork may be substantially arcuate to permit accommodation of a substantially circular battery within said watch.

Said wrist watch may include a driving circuit including a pair of transistors of different polarities, one of the two terminals of said coil being connected to the collector of each of said transistors.

A frequency adjusting device may be provided consisting of an adjustor member having an eccentric center of gravity and pivotably mounted on at least one of said mounting plates of said magnetic circuits for selective pivotable positioning in the plane of vibration of said tuning fork. The surface of said mounting board may be provided with a plurality of recesses, said adjustor member being provided with means for registration with said recesses for the selective positioning of said adjustor member. Further frequency regulation may be provided by including a member formed of magnetic material positioned in the magnetic field of the permanent magnets of each of said magnetic circuits for attraction by said permanent magnets, or providing a permanent magnet near the oscillating tines of said tuning fork for attracting said oscillating tines. Further improvements in the operation of the tuning fork according to the invention may be achieved by controlling the dimensions and positioning of the components of the magnetic circuit according to the invention.

A pair of mounting plates of the magnetic circuits according to the invention may be secured to the respective tines by means of one or more posts extending through said mounting plates and the end of said tine. A further post may be provided in a region of said mounting plates out of overlapping relation with said tine, said further post being providd with a region of increased diameter for defining a spacer for supporting said mounting plates in the desired spaced relation.

Accordingly, it is an object of this invention to provide a wrist watch having a tuning fork designed for ease of assembly and mass production, while being thin, thereby providing a relatively inexpensive thin wrist watch.

Another object of the invention is to provide a tuning fork construction for wrist watches provided with an easily regulated frequency adjusting arrangement, and wherein the frequency is stable even though the amplitude of the tuning fork is changed by reduction of battery voltage or fluctuation of load.

Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a top plan view of a wrist watch having the tuning fork construction according to the invention;

FIG. 2 is an enlarged top plan view of the end portion of one tine of the tuning fork of FIG. 1;

FIG. 3 is a sectional view taken along line A--A' of FIG. 2;

FIG. 4 is a top plan view of the frame of the coil of the embodiment of FIG. 1;

FIG. 5 is a circuit diagram of a driving circuit for use with the wrist watch having the tuning fork according to the invention;

FIG. 6 is a circuit diagram of another embodiment of the driving circuit for use with the tuning fork construction according to the invention;

FIG. 7 is a top plan view of a second embodiment of the tuning fork construction according to the invention;

FIG. 8 is a sectional view taken along line A.sub.1 -A.sub.2 -A.sub.3 of FIG. 7;

FIG. 9 is a partial enlarged side elevational view of the tuning fork of FIG. 7;

FIG. 10 is an enlarged partial perspective view of the mounting board of the magnetic circuit of the tuning fork embodiment of FIG. 7;

FIG. 11 is a side elevational view of another embodiment of the frequency adjusting mechanism according to the invention;

FIG. 12 is a top elevational view of a third embodiment of the tuning fork construction according to the invention;

FIG. 13 is a graph showing the amplitude and frequency characteristics of the tuning fork construction of FIG. 12;

FIG. 14 is a sectional view taken across the magnetic circuit of the tuning fork construction according to the invention showing the dimensional relationship between the portions thereof;

FIG. 15 is a partial top plan view showing still a further embodiment of the tuning fork construction according to the invention; and

FIG. 16 is a partial perspective view of the embodiment of FIG. 15.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIGS. 1 - 3, the wrist watch according to the invention depicted includes a voltage source of direct current 1, a tuning fork 2, a coil 3, mounting plates 4, a frame 5 for supporting coil 3, a ratchet wheel 6, and a block 7 incorporating the driving circuit for said coil connected by a terminal 8 to the negative pole of battery 1 and by terminal 9 to the coil 3. The diameter of the battery 1 is generally somewhat smaller than the radius of the movement of the wrist watch, and the tuning fork 2 is provided with arcuate tines in order to accommodate the substantially circular battery 1. The ends of each tine of said tuning fork is connected to a pair of mounting plates 4a and 4b by means of pins 10 as shown in FIG. 2. Said mounting plates are maintained in spaced overlapping relation. A permanent magnet 11a and 11b is mounted respectively on the inner surface of said mounting plates 4a and 4b. The polarities of permanent magnets 11a and 11b are disposed to form a closed magnetic circuit wherein the magnetic flux passes through the space between the magnets, through mounting plate 4b, through the intermediate portion of the tine of tuning fork 2, through mounting plate 4a, and back to said permanent magnet. If desired, only one permanent magnet may be provided in the magnetic circuit associated with each tine, mounted either on the upper or lower mounting plate.

The coil 3 is positioned in the space defined by permanent magnets 11a and 11b. Both the tines and the magnetic circuits associated therewith are formed symetrically for balanced oscillation. Coil 3 is fixed to frame 5, shown more particularly in FIG. 4, and the frame and coil assembly is inserted in the space at the end of the tuning fork between the mounting plate 4, leaving a proper spacing so as to avoid contact between said coil and the tuning fork and the components of the magnetic circuit. Frame 5 is then fixed to the plate of the watch. Conductive portions 12 and 13 are formed in the surface of frame 5 in order to receive the lead wires of coil 3, and to provide a conductive path between the driving circuit 7 and the battery 1. Frame 5 is preferably formed from a printed circuit board having a thin copper plate deposited thereon, the pattern of the desired conductive surface being defined in the thin copper plate by removing portions of said plate utilizing an etching reagent.

Referring now to FIG. 5, one embodiment of a driving circuit utilized to drive coil 3 is depicted, the electronic components of said driving circuit being being preferably mounted in block 7 of the watch according to the invention.

Said circuit consists of a battery 14 which could correspond to battery 1, a coil 15, which could correspond to coil 3, NPN type transistor 16, a PNP transistor 17 having its emitter connected to the emitter of transistor 16, a base resistor 18 connected between the base and collector of transistor 16, a trigger condensor connected between the base of transistor 16 and one end of coil 15, a variable resistor 20 connected between said end of coil 15 and the collector of transistor 17 for controlling the amplitude of oscillation, and a base resistor 21 connected between the base of transistor 17 and the other end of coil 15. Battery 14 is connected between said other end of coil 15 and the collector of transistor 16. This circuit is known as an astable locking oscillator. Other driving circuits utilizing a single coil may be utilized, and the tuning fork arrangement according to the invention is not limited to a particular driving circuit. A second embodiment of a circuit suitable for driving the tuning fork according to the invention is depicted in FIG. 6. Said circuit consists of a battery 22, a PNP type transistor 23, a NPN type transistor 24, a coil 25 which would correspond to coil 4 connected in series with variable resistor 30 between the collectors of transistors 23 and 24, a trigger condenser 26 connected between the base of transistor 24 and the collector of transistor 23, a base bias resistor 27 connected in parallel with trigger condenser 26, a base bias resistor 28 connected between the base of transistor 23 and the collector of transistor 24, and a decoupling condenser 29 connected between the base and collector of transistor 23. Decoupling condensor 29 serves to prevent the oscillation of harmonic waves, while variable resistor 30 is utilized for regulating the amplitude of oscillation. The circuit of FIG. 6 is particularly adapted for driving the tuning fork according to the invention through the use of a single coil. Variable resistor 30 may be dispensed with if not required.

It is frequently required to provide a frequency adjuster device for a time keeping vibrator such as the mechanical, electro-magnetically driven tuning fork according to the invention. Three approaches may be followed to regulate the frequency of a tuning fork. First, the equivalent length of the vibrating tines may be controlled. The second and third methods involve the regulation of the elasticity of the tuning fork, either by a mechanical or a magnetic method. The regulation of the equivalent length of the vibrating tines may be performed by turning a rotatable mass which has an eccentric center of gravity and is placed on the free end of the respective times. In the art, such frequency adjusters have taken the form of an adjusting member pivoted along an axis parallel to the vibrating plane of the tuning fork. Such an adjusting member is rotated in the vertical direction relative to the watch crystal, so that a large adjusting member cannot be utilized.

Referring now to FIG. 7, a tuning fork construction according to the invention is depicted having a frequency adjusting member provided with a pivot extending vertically relative to the vibrator plane. As such, it is particularly useful in thin wrist watches, since the mass and the eccentricity of the adjuster may be increased without affecting the thickness of the watch. The provision of the frequency adjusting mechanism depicted permits the manufacture of the tuning fork with lower tolerances. In the embodiment of FIGS. 7 and 8, the tuning fork assembly includes a tuning fork 31, a coil support 32, a screw 33 for fixing coil support 32 to the plate of the watch (not shown), a pair of mounting boards 35 secured to each tine of tuning fork 31 in spaced relation, a permanent magnet 36 mounted on the inner surface of each of said mounting boards formed of substantially semicircular shape, pins 37 for fixing each of said pairs of mounting boards 35 on tuning fork 31, and a frequency adjuster member 38 mounted on the upper surface of each of the upper mounting boards 35 by means of a screw 39.

As shown in FIG. 8, mounting boards 35a and 35b are mounted at the free end of one tine of tuning fork 31, while mounting boards 35c and 35d are mounted on the free end of the other tine of said tuning fork. Permanent magnets 36a, 36b, 36c, and 36d are mounted respectively on the mounting boards 35a, 35b, 35c, and 35d. The construction of the embodiment of FIG. 7 is substantially similar to the embodiment of FIG. 1, except with regard to the frequency adjustment arrangement. A plurality of spaced grooves 40 are formed in the upper surface of mounting plates 35a and 35c, said grooves extending radially relative to an axis defined by screw 39. As more particularly shown in FIG. 9, said grooves cooperate with the end 41 of the adjuster member 39 which project into said groove for the purpose of fixing the adjuster member 38 in any one of a plurality of preselected positions. The provision of the cooperative engagement between the adjuster member and the mounting plates prevents the accidental displacement of the adjuster member due to the motion of the wearer's arm, and further permits accurate and discrete adjustment of the frequency of the tuning fork.

Tuning fork 1 is preferably formed of an invariable elastic material, and formed with tines having an arcuate end portion for the purpose of accomodating a battery. The flux path and operation of the embodiment of FIG. 7 is substantially identical to that of the embodiment of FIG. 1.

The narrower the spacing between two neighbouring grooves 10, the more accurate the regulation which may be performed. It is possible to determine the precise variation in frequency with each incremental step of displacement of the adjuster member 38 relative to the grooves 40, for the purposes of calibrating frequency adjustment, and for the purposes of permitting such frequency adjustment without resorting to special measuring instruments. In the embodiment of FIG. 10, the mounting boards 42 are formed with a fan-shaped array of grooves 43, wherein said grooves are defined by a sawtooth wave. Apertures 44 are provided in mounting board 42 for the purposes of pinning the mounting board to the tine of the tuning fork, while aperture 45 is provided for fixing the adjusting member to the mounting boards. The embodiment of FIG. 10 offers the minimum spacing between grooves.

Referring now to FIG. 11, another embodiment of the frequency adjustment mechanism is depicted, wherein mounting board 46 has an elastic plate 48 secured thereto by means of fixing screw 47. A frequency adjuster mass 49 is fixed, as by welding, to the end of elastic plate 48, said elastic plate serving to bias said mass firmly within a recess 50 in the surface of mounting board 46, to further contribute to the resistance of the mechanism to shocks. In both this embodiment and the embodiment of FIGS. 8 and 9, adjustment is effected by opening the screw 39 or 47 and pivoting the adjusting member.

Referring now to FIG. 12, still another embodiment of the tuning fork construction according to the invention is depicted, wherein tuning fork 51 has a pair of mounting plates 55 secured to each tine thereof by means of pins 57, said pins being inserted in apertures extending through said mounting plates and tuning fork tines and being caulked at both ends for retention therein. A permanent magnet 56 is secured to the inner surface of each of the mounting plates 55 while a regulator member 58 is pivotally mounted on each of the upper mounting plates by means of a screw 59. A coil 54 is disposed in the space between the permanent magnets 56 and is carried by a coil frame 52 fixing the plate of the watch by screws 53. The structure, flux path and operation of the embodiment of FIG. 12 is substantially identical to the embodiments of FIGS. 1 and 7, and FIG. 8 represents a cross section taken along line A.sub.1 -A.sub.2 -A.sub.3 of FIG. 12. Referring to FIG. 8, the permanent magnets 36a and 36c have poles which repel each other and in the embodiment of FIG. 12, a magnetic plate 60 is inserted between these two permanent magnets for the purposes of compensation, said plates being fixed to the plate of the watch by means of screws 61. Since the polarities of the permanent magnets on the two lines of the tuning fork 51 repel each other, the rate of isochronism is increased when the amplitude of oscillation is increased.

Referring to FIG. 13, which shows a plot of frequency f vs. amplitude A, curve 62 shows the synchronous curve where the compensation plate 60 is not provided. If the end portion of the tuning fork is made small in size, and powerful permanent magnets are used, the slope of the synchronous curve increases, and a high precision watch utilizing a tuning fork cannot be achieved.

The synchronous curve 63 shows the effect of compensation plate 10. Since the permanent magnets attract one another and the compensation plate 60, the synchronous curve 63 depicted in FIG. 13 is such that the amplitude increases as frequency decreases. Curve 64 shows the compensated synchronous curve, which has only a slight upward curve. The compensation plate 60 furthers the leakage flux of the permanent magnets so non-compensation is desirable over overcompensation. The compensation plate 60 can also be formed of a permanent magnet. When so formed, said plate may not be provided between the permanent magnets 36a and 36c, because usually, the tuning fork is made of a material having a constant temperature coefficient of elasticity. Such material is generally magnetic, so that the permanent magnet compensation plate may be disposed near the tines of fork 51 and can attract said tine. In such an embodiment, the same compensating effect can be expected.

Where the embodiment of FIG. 12 is utilized, fluctuations in amplitude of oscillation due to reduction in battery voltage, temperature change or load fluctuation have only a slight effect on frequency, thereby insuring the reliability of the time piece over a long period.

One problem with the tuning fork construction according to the invention is that the magnetic leakage of the permanent magnets of the magnetic circuit is large as compared with the prior art cup constructions. This increased magnetic leakage results in the deterioration of the isochronism. Thus, if the amplitude of the tuning fork is increased, the frequency becomes high. Such magnetic leakage can be minimized if the components of the magnetic circuit are dimensioned as illustrated in FIG. 14, which shows a cross section through the magnetic circuit of one tine of the tuning fork construction according to the invention. Said embodiment consists of a pair of mounting plates 65c and 65d secured to a tine of tuning fork 67, and respectively carrying permanent magnets 66c and 66d. Each of the permanent magnets is mounted in a recess in the inner surface of the mounting plates, each of said mounting plates being formed with a rim projecting toward the facing mounting plates on the side thereof adjacent the other tine of said permanent magnet. Thus, said projecting rims serve to partially block the permanent magnets associated with each tine from the permanent magnets associated with the other tine. Said projecting rim extends about one-half of the thickness of the permanent magnet, the width of said rim, and therefore the spacing between the permanent magnet and the inner edge of the mounting plate is defined by the dimension f.

The thickness of the mounting plate in the region intermediate the permanent magnet and the region engaging the tine of tuning fork 67 is of a thickness a, while the region of said mounting plates adjacent the recess receiving permanent magnets 66 is of a thickness b. It is preferred that the thickness a be not much larger than the thickness b, in order to reduce the magnetic flux paths in the magnetic circuit. Accordingly, it is preferred that the value of dimension a less the dimension b be less than one-third of the thickness of the permanent magnet. For the same reason, it is preferred that the distance d between the tuning fork tine and the permanent magnet is larger than the distance e between the permanent magnets. At the least, the distance d should be more than one and one-half times the value of the distance e.

Isochronism is substantially improved if the distance f between the rim of the mounting plate and the permanent magnet is large. However, it is structurally difficult to make this dimension extremely large, so the value of the dimension f is selected by taking into account the distance e between the magnets or the isochronous characteristics. If the width of the projecting rim, as indicated by the difference between the value of the dimension c and the value of the dimension b, is about one-half of the thickness of the permanent magnet, then the repulsion of the respective permanent magnets associated with each tine caused by the leakage flux is reduced, and the isochronism of the tuning fork is improved. Similar dimensional relations would apply where only one permanent magnet is mounted on each of the magnetic circuits associated with each tine.

As noted above, the mounting plates are secured to the respective tines by means of pins which are retained in position by caulking on the opposed ends thereof. This approach offers a substantial advantage over the prior art wherein the magnetic circuit cups were secured to the tuning fork by welding as by silver solder, a process which proved difficult and, due to the high-heat treatment involved, was found to adversely affect the characteristics of the tuning fork as a vibrator. Referring now to FIGS. 15 and 16, a further embodiment of the mounting plates according to the invention is depicted, wherein said mounting plates are provided with projecting portions which extend beyond the outside of the tines of the tuning fork. The mounting plates 70 are secured by pins 71 and 72 to said tines. A further pin 73 extends through the projecting portions of the mounting plates 70, said further pin being provided with a central region 74 of increased diameter which serves to maintain the mounting plates 70 in spaced parallel relation. The provision of the additional pin 73 with the thicker region 74 adds substantial stabilaJy to the mounting of the mounting plates 70 to the relatively thin tines.

A wrist watch having the tuning fork construction according to the invention offers substantial advantages over the prior art tuning fork wrist watches. Firstly, since the coil and coil frame are inserted in the space between the mounting plates from the end of the tuning fork, the coil can be mounted and removed without displacing the tuning fork for the purposes of assembly and disassembly, thereby permitting rapid and automatic assembly.

Secondly, the magnetic field generated by the permanent magnets covers substantially the whole coil, so that the strength of the magnetic field on the surface of the coil is substantially uniform, and such strength can be increased, so that sufficient flux density can be obtained by a relatively thin magnet. The turns of the coil can be reduced by about one-third as compared with a conventional coil, thereby further contributing to the production of a thin wrist watch.

Thirdly, a single coil having only two terminals is utilized for both detecting and driving, so that the winding of the coil can be quickly performed, an advantage where mass production techniques are utilized. The prior art arrangements utilize two coils, only one being required in the arrangement according to the invention.

Fourthly, a portion of the tine of the tuning fork forms a part of the magnetic path in the respective magnetic circuits according to the invention. For this reason, it is not necessary to weld the tuning fork and the end portion of the tine by means of silver solder, as in the prior art arrangements, said end portion being readily secured by means of a pin or a screw in the arrangement according to the invention. As noted above, the tuning fork is preferably formed of a material having constant elasticity in the face of temperature changes, in order to maintain the frequency of the tuning fork at a constant level. It is not desirable to heat treat such material, so that the pinning arrangement according to the invention, as described above, avoids an otherwise undesirable potential damage to the tuning fork present in the prior art arrangement.

Accordingly, a wrist watch having the tuning fork construction according to the invention is particularly adapted for mass production, and a low-priced thin-type wrist watch utilizing a tuning fork as a time standard may be produced.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above construction without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Claims

What is claimed is:

1. An electronic wrist watch comprising a substantially U-shaped tuning fork having a pair of oscillating tines; a U-shaped magnetic circuit mounted on an end portion of each of said tines, each of said magnetic circuits consisting of a pair of spaced mounting plates secured to said end portion of one of said tines and at least two substantially semicircular permanent magnets, one being fixed to each of the mounting plates of each of said magnetic circuits in substantially overlapping relation in the space therebetween, the polarity of each of said permanent magnets being oriented so that the magnetic flux flows through each of said magnetic circuits along a path including said mounting plates and said end portion of the associated tine; and substantially circular coil means positioned in the space between the mounting plates of both of said permanent magnets in the path of the flux thereof, said semicircular permanent magnets of each of said magnetic circuits being aligned with their respective chords in facing relation to the chords of the semicircular permanent magnets of the other of said magnetic circuits, said coil means being positioned in substantially overlapping relation to said semicircular permanent magnets.

2. A tuning fork construction as recited in claim 1, including a substantially circular battery mounted in said wrist watch, the tines of said tuning fork including an arcuate region for accomodating said substantially circular battery.

3. A tuning fork construction as recited in claim 1, including a driving circuit having a PNP transistor and a NPN transistor, each of said transistors having a collector, said coil means consisting of a single coil connected between the collectors of said transistors.

4. A tuning fork construction as recited in claim 1, including frequency adjusting means having a frequency adjuster member having an eccentric center of gravity and means for mounting said frequency adjuster member on at least one tuning fork tine for selective pivotal displacement along a plane extending substantially parallel to the plane of oscillation of said tuning fork.

5. A tuning fork construction as recited in claim 4, wherein one of said frequency adjuster members is mounted on each of said tuning fork tines.

6. A tuning fork construction as recited in claim 5, wherein said mounting means includes a screw displaceably mounted in one of the mounting boards of each of said magnetic circuits.

7. A tuning fork construction as recited in claim 4, wherein said frequency adjusting means includes a plurality of spaced recesses in the path of pivotal displacement of said frequency adjuster member, said mounting means being adapted to selectively retain said frequency adjuster member in engagement with one of said recesses.

8. A tuning fork construction as recited in claim 7, wherein said mounting means retains a frequency adjuster member on a mounting board of each of said magnetic circuits, said recesses being formed in the surface of said mounting board.

9. A tuning fork construction as recited in claim 7, wherein said mounting means includes spring means for biasing said frequency adjuster member in a selected one of said recesses.

10. A tuning fork construction as recited in claim 1, including a compensation plate member positioned in the magnetic field of the permanent magnets of both of said magnetic circuits, said compensating plate member being formed of a magnetic material, said compensating plate member and said permanent magnet attracting each other.

11. A timepiece as recited in claim 10, including a frequency regulator member pivotably mounted on one of the mounting plates of each of said magnetic circuits for selective pivoting in a plane extending substantially parallel to the plane of oscillation of said tuning fork.

12. A tuning fork construction as recited in claim 1, including a compensating plate member positioned adjacent said oscillating tines of said tuning fork, said compensating plate member being formed of a permanent magnet, said permanent magnet and said oscillating tines attracting each other.

13. A tuning fork construction as recited in claim 12, including a frequency regulator member pivotably mounted on a mounting plate of each of said magnetic circuits for selective pivotable displacement in a plate extending substantially parallel to the plate of oscillation of said tuning fork.

14. A tuning fork construction as recited in claim 1, wherein each of said mounting plates carrying one of said permanent magnets is formed with a rim projecting toward the other magnetic plate in its magnetic circuit on the side thereof spaced from its associated tine, said permanent magnet being mounted adjacent said projecting rim and being spaced from said side of said mounting plate by said rim.

15. A tuning fork construction as recited in claim 14, the thickness of the region of said mounting plate including said rim being greater than the thickness of the region of said mounting plate against which said permanent magnet is secured, said rim projecting beyond said last-mentioned region of said mounting plate by about one-half of the thickness of said permanent magnets.

16. A tuning fork construction as recited in claim 15, wherein the distance between said permanent magnet and the tine associated therewith is greater than the width of the space in the path of the magnetic flux in which said coil means is inserted.

17. A tuning fork construction as recited in claim 16, wherein said distance between the tine of said tuning fork and said permanent magnet is more than one and one-half times the width of said space.

18. A tuning fork construction as recited in claim 17, wherein the width of said mounting plate in the region intermediate said tine and said permanent magnet less the width of said mounting plate in the region in which said permanent magnet is secured thereto is less than one-third of the thickness of said permanent magnet.

19. A tuning fork construction as recited in claim 14, wherein the distance between the tine of the tuning fork and the permanent magnet of each magnetic circuit is more than one and one-half times the width of the space in the flux path of said magnetic circuit within which said coil means is received.

20. A tuning fork construction as recited in claim 14, wherein the thickness of said mounting plate in the region intermediate said tine and said permanent magnet less the thickness of said mounting plate in the region at which said permanent magnet is secured thereto is less than one third of the thickness of said permanent magnet.

21. A tuning fork construction as recited in claim 1, wherein each of said magnetic plates and the associated tines of said tuning fork are formed with apertures therethrough in registration, said tuning fork construction including pins extending through said apertures for securing said mounting plates and tines together.

22. A tuning fork construction as recited in claim 1, wherein each of said pairs of mounting plates are formed with further apertures therethrough in registration with each other but out of registration with said tines, and including a further pin receives in said further apertures, said further pin being formed with a central region of increased thickness dimensioned to maintain said mounting plates in spaced substantially parallel relation.

23. A tuning fork construction as recited in claim 22, wherein said mounting plates are formed with a region projecting beyond said tines on a side thereof opposed from said permanent magnets, said further apertures being located in said projecting region.