US2037061A - Variable coil - Google Patents

Description

H. N. BLISS VARIABLE COIL April 14, 1936.

Filed Nov. 15, 1934 s sheets-sheet 1 '21-A 1g- ZL.

Y lvl w HROZ? /V BLISS INVENTOR.

ATTORNEYS.

H. N, BLISS VARIABLE COIL April 14, 1936.

Filed NOV. 15, 1954 3 Sheets-Sheet 2 HHUD /V BL/SS INVENTOR.

A TTORNEYS. A

.April 14, 1936. H. N. BLISS 2,037,061

VARIABLE COIL Filed Nov. 13, 1934 3 Sheets-Sheet 3 HROD. 7V BLISS INVENTOR.

A TTORNEYS.

Patented Apr. 14, 1936 UNITED STATES PATENT OFFICE VARIABLE COIL Application November 13, 1934, Serial No. 752,854

37 Claims.

This invention relates to electricity, and particularly to means for smoothly varying electrical inductance or resistance through fine gradations. In general it consists of a variable coil, of a simple and compact construction, Whose effective length can be readily changed by any small fraction of a turn, this result being attained without the use of extraneous reeling coils, so that the variable coil can be mounted as a unitary device. This invention is generally applicable to various scientific and commercial applications, wherever iine gradations in 'the length or number oi turns in a coil are desired; and it has a special application to the radio art, in that it makes possible the practical manufacture of short-wave receivers without the multiplicity of independent coils with which they are at present encumbered.

In the ordinary radio receiver, tuning to the various wave lengths has generally been done by turning the condensers to various degrees of capacity C. It has long been recognized that for ideal reception the inductance coils, (controlling the inductance L), should also be varied for various wave lengths, in order to maintain desirable L/C ratio; but as a practical matter it has been found diicult to make variable coils of reasonable size and good characteristics, and conse= quently the use of xed inductance coils has remained standard practice on commercial receivers.

Fixed inductances L approximating the desired value have been used in conjunction with variable condensers C, and in practice could cover a fair range without the L/C ratio becoming so bad as to spoil reception. While the sacrifice in reception could be tolerated in the ordinary broadcast range, it was found that the difficulties increased with the shorter wave lengths, so that the same coils could not be used, and it has been commercial practice to then substitute other xed coils, with attendant switches and complexity, so that a commercial universal or short-wave receiver has become a very complicated device.

The present invention therefore has a particular application to short-wave radio, in that it permits a few simple coils to take the place of many, and with better results, since the few variable coils attain with precision the desired L/C ratio which the many fixed .coils only approximated.

I am aware that variable coils utilizing the winding and unwinding of wire from one insulated core to an adjacent conducting core on another axis have been suggested, but the double cores were too bulky and the unsupported wire too fragile for commercial receivers. I am also aware that xed coils have been adjusted to secure uniformity in manufacture by sliding turns, from one end to the other, both ends being active, but that could only effect a small adjustment over a very limited range, so that additional coils had to be used as before. I am also aware that attempts have been made to short-circuit one end of a screwing helix, to make the other end 10 a variable inductance, but the short-circuiting devices, due to their self-inductance, made such devices unsuitable for the high frequencies found in radio work. I believe I am the first to produce a compact and rugged unitary variable coil 15 on a single axis capable of variation over a broad range without objectionable self-induction effects. This makes possible the simplifying of short-wave and universal radio sets as stated, and improved reception at all wave lengths.

Referring now to the drawings,

Fig. l is a general view in elevation of a preferred form of the invention.

Fig. 2 is a longitudinal cross-section on the line 2 2 of Fig. i.

Fig. 3 is an end View of the cylinder portion of Fig. l, with the external stationary coil omitted.

Fig. 4 is a detail view of one form of tensioning device which may be used, if desired.

Fig, 5 is a plan view of a modified form involving coaxial return winding.

Fig. 6 is a side elevation of the modification shown. in Fig. 5.

Fig. i is another modification showing a single diagonal transfer wheel. 35

Fig. 8 is a plan view of another modification somewhat similar to Fig. l but with internal sleeve transfer.

Fig. 9 is a vertical cross-section taken along the line 9--9 of Fig. 8. 40

Similar reference numerals refer to similar parts throughout the various views.

First taking up the form shown in Fig. 1, Fig. 2, and Fig. 3, the movable wire I is wound on an insulated sleeve 2 which is preferably made of rub berBakelite or some similar material, this insulated sleeve 2 preferably being provided with screw threads 3 to receive and guide the wire I in its winding. The end of the wire I is secured to a central pivot pin 4, having a suitable head 5 to which may be clipped an ordinary spring terminal to lead the current to or from the wire I.

The other end of the wire I is wound on a similar screw thread 6 on a core I preferably made of brass or other conducting material, 55

mounted on the shaft 8, which is provided with a screwlthread 8 of the same pitch as the threads 3 and 6 of the cores 2 and 1. The purpose of the thread 9 is to feed the cores 2 and I along at substantially the same rate as the wire I is wound or unwound on the cores 2 and 1, so that the lead-on or lead-off point of the wire may remain stationary. Thus, though the variable coil made by the wide I moves helically in growing or diminishing, its position can remain fixed, relative to other electrical elements, so that ity can be readily coupled electrically to any other adjacent fixed transformer coil.

The metallic core 'l short-circuits all the windings of the wire I in contact with it, as they wind, making them ineffective, so that the eective coil is that portionof the wire I carried on the inst-1'- lated core 2.

Various means may be used to transfer the wire I from the inactive to the active portion of the coil, one preferred form being shown in Fig. 1 and Fig. 2, consisting of a sleeve I0 of bakelite or other suitable insulating material surrounding the cores 2 and "I, and loosely fitting them so as to act as a running bearing support. 'I'his sleeve I0 is stationary and clamped in the'housing II which is screwed to the bracket I2. The bracket I2 is threaded to match the screw thread 9 on the shaft 8, which can be turned by any suitable means such as the knob I4.

The sleeve I0 is provided with openings or slots I5 and I6, through which the wire I may be lifted or peeled oif the threaded cores 2 and 1,-the material of the sleeve I0 being thinned down at the lift-off points to a fairly fine edge so as to receive the wire I smoothly with very little bending of the the wire. As the device is generallymsed for tuning, and operated frequently, it is important that sudden or abrupt bends of the wire be prevented. The sleeve I0 may also have a fine groove starting at thelift-off points to guide the wire I, such a guide groove preferably starting with the same pitch or helix as the screw thread 3, so as not to change the bend of the wire suddenly, and then progressing with a gradually increasing pitch so as to travel the desired distance axially with increased rapidity, and returning to a fine pitch again as it leads on to the other winding screw thread 6.

It is not necessary that the sleeve I0 be grooved all the way to guide the wire I, as after the wire l has been lifted off the inner rotatable cores and started on an expanded helix, it will seek its own path and run very satisfactorily over the polished outer surface of the bakelite tubing I0; but a groove may be provided all the way if desired.

The unit described can be used as a micro-variable resistance coil,-the current entering at the point 5, passing through a variable resistance according to the amount of wire I wound on the insulated sleeve 2, and then passing through the metallic core lI, shaft 8, screw 9, and bracket I2 to ground. But the chief application of the device is to the radio art as a variable inductance or transformer, in which case a stationary transformer winding 20 can be mounted on the insulated sleeve Ill in the proper relation to the helically variable winding on the core 2. Thus as the shaft 8 is turned the ratio of the transformer can be varied in any degree, down to a small fraction of a turn. By mounting both the variable coil and the stationary coil of the transformer away from all metal parts, self induction effects are prevented.

When the device is properly constructed, usually the normal elasticity of the wire I is sumcient to adapt itself to ordinary changes of temperature, so that the wire I remains snug but runs freely on the sleeve I0. But to cover extreme variations of temperature, or exceptionally long wire, it is sometimes desirable to insert an additional tensioning device, such as shown in Fig. 4 for example. This consists of a spring shoe 2l secured on the surface of the sleeve III at any point under. the transfer portion of the wire I, where it has been expanded into a coarse pitch helix. This shoe 2I tends to spring out and the wire I sliding over it is held to a tension so as to takeup expansion or contraction. This shoe also provides a point at which electricity may be fed into the wire I, instead of through the metallic core 1, if desired. Of course the shoe 2| can be used solely as a tension device and not as a conductor if desired, as it is mounted on an insulated base. If the shoe 2I is used to lead in electricity it becomes the determinate point beyond which the re-wind coil is ineffective, otherwise the point at which the wire I leaves the conducting core 'I becomes the determinate point. In any event the rewind portion of the coil is relatively inactive as compared with the end wound in the transformer, the rewind portion being used only to lead in the. current, or as a rewind drum; this makes possible the variation of the active end over a wide range, as the wire passing to the rewind end goes out of action.

With the very high frequencies used in radio work, it has been found highly desirable that no metal, or even the inactive end of a coil, be permitted anywhere near the active portion of the coil or transformer, due to the self-induction which may be set up. It is therefore important that the wire I be carried a substantial distance axially before it is short-circuited by the conducting core 6,-this axial distance being generally about equal to the diameter of the core, or more. If the short-circuiting metal 5 is at or c lose to the base of the effective variable coil, such currents will be induced in the metal 6 that the device will not function properly.

One of the reasons for the compactness of the device is that the same axis is used for the winding as for the re-Winding, instead of having one winding drum and another separate unwinding drum spaced at a distance as in prior devices.

where a conducting core in combination with a non-conducting core was involved. A modification embodying this advantage, though not quite as compact as the preferred form above described, is shown in Fig. 5', Fig. 6, and Fig. 7.

In these forms an external pulley 25 or pulleys 25 and 25 are used to transfer the wire axially of the cores, instead of the sleeve above described.

Taking up Fig. 5 and Fig. 6 more in detail, the core 2 is made of insulating material and the conducting core 'I are mounted on the shaft 8 substantially as described in connection with Fig. 1 and Fig. 2, and screwed along by the screw 9 in the bracket I2 in the same manner. `There being no external supporting sleeve I0 in Fig. 5 and Fig. 6, -the shaft 8 is extended through to an additional supporting bearing I2', insulated from the base, and the current may be led oif through that end of the shaft 8 and bearing I2' if desired instead of the pin 4. The stationary transformer winding 20 is supported around the base of the active helical winding in any suitable manner.

The insulated core 2 and the conducting core 'I are provided with threads 3 and 6 respectively to guide the wire I, and in Fig. 5 and Fig. 6 a pair of pulleys 25 and 25" mounted in a dlagonal direction are employed to transfer the wire I axially so that the conducting end may be suiiciently spaced from the non-conducting end. These pulleys 25 and 25" may be made of insulating material or mounted on insulated supports, unless it is ldesired to use one of them to conduct the current to the wire I.

When the shaft 8 is turned the wire I is wound and unwound from one end of the device to the other, thus varying the number of turns on lthe core 2 which forms the active coil. As the thread 9 moves the cores 2 and 'I longitudinally at the same rate that the wire I is wound on or off the threads 3 and 6 of the cores, the position of the wire I leading from the pulleys 25' and 25" to the core 2 remains unchanged, so that the variable coil made by the portion of the wire l in the threads 3 remains in the proper relation to the fixed transformer coil 20.

In the modification shown in Fig. 7 a single large diagonal pulley 25 is used in place of the pair of pulleys 25', 25". This single pulley 25 is arranged in a similar diagonal plane, and is shown supported on a movable bearing bracket 30 having a spring 3| to keep the wire I taut. Any similar tension device can be applied to Fig. 5 and Fig. 6 if desired.

The modification shown in Fig. 8 and Fig. 9 is somewhat similar to Fig. 1, except that the cores 2 and 'I are smooth surfaced instead of having screw threads, and the wire I goes internally in the sleeve 40 in a groove 4I of gradually variable pitch, instead of being lifted off the cores to travel externally of the sleeve. The sleeve du is clamped in a xed position in the housing Il', and the internal guide groove 4I takes the wire at the point 42 and with gradually increasing pitch carries it axially without binding until it approaches the brass end 1, when the pitch is gradually decreased until it becomes the same as that of the screw 9. It is thus possible to feed the wire smoothly without straining the wire I or unduly wearing the sleeve 4I).

In each of the foregoing modifications I have described a single unit, but it will be understood that in practice a number of units, usually two, three, four or more are generally required, and it will be obvious that they can be mounted along on the same shaft and rotated in unison, or geared or otherwise connected together. While I have in the foregoing described certain specific examples, it will be understood that they are merely for purposes of illustration, to make clear the principles of the invention, which is not limited to the particular forms shown, but is susceptible to various modifications and adaptations in different installations, as will be apparent to those skilled in the art, without departing from the scope of the invention as stated in the following claims.

I claim:

1. In a variable electrical coil, the combination of a rotatable core, wire adapted to be wound and unwound from one end portion of the core and rewound coaxially on the other end portion of the core, an insulated sleeve surrounding the core adapted to lift the wire off the core and pass it around the external surface of said sleeve in a helical path of smoothly varying pitch, the pitch gradually increasing and then gradually decreasing, so as to lead the wire on and off the core at substantially the same pitch that it is wound on the core, said rewind portion of the wire being inactive relative to the other portion, whereby the latter may serve as an active variable coil of broad range.

2. In a variable electrical coil, the combination of a rotatable core, wire adaptedto be wound and unwound from o'ne end portion of the core and rewound coaxially on the other end portion of the core, an insulated sleeve surrounding the core adapted to lift the wire off the core and pass it around the external surface of said sleeve in a helical path of smoothly varying pitch, the pitch gradually increasing and then gradually decreasing, so as to lead the wire on and off the core at substantially the same pitch that it is wound on the core, said rewind portion of the wire being inactive relative to the other portion, whereby the latter may serve as an active variable coil of broad range and a screw arranged to move the core bodily axially at the same rate that the winding progresses on the active coil.

In a variable coil, the combination of an insulated core, a conducting core arranged coaxially with said insulated core and rotatable in unison therewith, a sleeve of insulating material surrounding the cores, and a wire wound with a fine pitch on the insulated core, then lifted off the core and 'wound on the external surface of the sleeve with a smoothly varying pitch, starting with a fine pitch gradually increasing to a very coarse pitch, so as to travel rapidly axially, and then gradually decreasing to a fine pitch again, the wire then being led back on the conducting core at a considerable distance from where it left the insulated core, whereby the portion of the wire on the insulated core may be operated as a variable coil.

4. In a variable coil, the combination o an insulated core, a conducting core arranged coaxially with said insulated core and rotatable in unison therewith, a sleeve of insulating material surrounding the cores, and a wire wound with a ne pitch on the insulated core, then lifted off the core and wound on the external surface of the sleeve with a smoothly varying pitch, starting with a ne pitch gradually increasing to a very coarse pitch, so as to travel rapidly axially, and then gradually decreasing to a ne pitch again, the wire then being led back on the conducting core at a considerable distance from where it left the insulated core, whereby the portion of the wire on the insulated core may be operated as a variable coil and a screw arranged to move the cores bodily axially at the same rate that the Winding progresses on the variable coil.

5. In a variable coil, the combination of a fixed transformer winding, a variable transformer winding associated therewith comprising a wire wound with fine pitch and movable helical ly at that pitch so as to provide a variable numher of turns, means for moving the feeding portion of the Wire rapidly away from the variable and fixed transformer windings in a coarse helix of smoothly varying pitch, and coaxial means for rewinding the wire beyond said coarse helix at such a distance as to avoid interference with the transformer windings.

6. In a variable coil, the combination of a iixed transformer winding, a variable transformer winding associated therewith comprising a wire wound with fine pitch and movable helically at that pitch so as to provide a variable number of turns, means for moving the feeding portion of the wire rapidly away from the variable and fixed transformer windings in a coarse helix of smoothly varying pitch, coaxial means ior rewinding the wire beyond said coarse helix at such a distance as to avoid interference with the transformer windings and a screw arranged to move the variable transformer windings bodily axially at the same rate that said windings build up helically, so that the variable transformer windings malntain the proper position relative to the xed winding.

7. In a variable coil, the combination oi a sleeve of insulating material, a stationary transformer winding mounted on the end oi said sleeve, an insulated core rotatably mounted within said sleeve and provided with screw thread grooves oi' iine pitch, a wire wound on said core in said grooves, then lifted oiI said core and on to the sleeve with a helical path gradually but rapidly increasing to a very coarse pitch so as to carry the wire rapidly away from the transformer windings, a rewinding core coaxial with the first mentioned core, to which the wire returns after traversing the sleeve, said sleeve spacing the transformer windings from the rewinding portion so that the latter is relatively inactive as regards the former.

8. In a variable coil, the combination of a sleeve of insulating material, a stationary transformer winding mounted on the end of said sleeve, an insulated core rotatably mounted within said sleeve and provided with screw thread grooves of iine pitch, a wire wound on said core in said grooves, then lifted oi said core and on to the sleeve with a helical path gradually but rapidly increasing to a very coarse pitch so as to carry the wire rapidly away from the transformer windings, a rewinding core coaxial with the first mentioned core, to which the wire returns after traversing the sleeve, said sleeve spacing the transformer windings from the rewinding portion so that the latter is relatively inactive as regards the former and means for moving the cores axially at the same rate that the wire is wound on them, so that the wire may be taken on and off the cores by the sleeve at xed points.

9. In a variable coil, the combination of a rotatable core, a wire coiled around one end of said core to form an active coil variable over a broad range, said wire being also wound on the other end of said core with its turns all of substantially the same potential so as to be substantially inactive as a coil and means for lifting the wire off the core and carrying it rapidly between the active and inactive coils so as to minimize inter ference between them.

10. In a variable coil, the combination of a rotatable core, a wire coiled around one end of said core to form an active coil variable over a broad range, said wire being also wound lon the other end of said core with its turns all of substantially the same potential so as to be substantially inactive as a coil and means for carrying the wire rapidly from one coil to the other with gradual bends of radii of curvature approximating that of the core, so as to provide a smooth running coil suitable for continual variation.

11. In a variable coil, the combination of a rotatable core, a wire wound on said core so as to form an active broadly variable coil at one end and a relatively inactive coil at the other end, and means for carrying the wire between the active variable coil and the inactive coil at a greater axial rate than the active coil progresses in winding and unwinding.

12. In a variable coil, the combination of a rotatable core, a wire coiled around one end ot said core to form an active coll variable over a broad range, said wire being also wound on the other end of said core with its turns all of substantially the same potential so as to besubstantially inactive as a coil and means for moving the core bodily axially at thesame rate that the variable coil progresses in its winding, so that the general location oi' the variable coil remains substantially ilxed.

13. In a variable coil, the combination of a rotatable core,'a wire coiled around one end of said core to form an active coil variable over a broad range, said wire being also wound on the 4other end of said core with its turns all ot substantially the same potential so as to be substantially inactive as a coil, means for lifting the wire oi the core and carrying it rapidly between the active and inactive coils so as to minimize interference between them and means for moving the core bodily axially at the same rate that the variable coil progresses in its winding, so that the general location of the variable coil remains substantially xed.

14. In a variable coil, the combination o! a rotatable core, a wire coiled around one end of said core to form an active coil variable over a broad range, said wire being also wound on the other end of said core with its turns all of substantially the same potential so as to be substantially inactive as a coil, means for carrying the wire rapidly from one coll to the other with gradual bends of radii or curvature approximating that of the core, so as to provide a smooth running coil suitable for continual variation and means for moving the core bodily axially at the same rate that the variable coil progresses in its winding, so that the general location oi the variable coil remains substantially ilxed.

15. In a variable coil, the combination of a rotatable core, a Wire wound on said core so as to form an active broadly variable coil at one end and a relatively inactive coil at the other end, means for carrying the wire between the active variable coil and the inactive coil at a greater axial rate than the active coil progresses in winding and unwinding and means for moving the core bodily axially at the same rate that the variable coil progresses in its winding, so that the general location of the variable coil remains substantially fixed.

16. In a variable coil, the combination of a rotatable core, a wire wound to form an active variable winding on one end of the core, said wire being wound as a relatively inactive winding on the other end of the core, so that the inactive end serves as a rewinding portion as the active end is wound or unwound, said rewinding portion being spaced axially from the active winding by a distance substantially equal to the diameter of the core, so as to minimize interference between them.

17. In a variable coil, the combination of a core of conducting material, a core of non-conducting material, said cores being arranged lcoaxial with each other and simultaneously rotatable, and wire arranged to wind and unwind from one of said cores to the other, whereby the turns of wire on the conducting core are short-circuited and the number of effective turns on the nonconducting core are varied.

18. In a variable coil, the combination of a core of conducting material, a 'core oi' non-conducting material, said cores being Varranged coaxial With each other and simultaneously rotatable, and wire arranged to wind and unwind from one of said cores to the other, whereby the -turns of wire on the conducting core are shortcircuited and the number of effective turns on the non-conducting core are varied, and means for moving the last mentioned core axially at substantially the same rate as the turns are. wound on it, so that the variable 'coil remains in a substantially fixed position.

19. In a variable coil, the combination of a coro of conducting material, a core of non-conducting material, said cores being arranged coaxial with each other and simultaneously rotatable, and wire arranged to wind and unwind from one of said cores to the other, whereby the turns of wire on the conducting core are short-circuited and the number of effective turns on the non-conducting core are varied and means external to said cores for guiding the Wire from one of said cores to the other.

20. In a variable coil, lthe combination of a. core of conducting material, a core of non-conducting material, said cores being arranged' coaxial with each other and simultaneously rotat able, and wire arranged to wind and unwind from one of said cores to the other, whereby the turns of wire on the conducting core are short-circuited and the number of effective turns on the non-conducting core are varied, means for moving the last mentioned core axially at substantially the same rate as the turns are wound on it, so that the variable coil remains in a substan tially fixed position and means external to said cores for guiding the wire from one of said cores to the other.

2l. In a variable coil, the combination of a core of non-conducting material, wire arranged to coil on and off said core to form a variable number of turns, and means for moving the core axially at substantially the same rate as the wire is coiled Aon it so that the variable coil remains in a substantially fixed position.

22. In a variable coil, the combination of a rotatable core, wire arranged to coil on said core to form a variable number of effective turns, said turns being closely spaced, means'for carrying the wire away from one end of the coil with a gradually increased axial travel per turn of the core, so that it is rapidly spaced from the effective coil after leaving it, and then gradually reduced to the original pitch, and means for rendering ineifective these windings at the second end of the coil, whereby the coil is made variable by rotating the core.

23. In a variable coil, the combination of a rotatable core, wire arranged to coil on said core to form a variable number of effective turns, said turns being closely spaced, means for moving the coil bodily axially at substantially the same rate as the closely spaced turns are wound axially, so that the eiective variable coil retains a substantially iixed location, means for 'carrying the wire away from said variable coil with an increased axial travel per turn of the core, so that it is rapidly spaced from the coil after leaving it, and means for rendering ineffective the turns after they are so carried away, whereby the eective length of the coil is made variable by rotating the core.

24. In a variable coil, the combination of a rotatable core, a second rotatable core, saidcores arranged co-axial with each other and simultaneously rotatable, wire arranged to wind and unwind from one of said cores to the other, and means for conducting electricity to the Wire at a determinate point, means for rendering the wire on one side of said point ineifective on one of said cores and effective on the other side of said point on the other of said cores, whereby the effective winding may be varied by rotating the cores.

25. In a .variable coil, the combination of a rotatable corea second rotatable core, said cores arranged 'co-axial with each other and simultaneously rotatable, wire arranged to wind and unwind from one of said cores to the other, means for conducting electricity to the wire at a determinate point, so that the effective length of wire coiled on one of said cores is made variable by rotating the cores, means for making the wire coiled on the other core ineffective, and means for moving the effective core bodily axially at the same rate as the winding progresses, so that the variable coil remains substantially in the same location.

26. In a variable coil, the combination of a rotatable core, a second rotatable core, said cores arranged co-axial with each other and simultaneously rotatable, wire arranged to wind and unwind from one of said cores to the other, means for conducting electricity to the wire at a determinate point, so that the effective length of wire coiled on one of said cores is made variable by rotating the cores, means for makingthe wire coiled on the other core ineffective, means for moving the cores bodily axially at substantially the same rate as the winding progresses, so that the base of the variable coil remains substantially in the same location and means external to said cores for guiding the wire from one of said cores to the other.

27. In a variable coil, the combination of an effective helix of wire wound with a fine pitch, a transfer portion from the base of said effective helix, said transfer portion having a greatly augmented component of axial travel, and a re-wind portion of substantially fine pitch, said re-wind portion of said wire being located substantially axially relative to the effective portion, and means for conducting electricity to the effective coil at a point intermediate the two ends of the wire, and means for rendering the wire on the other side of said point ineffective.

28. In a variable coil, the combination of a rotatable core of insulating material, a coil of wire wound on said core, means for diverting the wire from one end of said coil and transferring it with greater axial travel per rotation of the core, a second winding core on the same axis as the first mentioned core and rotatable therewith, to which the wire after being diverted is transferred for re-winding and means for moving the coil bodily axially at the same rate as the winding progresses, so that the variable coil remains substantially in the same location.

29. In a variable coil, the combination of a rotatable core of insulating material, a variable coil of wire wound on said core, means for diverting the wire from one end of said coil and transierring it with greater axial travel per rotation of the core, a second winding core on substantially the same axis as the first mentioned core and rotatable therewith, to which the wire after being diverted is transferred for re-winding, and means for introducing electricity to said wire at said transfer portion and at a determinate distance from the diverting end of said first mentioned coil.

30. In a variable coil, the combination of a rotatable core, 'a coil of wire arranged to wind on one end o! the core and simultaneously unwind from the other end, transfer means located between the two ends whereby the coil .at one end is separated from the coil at the other and means for moving the core bodily axially at substantially the same rate as the winding progresses, so that the variable coil remains substantially in the same location.

31. In a variable coil. the combination of a rotatable core, a coil of wire arranged to wind on one end o1' said core and simultaneously unwind from the other, and a sleeve around said core between the two ends, said sleeve having means for transferring the wire from one end to the other with increased pitch, during the transfer, whereby the two coiling ends are separated from v each other.

32. In a variable coil, the combination of a rotatable core, a coil of wire arranged to wind on oneend of said core and simultaneously unwind from the other, a sleeve around said core between the two ends, said sleeve having means for transferring the wire from one end to the other with increased pitch, during the transfer, whereby the two Acoiling ends are separated from each other and means for moving the core bodily axially at substantially the same rate as the winding progresses, so that the variable coil remains substantially in the same location.

33. In a variable coil, the combination of a rotatable core, a coil of wire arranged to wind on one end ot said core and simultaneously unwind from the other, a sleeve around said core between the two ends, said sleeve having means for transferring the wire from one end to the other with increased pitch, during the transfer, whereby the two coiling ends are separated from each other and means for conducting electricity to said coil at a determinate distance from the active portion of said coil.

34. In a variable coil, the combination of a core,

a coil of wire wound on said core with relatively tine pitch at both ends, means for guiding the a coil of wire wound on said core with relativelyI ilne pitch at both ends, means for guiding the wire with relatively coarse pitch between the two ends, means for rotating the core so that the wire is wound and unwound from one end to the other, and means i'or conducting electricity to said coil at a determinate point between the ne pitch portions, so that the eective length of the coil is made variable by rotating the core.

36. In a variable coil, the combination of a core,A

a coil of wire wound on said core with relatively fine pitch at both ends, means for guiding the wire with relatively coarse pitch between the two ends, means for rotating the core so that the wire is wound and unwound from one end to the other, means for conducting electricity to said coil at a determinate point between the ine pitch portions, so that the eil'ective length of the coil is made variable by rotating the core, and means for moving the core bodily axially simultaneously with its rotation.

37. In a variable coil, the combination of a helical coil of wire of coarse pitch intermediate two portions of iine pitch, means for conducting electricity to the coarse pitch portion at a determinate distance from one ilne pitch portion, that one line pitch portion being movable helically along its own length to form an active broadly variable helical coil, While the fine pitch portion on the other side of the coarse pitch is relatively inactive as a coil.

HAROLD N. BLISS.

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