US2379282A - Recorder - Google Patents

Description

June 26, 1-945. R. DALLY ETAL RECORDER Filed Aug. '7, 1943 Patented June 26, 1945 RECORDER Roy Daily, Francis H. Goldsmith, and David J. Monroe, Racine, Wis., assignors to Webster Electric Company, Racine, Wis., a corporation ci Delaware Application August 7, 1943, Serial No. 497,779

i? Ciaims. (Cl. 179-100.41)

The present invention relates to recorders for recording sound or other vibratory effects upon records, such as phonograph records. 'I'he invention is equally applicable to recorders of the engravingtype which utilize a cutting tool or recorders of the embossing type. the stylus of which does not remove material from the record, but merely displaces it.

One of the objects of the invention is the provision oi' an improved recording unit which is adapted to be used at extreme temperatures, such as, for example -50 degrees C. to +120 degrees C.

Another object of the invention is the provision of an improved recorder unit which has staible characteristics at all of a relatively wide range of temperatures. such as those which might be encountered in winter in arctic regions and those which might be encountered in the hottest seasons in the tropical zone.

The recorders of the prior art which utilize rubber or other similar resilient material as a supmrt for the armature or for damping material are not suitable for use at extremely cold temperatures because under such conditions the rubber freezes or becomes more rigid so that it is practically solid, and ordinary damping materiais wouid aiso freeze or become substantially solid.

Under such conditions, knife edges will frost and change their characteristics, due to the collection oi moisture, and the recordersof the prior art do not have stable characteristics at the extremes of temperature for which the present invention is adapted.

Another object of the invention is the provision of an improved recorder which records sound with high fidelity and with a minimum amount of distortion or divergence from fidelity, so that the sound may be faithfully reproduced by suitable high fidelity reproduction mechanism.

Another object of the invention is the provision of an improved recorder of the class described, in which there is a better mechanical response for a minimum amount of electrical input than with the devices of the prior art, and in which the performance of the recorder will not vary over a long period of use under the most adverse conditions.

Another object of the invention is the provision ofen improved recorder construction which is simple, compact, sturdy, and which has a minimum number of parts so that it may be manufactured at a low cost and so that it may be placed within the range of a vast number of users.

Another object of the invention is the provision oi' an improved amature construction which takes the form of a tuned reed having a. minimum amount of mass and utilizing a minimum amount of material.

Other objects and advantages of the invention will be apparent from the following-description and accompanying drawings, in which similar characters of reference indicate similar parts throughout the several views.

Referring to the drawings accompanying the specification,

Fig. l is a side elevational view showing a tone arm, turntable, and recorder unit embodying the present invention;

Fig. 2 is a sectional view of the recorder unit, taken along the plane of the line 2-2 oi' Fig. 4;

Fig. 3 is a vertical sectional view, taken on the plane of the line 3--3 ci.' Fig. 4, looking in the direction of the arrows;

Fig. 4 is an end elevational view of the recorder unit, showing the left end of the unit of Fig. i;

Fig. 5 is a, rear end elevational view of the recorder unit;

Fig. 6 is a fragmentary bottom plan view of that portion of the recorder unit, showing the stylus:

Fig. 7 is a sectional view of the recorder unit, taken on the plane of the line i-i ci Fig. .2, looking in the direction of the arrows:

Fig. 8 is a vertical sectional view of the re- A corder unit, taken on the plane of the line 8 8 of Fig. 7, looking in the direction oi' the arrows;

Fig. 9 is a vertical sectional view, taken on the plane of the line 9-9 of Fig. '1;

Fig. 10 is a vertical sectional view, taken on the plane of the line I0--I0 of Fig. 8, looking in the direction of the arrows:

Fig. 11 is a. fragmentary sectional view similar to Fig. 10, showing the mode of attachment of the stylus to the armature when the recorder is of the engraving type.

Referring to Fig. 1, 20 indicates a cabinet or other support for a tone arm column 2|, which supports a. tone arm 22 for pivotal movement on a vertical axis. The support carries a turntable 23 on a. turntable shaft 24 for supporting the rec--v ord 25, and the tone arm 22 has the recorder unit 26 pivotally mounted upon the tone arm by means of the pins or shafts 21.

In other forms of tone arm which are mounted on the column 2l for vertical and horizontal movement, the recorder` unit may be fixedly secured to the end of the tone arm.

"me recorder una :s mss. 2 to' 4) mommy comprises an outer housing Il, which is formed by a lower supporting plate 2l and a rectangular housing member Il.

The mechanism of the unit preferably comprises a permanent magnet 3| (Fig. 2) in the form of a rectangular block of magnetic material of high coercive force, such as, for example, Alnlco, provided with pole pieces 32, Il (Fig. 2), spool Il, electrical coils Il, and amature It (Fig. 8)

The magnet Il is preferably provided with ground plane surfaces I1, 3l for engaging the pole pieces l2. n so that the air gap may be accurately predetermined, and the magnet is preferably formed with grooves 80, 4l (Fig. 1) located in each of its opposite sides for receiving the screw bolts ll, 42.

Grooves It and Il may have flat side walls and half cylindrical end portions. and are preferably slightly larger than the screw bolts to permit of some lateral and longitudinal adjustment. 'I'he Alnico of which the magnets are made is preferably cast in bars having oppositely disposed grooves Il and Il, and the bars are cut of! to length and provided with the accurately ground plane surfaces l1 and I8.

The pole pieces 32, are similar in construction, but are oppositely located right hand and left hand members. 'I'hey may be constructed of any suitable metal, such as cold rolled sttel or soft iron, and may either begcast or made of sheet metal. Each pole piece has a rectangular body portion Il, which may be of the same width as the thickness of the magnet block 3| The body "portion 43 of..each pole piece has a pair of spaced through Il, 4l, 48, and 41, one of which is countersunk for receiving the screw bolts ll, I2 and the heads thereof.`

'I'he apertures 41 in the pole piece 32, for example, may be threaded to receive the threaded end Il of the screw bolts Il, 42, and by means of these screw bolts the two pole pieces are frictionally clamped on the magnet block Il.

The apertures I4. Il, and l1, l1 are suitably spaced vfrom each other so that the screw bolts are located in the grooves 39 and M in the magnet block Il with a slight amount of vclearance between the bolts and the walls of the grooves for adjustment.

Each pole piece is preferably provided at its rear end (Fig. 2) with a groove 49 of rectangular cross-section slightly spaced from the end and adapted slidably to receive the rear wall l0, comprising a sheet of insulating tlber for supporting the connectors 5|, I2. The fiber sheet is slightly deeper than the width of the pole piece bodies 43, and is provided with laterally projecting ears 53, 54, which eiect a complete closure of that end of the housing in the assembly, as shown in Fig. 5.

The connectors il, 52 maybe similar in construction, and each may consist of a short strip of sheet metal. having a 'if-shaped end portion 5l for engagement with a conductor, the strip being secured to the fiber end wall l by a tubular rivet I6.

At its opposite end each pole piece l! or I3 is preferably formed with a pair of inwardly extending parallel flanges 51, I8, l, 80, which serve as the pole pieces proper, and which when assembled with`the armature 8l and the magnet 3i provide an equal air gap clearance at 6I.' il at the iixed end of the armature I6, and also at asvaasa 83, 64, which indicate the air gaps at the movable end of the armature 36.

The pole piece flanges l1, ll, Il, l0 are substantially rectangular in shape, the two upper flanges 51 and Il being longer than the lower flnages Il and l0 in order to provide the same air gap at It, Il on each side of the thin portion of the armature I0 as is provided on each side of the relatively thick portion of the armature I6 adjacent the pole piece flanges III, Il.

When the pole pieces are formed of sheet metal, the outer corners at Il, between the body flange and the pole piece flanges, will be rounded; but the inner corners Il will be ground or otherwise cut out to substantially rectangular form so as not to interfere with the seating of the spool u, which is to be placed between them.

The spool Il consists of an insulating spool which is preferably made of initially plastic moldable material, and it has a pair of parallel end flanges I1, Il (Fig. 8) carried by a central tubular portion II. 'Ihe tubular portion II has a through aperture 10 of substantially rectangular shape for passing the1 armature 3l with a suitable clearance so that the armature is not engaged in any way by the spool.

The spool 34 carries the insulated coils of conducting wire Il, preferably of enameled wire. since a greater efficiency may be attained in some cases by such wire. 'I'he coils Il may consist of a great number of turns of fine wire or a few turns of relatively heavy wire, since the impedance and the conductivity of the coil'may be made any desired amount to suit the recorder for different purposes.

'I'he ends of the coil ll terminate in conductors 1| 12 (Fig. 2), which are connected to the connectors 5I, l2 by means of a drop of solder in the tubular rivet. The upper spool ange 0I (Fig. 10) and the lower spool-nange 81 are preferably provided with a relatively shallow groove 13, which extends diametrically across the upper and lower faces of these flanges respectively, for receiving the pole piece flanges l1, I8, l, 6l.

In some embodiments of the invention only the .upper face of the spool may be provided with a groove 13; or in others only the lower face of ,the spool.

The cross section of this shallow groove 13 is substantially rectangular and complementary to the shape of the pole piece flanges l1, Il so that the pole pieces must be slid laterally on the spool I4, and 'the spool fits inside the pole piece flanges l1-6l when the pole piece flanges l1, Il are in the groove 1t.

Thus the spool 34 and the coil which it carries are positively anchored with respect to the pole pieces, which is of particular importance in a recorder where there is a reaction against the coils when force is exerted on the armature and stylus.'

'I'he width of the spool flanges 8l between the pole piece bodies 43 is such that the flanges ll nt between the pole pieces when they are secured to the magnet block ll, anchoring the spool against lateral movement as well as longitudinal movement.

The magnet and spool assembly thus far described is preferably carried by the bottom plate 29, which is constructed of nonmagnetic material. such as brass, aluminum. or some other suitable material.

4This bottom plate may consist of a rectangular sheet of such metal, which is formed at its forward end with the upwardly extending flange H1, forming the end wall of the housing,

Adjacent this end the bottom plate 20 is provided with an elongated aperture 10, having parallel sides and rounded ends; and the aperture 10 preferably extends, as shown in Fig. 6, toward the left, beyond the pole piece flanges and 60 so that the stylus 9| may clear these pole piece flanges to permit its being lifted out of the pole piece. assembly, as the stylus itself may thus be made, if necessary, slightly wider than the total air gap.

If it were not for the clearance at the left end of the aperture 16 in Fig. 6, the stylus, when thicker than the total air gap between the ilanges 58 and 60, would have to be removed'through the bottom of the housing, whereas with the present structure it may be lifted out of the top oi' the housing,

The bottom plate 29 may be secured to the magnet and pole piece assembly by means of screw bolts 11, 18 (Fig. 6), which pass through this bottom plate and are threaded into the lower pole piece flanges 58 and 60, as shown in Fig. 8.

This secures the pole piece flanges 50. 60 in sic--V curate predetermined spaced relation to each other so as to determine the accuracy of the air gaps at 63, 64.

'I'he'armature 36 is preferably of the reed type, having at least a lower portion ||2 which is relatively thin like a reed and adapted to vibrate at a predetermined frequency due to its resilient characteristics.

At the points |0| the thickness of the armature is increased, providing a body portion 91, which may be of uniform thickness from the points lill to the upper end.

The armature body 91 may be of substantially rectangular shape in cross-section and elevation, and it is provided at its upper end with a pair of centrally located screw bores |00 for receiving tbe screw bolts 902, which secure the armature 36 to a cross bar 903. The cross bar 103 has a transverse groove |00 of rectangular section adapted to receive the armature body 91, which issecured to this groove by the screw bolts iti.

In order to secure the pole pieces in accurately spaced position the cross bar 03 is preferably formed with a pair of depending ribs 3l, 82, which are adapted to nt into the complementary grooves 8d, d5 formed in the upper pole piece flanges 51, A59.

The armature supporting bar |03 also has countersunk apertures H05 for receiving the screw bolts @06 and the heads thereof, and the screw bolts are threaded into the threaded bores sul. Tlere is a clearance around the screw bolts ld in the bores |05, and it is the ribs B2 and grooves 84, 85, which have an accurate fit, that determine the spacing of the pole piece anges el, 59, the parts being held in xed position by the screw bolts. This structure has the additional advantage that, if desired, the complete armature assembly, comprising the armature 36 and the supporting bar |03, may be removed; or, if desired, only the armature 36 may be removed by loosening tle screw bolts which hold it;

In some embodiments of the invention the bottom plate 29 and the lower pole piece flanges 58. t0 may also be provided with grooves and ribs di, 82, 84, 85; but the accurate spacing may be accomplished by the use of these elements only on the upper or the lower side of the pole pieces.

The shape of the end portion H2 of the stylus is shown in Pig. 'I for the embossing type of recorder, in which the stylus is secured at an angle. In this embodiment of the invention the end portion H2 of the armature tapers toward the point |00 adjacent the front side of the armature, and the stylus 8| .comprises a suitably pointed member, preferably of an alloy of the platinum group, which is welded on the end of the rib portion ||2 with its point extending at a predetermined angle of from twenty-eight to thirty degrees, for example.

In the embodiment of Fig.11, where the recorder is oi' the cutting or engraving type, the

stylus 9| is vertical, or nearly vertical, and it may be located centrally of the rib portion H2. The armature 36 may be made of any suitable nonferrous or magnetic metal. such as cold rolled steel: and in some embodiments of the invention a suitable non-metallic stylus, such as a sapphire, could be attached by means of a holder of metal which is welded to the reed.

The armature assembly is so proportioned and constructed that it will be.tuned to vibrate at any desired resonant frequency, such as, for example, 5,000 cycles per second; but most of the movement takes place at the free end of the armature.

In some embodiments of the invention the reed or armature 36 can be riveted or welded to the supporting top plate or bar |03. 'I'his bar, however, must be made of nonmagnetic material, such as, for example, brass or aluminum; otherwise it would short circuit the flux, which would pass through the bar, rather than through the air gap.

The box-like portion of the housing 30 may consist of a piece of nonmagnetic sheet metal like the bottom plate 29, and which has a top flange 92 and a pair of "depending side ilanges 93, 94 at right angles to the top flange and iitting on the edges of the bottom plate 29.

`The side flanges @3 carry inwardly extending guide anges 95, which t under the bottom plate, and the size of the housing member 30 is suchthat it may be slid endwise on the recorder assembly with the anges 95 under the bottom plate.

The left end of the housing is thus closed by the end plate |I1, and the right end is closed by the insulating plate 5t. The housing and the pole piece members are preferably provided with a pair of oppositely located apertures 96 for receiving pins or other members by means of which the housing is pivoted on the tone arm at 22.

As the present armature is mounted without any knife edges or without being secured in resilient material, such as rubber, there is no possibility of the movement ofthe armature being hampered by the knife edges frosting or by a rubber mounting becoming solid at low temperatures. The characteristics of the present recorder are substantially stable throughout a Wide range of temperatures, from very high temperatures to very low temperatures; and thus the present recorder is adapted to be used under the most adverse conditions.

Its armature and stylus may be removed as a unit and replaced whenever necessary; and when such a replacement is made. there is no danger of the spacingr at the air gap becoming deranged.

While we have illustrated a preferred embodiment of our invention. many modifications may be made without departing from the spirit of the invention, and we do not Wish to be vlimited to desire to avail ourselves of all changes within the.

scope of the appended claims.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent of the United States is:

l. In a recording unit for recording sound and the like, the combination of a permanent magnet of high coercive force comprising a body of magnetic material having plane oppositely disposed faces, a pair ,of pole piece members, each pole piece member having a plane body portion adapted to engage said faces-securing means passing through the pole piece members for securing the said pole piece members to the said magnetic body by frictional clamping action, an armature carried by said pole piece members and adapted to vibrate between the opposed pole faces thereof, said armature comprising a reed-like member having a stylus canied by its free end, and said reedlike member being carried by a supporting bar of non-magnetic material, threaded means for securing said supporting bar to said pole pieces, and shoulder means carried by said pole pieces and by said bar for determining the spacing of said armature from the pole faces.

2. In a recording unit for recording sound and the like, the combination of a permanent magnet of high coercive force comprising a body of magnetic material having plane oppositely disposed.

faces, a pair of pole piece members, each pole piece member having a plane body portion adapted to engage said faces, securing means passing through said pole piece members for securing said pole piece members to said magnetic body by frictional clamping action. and an armature carried by said pole piece members and adapted to vibrate between opposed pole faces thereof, said armature being flxedly carried by a supporting bar and said bar being iixedly secured to said pole piece members, said amature comprising a reed-like member having a stylus carried by its free end.

3. In a recording unit for recording sound and the like, the combination of a permanent magnet of high coercive force comprising a body of magnetic material having plane oppositely disposed faces, a pair of pole piece members, each pole piece member having a plane body portion adapted to engage said faces, said magnetic body having oppositely disposed grooves, securing means located in said grooves and passing through said pole piece members for securing said pole piece members to said magnetic body by frictional clamping action, and an armature carried by said pole piece members and adapted to vibrate between opposed pole faces thereof, said armature being fixedly carried by a supporting bar and said bar being ilxedly secured to said pole piece members, said armature comprising a reed-like member having a stylus canied by its free end.

4. In a recording unit for recording sound and the like, the combination of a permanent magnet of high coercive force comprising a body of magnetic material having plane oppositely disposed faces. a pair of pole piece members, each pole piece member having a planebody portion adapted to enslse said faces, said magnetic body hav ing oppositely disposed grooves, securing means located in said grooves and passing through said pole piece members for securing said pole piece members to said magnetic body by frictional clamping action, and an armature carried by said pole piece members and adapted to vibrate between opposed pole faces thereof, said armature being ixedly carriedby a supporting bar and said bar being fixedly secured to said pole piece members, said pole piece members and said bar having complementary ribs and grooves on op'- posite sides of said armature for locating the faces of the pole pieces in predetermined spaced relation to the armature.

5. Iny a recording unit for recording sound and the like, the combination of a permanent magnet of high coercive force comprising a body oi magnetic material having vplane oppositely disposed faces, a pair of pole piece members, each pole piece member having a plane body portion adapted to engage said faces, said magnetic body having oppositely disposed grooves, securing means located in said grooves and passing through said pole piece members for securing said pole piece members to said magnetic body by frictional clamping action, and an armature carried by said pole piece members and adapted to vibrate between opposed pole faces thereof, said armature being ilxedly carried by a supporting bar and said bar being fixedly securedto said pole piece members, said pole piece members and said bar having complementary ribs and grooves on opposite sides of said armature for locating the faces of the pole pieces in predetermined spaced relation to the armature, and threaded means for securing said bar to said pole piece members.

6. In a recorder assembly the combination of a block of magnetic material of high coercive force with a pair of pole piece members and threaded means for clamping said pole piece members frictionally on said block, said pole piece members having upper and lower pole flanges extending toward each other and provided with pole faces, an armature comprising a reed of magnetic material nxedly mounted at one of its ends on .said pole piece members, and an insulated energizing coil carried by said pole piece members and surrounding said armature.

7. In a recorder assembly the combination of a block of magnetic material of high coercive force with a pair of pole piece members and threaded means for. clamping said pole piece members frictionally on said block, said pole piece members having upper and lower pole flanges extending toward each other and provided with pole faces, an armature comprising a reed of magnetic material ilxedly mounted at one of its ends on said pole piece members, and an insulated energizing coil carded by said pole piece members and surrounding said armature, said coil being carried by a spool, said spool fitting between said pole piece flanges.

8. In a recorder assembly the combination of a block of magnetic material of high coercive force with a pair of pole piece members and threaded means for clamping said pole piece members frictionally on said block, said pole piece members having upper and lower pole flanges extending toward each other and provided with pole faces, an armature comprising a reed of magnetic material xedly mounted at one of its ends on said pole piece members, and an insulated energizing coil carried by said pole piece members and surrounding said armature. said coil being carried by a spool, said spool ntting between said pole Piece flanges. and said spool being provided with grooves for receiving predetermined of said pole piece ilsnges to anchor the spool in place.

9. In a magnetic sound apparatus, the combination of a magnetic structure including a source of magneto motive force and a pair of substantially parallel legs, provided Awith substantially U-shaped pole pieces extending toward each other and resiliently supported by said legs. with an energizing coil supported between said `lJOle pieces, an armature located between said through bolts threaded into the pole pieces andprovided with a clearance surrounding said bolts in said bridge member', said pole pieces and bridge member being provided with complementary transversely extending grooves and ridges for definitely determining the position of the pole pieces and the extent of the air gap between said armature and each pole piece, said ridges and grooves being forced into engagement with each other and secured by said through bolts against release.

10. In a recorder assembly the combination of a block of magnetic material of high coercive force with a pair of pole piece members and threaded means for clamping said pole piece members frictionally on said block, said pole piece members having upper and lower pole ilanges extending toward each other and provided with pole faces, an armature comprising a. reed of magnetic material ilxedly mounted at one oi its ends on said pole piece members, and an insulated energizing coil carried by said pole piece members and surrounding said armature, said armature being carried by a supporting member of non-magnetic material carried by the pole piece members, said pole piece members and nonmagnetic member being provided with a groove and complementary rib on each side of the armature whereby the amature is accurately spaced from the pole piece faces.

l1. In a recorder assembly the combination of a block of magnetic material of high coercive force with a pair of pole piece members and threaded means for clamping said pole piece members frictionally on said block, said pole piece members having upper and lower pole flanges extending toward each other and provided with pole faces, an armature comprising a reed of magnetic material xedly mounted at one of its ends on said pole piece members, and an insulated energizing coil carried by said pole piece members and surrounding said armature, said armature having a stylus welded to its free end and extending beyond the pole pieces.

12. In a recorder assembly the combination of a block of magnetic material of high coercive force with a pair of pole piece members and threaded means for clamping said pole piece members frictionally on said block, said pole piece members having upper and lower pole anges extending toward each other and provided with pole faces, an armature comprising a reed of magnetic material flxedly mounted at one of its ends on said pole piece members, and an insulated energizing coil carried by said pole piece members and surrounding said armature, said coil being carried by a spool, said spool lltting between said pole piece flanges; said spool having an enlarged aperture to provide clearance at one side of said pole piece members for passage of said stylus whereby the armature and its supporting member may be removed as a unit from the opposite side.

13. In a magnetic sound apparatus, the comation of a magnetic structure including a source of magneto motive force and a pair of substantially parallel legs, provided'with substantially U-shaped pole pieces extending toward each other and resiliently supported by said legs. with an energizing coil supported between said polev pieces, an armature located between said pole pieces and projecting therefrom at its free end, said armature having a stylus at its free end and a non-magnetic bridge member supporting said armature between said pole pieces, with its supported end in iixed position, said bridge member and the adjacent parts of said pole pieces being provided with apertures for receiving through bolts threaded into the pole pieces and provided with a clearance surrounding said bolts in said bridge member; said pole pieces and and grooves being forced into engagement witheach other and secured by said through bolts against release, said armature comprising a member of magnetic metal having that portion which is xedly secured to the bridge piece of substantial thickness and being reduced in thickness from its free end to a point inside the said coil to form a thin reed-like portion at its free end for resonant vibratory motion. 14. In a magnetic sound apparatus, the combination of a block of magnetic material forming a permanent magnet with apair of pole piece members and threaded means for clamping said pole piece members against said permanent magnet. said pole piece members having pole flanges extending toward each other and the pole flanges on each pole piece member being spaced from each other, and an armature flxedly secured to said pole piece members and adapted to vibrate between said pole anges, said armature being carried by a non-magnetic metal member secured to a pole piece ange on each pole piece member, said non-magnetic member and the adjacent pole piece flanges being provided with engaging shoulders on each side of the armature for predetermined air gap spacing between the armature and the pole piece flanges. y

15. In a magnetic sound apparatus, the combination of a block of magnetic material forming a permanent magnet with a pair of pole piece members and threaded means for clamping said vpole piece members against said permanent magnet, said pole piece members having pole iianges extending toward each other and the pole flanges on each pole piece member being spaced from each other, an armature ilxedly secured to said pole piece members and adapted to vibrate between said pole ilanges, said armature being carried by a non-magnetic metal member secured to a pole piece flange on each pole piece member, said non-magnetic member and the adjacent pole piece iianges being provided with engaging shoulders on each side of the armature for predetermined air gap spacing` 16. In a magnetic sound apparatus. the combinationI of e. block of magnetic material'forming a permanent magnet with a pair of pole piece members and threaded members for clamp ing said 901e piece members against said permanent magnet. said pole piece members having pole flanges extendingtowardeachotherand the pole nanges on each lpole piece member being spaced from each other, an amature tixedlysecuredtosaidpolepiecemembersand adapted to vibrate between said pole nanges. said amature being carried by a non-magnetic metal member secured to spele piece flange on each pole piece member, said nonmagnetic member and the adjacent pole piece iianges being provided with engaging shoulders on each side oi' the amature for predetermined air gap spacing between the armature and the pole piece iianges, a housing comprising a non-magnetic metal plate secured to one side of said apparatus and having an aperture for passage oi' the armature, a box-like metal member ot non-magnetic material slidably mounted on said plate and adaptedtocoveritssidesandtop,andanend closure comprisingaplateot insulatingmaterial carrying connectors for said apparatus. saidinsuiatingplatebeingslidablymountedin 5. slots 'm said pole piece members.

17. An armature structure for recorders, eomprising a bridge member formed with a tramversely extendinggroove oirectangularcrol section.andanarmatureofmagneticmateriai 10 having a iixed end portion of substantialiy'rectangular cross section ntting in said groove. threaded means for securingsaid armature ln said groove. said armature being ot substantially rectangularshapeinelevatiomandbeingre- 15 duced in thicknessintermediat@ its ends, form- ROY DAILY. More H. oomsm'rn.- D. J. Mormon.

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