US2963970A

US2963970A - Electric detonator

Info

Publication number: US2963970A
Application number: US609297A
Authority: US
Inventors: Schaadt Franz; Hegelmayer Adolf
Original assignee: WASAG CHEMIE A G FA
Current assignee: WASAG CHEMIE A G FA; WASAG-CHEMIE AG Firma
Priority date: 1956-09-11
Filing date: 1956-09-11
Publication date: 1960-12-13
Anticipated expiration: 1977-12-13

Description

Dec. 13, 1960 F, SCHAADT ETAL 2,963,970

ELECTRIC DETNATOR Filed Sept. 11, 1956 3 Sheets-Sheet 1 Im/enfon- Frarlz SehanT' Adolf Hegelmager Dec. l3, 1960 F. scHAADT ET AL 2,963,970

A ELECTRIC DETONATOR Filed Sept. 11, 1956 3 Sheets-Sheet 3 fwn/111100,01' ,3

[27s/enfer: FYGLHZ- SchaaJf' United States Patent ELECTRIC DETONATOR Franz Schaadt and Adolf Hegelmayer, Haltern, Germany, assignors to Firma Wasag-Chemie A.G., Essen, Germany Filed Sept. 11,1956, Ser. No. 609,297

14 Claims. (Cl. 102-28) The present invention relates to electric detonators, and more particularly to an electric detonator in which between a pair of terminals a plurality of spark gaps are formed for detonating a charge arranged adjacent the spark gaps of the detonator.

Electric detonators having very short reaction times, such as 5 seconds, and low energy consumption, such as 104 watts-seconds, are required for detonating explosive charges in projectiles which hit a target at high speed.

According to a known electric detonator, two terminals are provided between which a metallic layer is arranged which is melted and forms a spark gap when a voltage is applied to the terminals.

-f However, the electric detonators according to the known art have certain disadvantages, and particularly it is very diicult to produce in an economical manner absolutely reliable detonators of the known art.

It is one object of the present invention to overcome the disadvantages of the known electric detonators, and to provide an electric detonator which reliably operates, has a short reaction time, and requires only a small voltage for being energized.

It is another object of the present invention to provide an electric detonator in which a plurality of spark gaps is formed between two terminals.

It is a further object of the present invention to provide an electric detonator in which a plurality of spark gaps is simultaneously formed between very small conductive areas which are separated by very thin insulating means from each other.

It is also an object of the present invention to provide a reliably operating electric detonator which can be inexpensively manufactured.

With these objects in view, the present invention mainly consists in an electric detonator which comprises a pair of terminals having a pair of opposite spaced surfaces; and insulated thin conductor means constituting bridging means located between the surfaces of the terminals. In accordance with the present invention the conductor means may include a plurality of thin insulated wires. According to another embodiment, the conductor means includes a coil wound of superimposed conductive and insulating bands. Consequently, the conductor means have in a plane transverse to the terminals, a plurality of very small conductive end face or edge areas which are separated by thin insulating cross sectional areas whereby a plurality of spark gaps is formed between the conductive cross sectional areas of the conductor means and the terminals when a voltage is applied to the terminals. Such spark gaps merge, and form arcs sufficient for igniting a detonator charge located adjacent the electric detonator of the present invention.

Preferably, the detonator charge is arranged adjacent one end of the conductor means, and the sparks are formed at such end to reliably ignite the charge.

In the embodiment of the present invention in which the detonator coil is wound of superimposed conductive and insulating bands, it is advantageous to use a bilar winding to reduce the inductive resistance of the coil.

The formation of the arcs is improved if the conductor means are cut oif at one end face thereof whereby burrs are formed which partly bridge the insulating areas which separate the conductive areas. Most of the spark gaps formed in such manner have high resistances. However, it cannot be avoided that some of the spark gaps have low resistance, and in this case a small amount of energy supplied by a condenser to the terminals would not be suicient to produce sparks. Consequently, in accordance with the present invention the bridges having low resistance are burned off by connecting once, or several times, condensers having a suitable higher capacity to the termnials. The discharge of such condensers eliminates the low resistance bridges, and all remaining bridges have a high resistance, about 1,000 ohms, so that even the discharge of a condenser having a low energy of 10-4 watts-seconds produces a hotmspark.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however,'both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Fig. 1 is a sectional view of a detonator according to the present invention provided in a detonating cap;

Fig. 2 is a cross sectional view of one embodiment of the present invention;

Fig. 3 is a cross sectional View of another embodiment of the present invention;

Fig. 4 is a developed view of one element used in the embodiment of Fig. 3;

Fig. 5 is a cross sectional View of a modified embodiment of the present invention;

- Fig. 6 is a developed view of one element of the embodiment shown in Fig. 5;

Fig. 7 is a cross sectional view of a further modified embodiment of the present invention;

Fig. 8 is a side view illustrating the rst step in the manufacture of a detonator of the type shown in Fig. 5;

Fig. 9 is a cross sectional view illustrating a further step in the manufacture of the detonator according to Fig. s;

Fig. 10 is a side view illustrating a rst step in the manufacture of a detonator according to Fig. 1l; and

Fig. l1 is a schematic cross sectional view illustrating a further step in the manufacture of a modified embodiment of a detonator according to the present invention.

Referring now to the drawings, and more particularly to Fig. l, in a detonator cap 7 are arranged the charge 8 and the detonator charge 9. Directly adjacent the detonator charge 9 is arranged the electric detonator according to the present invention which includes an outer terminal 3, an inner terminal composed of the terminal parts 2 and 15, and the spark producing bridging means 1 according to the present invention which will be described in greater detail hereinafter. The outer terminal 3 is insulated from the inner terminals 15 and 2 by

insulators

6 and 5. Insulator 6 has an annular shoulder 6a on which the disc-shaped insulator 5 abuts. Part 2 of the inner terminal has a recess 2a into which the part 15 projects. Part 15 of the inner terminal passes through a bore 5a of the insulator 5 and constitutes the core of the electric detonator proper.

Terminals

3 and 15 have opposite parallel annular surfaces on which bridging means 1 abuts.

When a voltage is applied to the cap 7 and to the terminal 2, the electric detonator 1 is activated since the cap 7 is electrically connected to the outer terminal 3,

In accordance with the present invention, the electric detonator 1 is so constructed that particularly along the annular bridging surface 1a a number of sparks are formed which are sufficient to detonate the adjacent charge. Various embodiments of the detonator 1 according to the present invention will now be described with reference to the other figures.

In the embodiment of Fig. 2 a plurality of insulated very thin wires extends parallel to the core 15 in the space between the outer terminal 3 and the inner terminal 15. Since the wires 10 extend parallel to the annular faces a and 3a, the

terminals

3 and 15 are separated from each other by the conductive end face areas of the wires 10 and by the insulating end face areas of the wires 10 which separate and insulate the conductive cross sectional areas from each other. When a voltage is applied to the

terminals

2, 15 and 3, sparks form particularly along the surface 1a, see Fig. l, between conductive cross sectional areas so that a bridge of series connected sparks connects the terminals. The sparks merge and produce suflicient heat to ignite the adjacent charge 9. The wires 10 are preferably of the type used for stranded wires for conducting high frequency currents. When the wires 10 are cut off along the bridging surface 1a, burrs are formed on each wire whereby the insulation of the respective wires is partly bridged so that the formation of sparks is facilitated.

In the embodiment illustrated in Fig. 3, the bridging means 11 is formed by a coil which is spirally wound about the core 15. The coil consists of a conductive band on which an insulating band is superimposed in the manner of a known condenser construction. The arrangement is such that the layers of conductive material and the layers of insulating material extend parallel to the annular surfaces of the terminals. The lateral edges of the conductive band and of the insulating band are flush along the bridging surface la, while along the face 1b, see Fig. l, the insulating band slightly projects in axial direction beyond the conductive band. Thereby the formation of sparks at the end of the detonator remote from the charge 4is prevented, whereas the formation of sparks adjacent the detonator charge is facilitated. Particularly, if the spiral-shaped coil 11 is produced by cutting a longer coil into two detonator coils, burrs are created along the surface la `so that the insulating band layers are partly bridged for facilitating the formation of spark gaps.

While the conductive band of the detonator coil constitutes a conductive connection between the inner terminal 15 and the outer terminal 3, due to the high inductivity of the spiral-shaped coil, the current does not ow through the conductive band, but a discharge takes place along radial paths on the face 1a of the coil.

It will be understood that such spark formation is facilitated by the burrs which reduce the thickness of the insulating layers.

If only very few windings are provided, the inductive resistance of the detonator coil may become too small so that the current would ilow along the conductive band. Such conductive band 11 is continuous in the embodiment of Fig. 3, as also shown in Fig. 4. To interrupt the direct connection between the outer terminal and the inner terminal, the conductive band means is formed in the embodiment shown in Figs. 5 and 6 of a plurality of band portions 12. Such band portions are superimposed on an insulating band 12a, so that the ends of adjacent conductive band portions 12 are spaced from each other as best -seen in Fig. 6. Since due to the interruptions between the conductive band portions no current can flow directly from the outer terminal 3 to the, inner terminal 15, the voltage is discharged along radial spark gaps formed between the cross sectional areas of the conductive band means and the terminals.

According to the modified embodiment shown in Fig. 7, the outer terminal is not a solid tubular .IIlial mlfm ber 13, but is formed of a spiral-shaped winding of a conductive band 13. In the embodiment of Fig. 7, superimposed conductive and insulating bands are wound in a spiral-shaped coil about the core 15, and after sufcient layers of conductive and insulating materials are provided, the insulating band is cut off, and the conJ ductive band is further wound to form a spiral-shaped winding which constitutes the outer terminal 13a. Since the insulating bland is omitted in the winding 13a, all conductive layers 13 are connected to each other and constitute a terminal 13a. It will be understood that in the embodiment of Fig. 7, the bridging surface 1a may also be cut to form the advantageous burrs, while the insulating band may project beyond the conductive band at the other end of the detonator coil.

Figs. 8 and 9 illustrate the manufacture of a detonator coil according to the present invention. On the core 15 is wound a metal band 13 and an insulating band 16 which is superimposed on the conductive band 13. Another conductive band 14 is separated from the conductive band 13 by a space 17, so that in the finished coil the direct conductive connection between the inner terminal 15 and the outer terminal 13a is interrupted. Fig. 9 shows the winding of the coil in a schematic manner with the loops of the coil spaced from each other. A plurality of spark gaps 18 extending in radial direction with respect to the coil and the core are indi' cated by the reference numeral 18.

Figs. l0 and l1 illustrate a modied embodiment in which two

conductive bands

25 and 28 are arranged alternatively with two insulating

bands

26 and 27. Similar to the arrangement of Figs. 8 and 9, the inner ends of the respective bands are staggered. First, the conductivev band 25 is wound on the core 15 then the insulating band Z6 is added in a position in which the metal band 25 is superimposed upon the insulating band 26, then the insulating band 27 and the conductive band 2S are added. Fig. ll illustrates the rst part of the coil in a schematic manner with the windings spaced from eachl other, and it will be seen that a plurality of spark gaps 29 indicated by radial lines are formed between the cross sectional areas of the conductive bands and of the insulating bands. In this embodiment as well, the end portion of the coil adjacent the detonating charge is preferably cut off so that burrs are formed on the lateral edges of the conductive metal bands which partly bridge the insulating bands and facilitate the formation of sparks along the cut bridging surface 1a. The insulating layers may project at the other end of the coil beyond the lateral edges of the conductive layers to prevent spark formation at the end of the coil remote from the detonator charge 9.

it will be understood that each of the elements dcscribed above, or two or more together, may also find a useful application in other types of electric detonators differing from the types described above.

While the invention has been illustrated and described as embodied in an electric detonator in which a plurality of spark gaps is formed between very small cross sectional areas of a conductor means, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute esscnial characeristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1 Eletric detonator comprising, in combination, a

pair of spaced terminals insulated from each other and having opposite annular surfaces forming a wide gap; and a bridging means including a thin elongated conductive band means and a thin elongated insulating band means superimposed upon said conductive band means, said conductive band means and said insulating band means being wound in superimposed positions about one of said terminals to form a spiral-shaped coil located between and abutting against said surfaces of said terminals, said conductive band means having a conductive edge means located in a surface transverse to said opposite surfaces of said terminal, and said insulating band means having an insulating edge means located in said transverse surface adjacent said conductive edge means and separating and insulating portions of said conductive edge means, said insulating and conductive edge means forming a bridging surface extending between said terminals transverse to said annular surfaces whereby a series of spark gaps connecting said terminals is formed by said edge means, and said wide gap is bridged by sparks passing between said terminals over said portions of said conductive edge means and across said insulating edge means along said bridging surface when a voltage is applied to said terminals while no current passes between said insulated terminals through said insulating band means.

2. Electric detonator comprising, in combination, a pair of spaced terminals insulated from each other and having opposite surfaces forming a wide gap; and a bridging meansincluding a plurality of thin insulated wires extending parallel to said surfaces and including conductive wire material and layers of solid insulating material enveloping said wire material, respectively, said bridging means being located in said gap abutting against said surfaces of said terminals, said conductive material having conductiveend face means located in a surface transverse to said opposite surfaces of said terminal, and said layers of insulating material having annular insulating end face means located in said transverse surface adjacent said conductive end face means and separating and insulating said conductive end face means from each other, said said end face means of said conductive and insulating materials forming a continuous bridging surface extending between said terminals transverse to said opposite surfaces whereby a series of spark gaps connecting said terminals is formed by said end face means and said wide gap is bridged by sparks passing between said terminals over said conductive end face means-and across said insulating end face means along said bridging surface when a voltage is applied to said terminals while no current passes between said insulated terminals through said layers of insulating material.

3. Electric detonator comprising, in combination, a pair of spaced terminals insulated from each other and having opposite annular surfaces forming a wide gap; and a bridging means including a plurality of thin insulated Wires extending parallel to and in axial direction of said annular surfaces and including conductive wire material and layers of solid insulating material enveloping said wire material, respectively, said bridging means being located in said gap abutting against said annular surfaces of said terminals, said conductive material having conductive end face means located in a surface transverse to said opposite surfaces of said terminal, and

said layers of insulating material having annular insulating end face means located in said transverse surface adjacent said conductive end face means and separating and insulating said conductive end face means from each other, said end face means of said co-nductive and insualting materials forming a continuous bridging surface extending between said lterminals transverse to said opposite surfaces whereby a series of spark gaps connecting said terminals is formed by said end face 6 between said terminals over said conductive end face means and across said insulating end face means along said bridging surface when a voltage is applied to said terminals while no current passes between said insulated terminals through said layers of insulating material.

4. fElectric detonator comprising, in combination, a pair ofv spaced terminals insulated from each other, at least one of said terminals including a first conductive band portion wound to form a spiral-shaped winding having a substantially annular surface, the other of said terminals having an annular surface located opposite and spaced from said annular surface of said terminal and forming a wide gap with the same; a second conductive band portion integrally connected with said first conductive band portion and forming a continuous conductive band with the same; a thin insulating band superimposed upon said second conductive band portion and being wound with the same to form a spiral-shaped coil. located between and abutting against said annular surfaces of said terminals so that conductive layers and insulating layers extend substantially parallel to said annular surfaces, said second conductive band portion having conductive edge means located in a surface transverse to said opposite surfaces of said terminal and said insulating band having insulating edge means 1ocated in said transverse surface adjacent said conductive edge means and insulating portions of said conductive edge means from each other, said conductive and insulating edge means forming a bridging surface extending between said terminals transverse to said annular surfaces whereby a series of spark gaps connecting said terminals is formed by said edge means and said wide gap is bridged by sparks passing between said terminals over said conductive edge means and across said insulating edge means along said bridging surface when a voltage is applied to said terminals while no current passes between said terminals through said insulating band.

5. Electric detonator comprising, in combination, a pair of spaced terminals insulated from each other, at least one of said terminals including a irst conductive band portion wound to form a spiral-shaped Winding having a substantially annular surface, the other of said terminals having an annular surface located opposite and spaced from said annular surface of said terminal and forming a wide gap with the same; -a second conductive band portion integrally connected with said rst conductive band portion and forming a continuous conductive band with the same; a third conductive band portion having an end spaced a short distance from one end of said second conductive band portion; a thin insulating band superimposed upon said second and third conductive band portions and being wound with the same to form a spiralshaped coil located between and abutting against said annular surfaces of said terminals so that conductive layers and insulating layers extend substantially parallel to said annular surfaces, said second and third conductive band portions having a conductive edge means located in a surface transverse to said opposite surfaces of said terminal and said insulating band having insulating edge means located in said transverse surface adjacent said conductive edge means and insulating portions of said conductive edge means from each other, said conductive and insulating edge means forming a bridging surface extending between said terminals transverse to said annular surfaces whereby a series of spark gaps connecting said terminals is formed by sad.edge means and said wide gap is bridged by sparks passing between said terminals over said conductive edge means and across said insulating edge means along said bridging surface when a voltage is applied to said terminals while no current passes between said terminals through said insulating band.

6. Electric detonator comprising, in combination, a pair of spaced terminals insulated from each other and having opposite surfaces forming a wide gap; and a bridging lmeans and said wide gap is bridged by sparks passing means including a plurality of thin insulated wires extending parallel to said surfaces and including conductive wire material and layers of solid insulating material enveloping said wire material, respectively, said bridging means being located in said gap abutting against said surfaces of said terminals, said conductive material having conductive end face means located in a surface transverse to said opposite surfaces of said terminal having burrs, and said layers of insulating material having annular insulating end face means located in said transverse surface adjacent said conductive end face means and separating and insulating said conductive end face means from each other and being partly covered by said burrs, said end face means of said conductive and insulating materials forming a continuous bridging surface extending between said terminals transverse to said opposite surfaces whereby a series of spark gaps connecting said terminals is formed by said end face means and said wide gap is bridged by sparks passing between said terminals over said conductive end face means and across said insulating end face means along said bridging surface when a voltage is applied to said terminals while no current passes between said insulated terminals through said layers of insulating material.

7. Electric detonator comprising, in combination, a

pair of spaced terminals insulated from each other and having opposite annular surfaces forming a wide gap; and a bridging means including a thin elongated conductive band means and a thin elongated insulating band means superimposed upon said conductive band means, said conductive band means and said insulating band means being wound in superimposed positions about one of said terminals to form a spiral-shaped coil located between and abutting against said surfaces of said terminals, said conductive band means having a conductive edge means located in a surface transverse to said opposite surfaces of said terminal and having burrs, and said insulating band means having an insulating edge means located in said transverse surface adjacent said conductive edge means and separating and insulating portions of said conductive edge means and being partly covered by said burrs, said insulating and conductive edge means forming a bridging surface extending between said terminals transverse to said annular surfaces whereby a series of spark gaps connecting said terminals is formed by said edge means, and said wide gap is bridged by sparks passing between said terminals over said portions of said conductive edge means and across said insulating edge means along said bridging surface when a voltage is applied to said terminals while no current passes between said insulated terminals through said insulating -band means.

8. Electric detonator comprising, in combination, a pair of spaced terminals insulated from each other and having opposite annular surfaces forming a wide gap; and a bridging means including a thin elongated conductive band means including two conductive bands and a thin elongated insulating band means including two insulating bands, said conductive bands being alternately arranged with said insulating bands so that each insulating band is superimposed on one of said conductive bands, said conductive band means and said insulating band means being wound in superimposed positions about one of said terminals to form a spiral-shaped coil located between and `abutting against said surfaces of said terminals, said conductive band means having a conductive edge means located in a surface transverse to said opposite surfaces of said terminal, and said insulating band means having an insulating edge means located in said transverse surface adjacent said conductive edge means and separating and insulating portions of said conductive edge means, said insulating and conductive edge means forming a bridging surface extending between said terminals transverse to said annular surfaces whereby a series of spark gaps connecting said terminals is formed by said edge means, and said wide gap is bridged by sparks passing between said terminals over said portions of said conductive edge means and across said insulating edge means along said bridging surface when a voltage is applied to said terminals while no current passes between said insulated terminals through said insulating band means.

9. Electric detonator comprising, in combination, a pair of spaced terminals insulated from each other and having opposite annular surfaces forming a wide gap; and a bridging means including a thin elongated conductive band means and a thin elongated insulating band means superimposed upon said conductive band means, said conductive band means and said insulating band means being wound in superimposed positions about one of said terminals to form a spiral-shaped coil located between and abutting against said surfaces of said terminals, said conductive band means having at one end of said coil a conductive edge means located in a surface transverse to said opposite surfaces of said terminal, and said insulating band means having at said one end of said coil an insulating edge means located in said transverse surface adjacent said conductive edge means and separating and insulating portions of said conductive edge means, said insulating and conductive edge means forming a bridging surface at said one end of said coil extending between said terminals transverse to said annular surfaces whereby a series of spark gaps connecting said terminals is formed by said edge means, and said wide gap is bridged by sparks passing between said terminals over said portions of said conductive edge means and across said insulating edge means along said bridging surface when a voltage is applied to said terminals, said conductive band means having at the other end of said coil another conductive edge means, and said insulating band means having at said other end of said coil another insulating edge means, said other insulating edge means projecting beyond said other conductive edge means and forming an insulating wall between the same to prevent formation of spark gaps on the respective end of said coil while no current passes between said insulated terminals through said insulating band means.

l0. Electric detonator comprising, in combination, a pair of spaced terminals insulated from each other and having opposite annular surfaces forming a wide gap; and a bridging means including a thin elongated conductive band means and a thin elongated insulating band means superimposed upon said conductive band means, said conductive band means including at least two conductive bands having confronting ends spaced a short distance from each other, said conductive band means and said insulating band means being wound in superimposed positions about one of said terminals to form a spiral-shaped coil located between and abutting against said surfaces of said terminals, said conductive band means having a conductive edge means located in a surface transverse to said opposite surfaces of said terminal, and said insulating band means having an insulating edge means located in said transverse surface adjacent said conductive edge means and separating and insulating portions of said conductive edge means, said insulating and conductive edge means forming a bridging surface extending between said terminals transverse to said annular surfaces whereby a series of spark gaps connecting said terminals is formed by said edge means, and said wide gap is bridged by sparks passing between said terminals over said portions of said conductive edge means and across said insulating edge means along said bridging surface when a voltage is applied to said terminals While no current passes between said insulated terminals through said insulating band means.

l1. An electric detonator comprising, in combination, a pair of spaced terminals insulated from each other and having opposite faces forming a wide gap; and a bridging means located in said gap between said faces and including electrically conductive elements extending sub- Stantally in the same direction as said faces of said terminals and having conductive end faces located in a surface transverse to said opposite faces, and layers of a solid insulating material extending substantially in the direction of said opposite faces between said conductive elements, said layers of insulating material having insulating edges located adjacent said endy faces in said transverse surface and separating and insulating said conductive end faces from each other so that said conductive end faces of said conductive elements and said edges of said layers of insulating material form a continuous bridging surface extending between said opposite faces transverse to the same and to said layers of insulating material whereby a series of spark gaps connecting said terminals is formed by said end faces of said conducitve elements so that said wide gap is bridged by sparks passing between said terminals from conductive end face to conductive end face and across said edges fof vsaid layers of insulating material over said bridging surface when a voltage is applied to said terminals, while no current passes between said terminals through said layers of insulating material.

l2. An electric detonator as set forth in claim 11 wherein said end faces of said conductive elements, and

said edges of said layers of insulating material are located in a plane so that said bridging surface4A is planar.

13. An electric detonator as set forth in" claim 11 wherein said opposite faces of said terminals are circular and concentric; and wherein said end faces of said conductive elements and said edges of said layers of insulating material are located in a plane perpendicular to said opposite faces of said terminals so that said bridging surface is planar.

14. An electric detonator as set forth in claim 11 wherein said end faces of said conductive elements have transverse burrs projecting over part of said edges of said layers of insulating material to facilitate the crossing of said edges of said layers by sparks.

References Cited in the tile of this patent UNITED STATES PATENTS 1,425,476 Henkle Aug. 8, 1922 2,086,548 Handforth July 13, 1937 2,261,436 Fanger Nov. 4, 1941 2,342,842 Corbin Feb. 29, 1944 2,764,091 Hudson Sept. 25, 1956 2,818,020 Burklund Dec. 3l, 1957l