AN ELECTRIC ARC EXTINCTION CHAMBER Background of the invention The invention relates to the field of chambers and 5 devices for extinguishing electric arcs. Circuit breaker devices for low voltages (U_AC~lOOO volts (V) and U DC~l500V), generally enable an electric arc to be extinguished in air. The advantage of this technique compared with extinguishing the arc in a 10 vacuum, in sulfur hexafluoride (SF6), or in oil, or indeed compared with devices making use of an insulated gate bipolar transistor (IGBT), lies in being simple to fabricate and use, and consequently in being of low cost. Breaking current on a direct current (DC) 15 electricity network necessarily involves generating a back electromotive force (emf) of potential that is greater than the potential of the source to be interrupted. This is the major difficulty for breaking DC. In the context of techniques for breaking in air, 20 the electric arc generated when opening the switch in air is used as means for generating a back emf. The main techniques of breaking in air are discussed below. The arc lengthening technique serves to lengthen and 25 thus cool the arc while opening the switch. Nevertheless, this principle can be found to have poor performance on overload. The technique of lengthening and splitting the arc combines lengthening the arc with splitting it in an 30 extinction chamber. Depending on the current to be broken, it is possible that splitting might not come into effect and there can exist critical levels of current for which the arc stagnates at the inlet to the chamber. This principle has the advantage of behaving well on 35 overload since the splitter plates support the arc and enable it to be cooled effectively. il t[ ~ i i il 2 The technique of lengthening by magnetic blowout uses a permanent magnet that tends to blow the arc out magnetically. Such magnetic blowout lengthens the arc to a great extent and cools it effectively. Nevertheless, 5 this extinction principle can be limited at high currents since the cooling of the arc can be degraded as a result of lengthening being less effective at such a level of current. Furthermore, and by way of example, extinction can 10 be made more difficult in the field of photovoltaic (PV) installations because the panels being used deliver voltages that increase from year to year in order to reduce the costs of such installations. In the content of such applications, it is known to connect a plurality 15 of switches in series in order to increase the breaking capacity of the resulting device. Nevertheless, that solution is not entirely satisfactory. Other applications, e.g. in the railway field, can also require the use of devices having considerable 20 breaking capacity on a DC network so as to enable overload voltages to be broken. It is thus desirable to improve existing electric arc extinction devices by improving their arc extinction capacity. It is also desirable to obtain circuit breaker 25 devices that can be used for splitting an electric arc generated after passing a direct current or an alternating current between electrical contacts. There thus exists a need to have novel extinction chambers and novel breaker devices presenting improved 30 circuit-breaking capacity. There also exists a need to have novel breaker devices suitable for facilitating penetration of an electric arc into the depth of the extinction chamber. There also exists a need to have novel breaker 35 devices and novel extinction chambers capable of splitting an electric arc after a direct current or an 3 alternating current has been flowing between electrical contacts. Object and summary of the invention 5 To this end, in a first aspect, the invention provides an electric arc extinction chamber comprising: · a stack of electric arc splitter plates, the splitter plates defining an inlet of the extinction chamber that is to be present facing electric contacts, 10 and a back of the extinction chamber; and · at least one permanent magnet present inside the extinction chamber in a central zone in the width direction of the extinction chamber and beside .its back, the magnet presenting magnetization having a non-zero 15 component along an axis extending between the .inlet and the back of the extinction chamber. The central zone in the width direction of the extinction chamber corresponds to the zone of the .inside of the extinction chamber defined by planes of equation 20 xa = 0.25L and xb = 0. 75L, where L designates the width of the extinction chamber and where xa and xb are measured along the width of the extinction chamber, taking one of the ends of the splitter plates as the origin. The magnet .is also situated beside the back of the 25 extinction chamber, i.e. the magnet is closer to the back of the extinction chamber than to the inlet of the extinction chamber, and the magnet generates a magnetic field of intensity that increases on going from the inlet towards the back of the extinction chamber. 30 The .invention advantageously makes .it possible to provide extinction chambers presenting improved extinction capacity. In an embodiment, the magnet may be held in an electrically insulated magnet support. 35 In an embodiment, the magnet support may be assembled by engagement with one or more splitter plates. '~ ' 4 Such a characteristic is advantageous since it makes it possible to place the magnet as close as possible to the back of the extinction chamber and for the magnet to have a stationary position relative to the splitter 5 plates. In an embodiment, the extinction chamber may further include a flux channeling element present inside the extinction chamber. The flux channeling element is constituted at least 10 in part by a magnetic part extending towards the inlet of the extinction chamber, e.g. a part of elongate shape. 15 The presence of a flux channeling element is advantageous since it contributes to "stretching'' a maximum of the magnetic field line generated by the magnet_· towards the inlet of the extinction chamber; .. The flux channeling element thus serves to further improve the attraction of an electric arc towards the back of the extinction chamber. The flux channeling element may be placed facing the 20 magnet. The flux channeling element may be held in the magnet support, and for example it may be in contact with the magnet. Nevertheless, as can be seen from the description below, such a configuration is not essential. 25 Preferably, the extinction chamber is symmetrical about a plane of equation x = 0.5L, where L designates the width of the extinction chamber and where x is measured along the width L of the extinction chamber, taking one of the ends of the splitter plates as the 30 origin. 35 Such a configuration is advantageous since it makes it possible to have an extinction chamber of extinction capacity that is unaffected by the direction in which the electric arc moves when the contacts open or by the polarity with which the breaker device is connected. i I I i I I I I i li 5 This configuration is particularly advantageous with DC because it is invariant relative to the polarity with which the breaker device is connected. In an embodiment, the height of the magnet may be 5 greater than or equal to half the height of the stack of splitter plates. Under such circumstances, the height of the magnet may be less than, or equal to, or greater than the height of the stack of splitter plates. In a variant, the height of the magnet may be less than half 10 the height of the stack of splitter plates. In an embodiment, a single magnet may be present inside the extinction chamber. In a variant, a plurality of permanent magnets may be present inside the extinction chamber, at least one 15 magnet of said plnrality of magnets being' pres·ent .in· the central zone in the width direction of the extinction chamber and beside its back. Under such circumstances, the magnets of this plurality of magnets may optionally be in contact with one another. The magnets of the 20 plurality of magnets may have the same magnetization direction, but that is not essential. In an embodiment, the majority, or even all, of the magnets in this plurality of magnets may be present in the central zone in the width direction of the extinction chamber and 25 beside its back. 30 35 In an embodiment, the extinction chamber may include one or more electrically insulating electric arc guide cheeks, the guide cheeks being situated at the inlet of the extinction chamber and covering the ends of the splitter plates in full or in part. The presence of one or more guide cheeks is advantageous insofar as they serve to prevent the arc from attaching to the ends of the splitter plates, thereby further improving extinction performance by increasing the lengthening of the arc and thus the voltage of the arc. 6 In an embodiment, the guide cheek(s) may be secured to the magnet support, and for example they may be made integrally therewith. The pre~ent invention also provides a circuit 5 breaker device comprising: an extinction chamber as defined above; and a contact zone in which there are present at least one stationary contact and at least one movable contact that is movable relative to the stationary contact, the 10 contacts being.suitable for being_ put into contact with each other and for being separated from each other, the stationary contact being present facing the inlet of the extinction chamber. In an embodiment, the movable contact may be 15 conf·igured to: move· in rotation about an axis of rotation while the contacts are being separated. In an embodiment, the device may further include an arcing horn present facing the stationary contact, the width of the arcing horn being greater than the width of 20 the stationary contact. Because of the presence of the permanent magnet in the extinction chamber, an arc generated between the contacts tends to have a non-zero movement component along the width of the extinction chamber. Thus, e.g. 25 when the movable contact is moved in rotation about an axis of rotation while the contacts are separating, the arc that is generated tends to be deflected with a nonzero component along the axis of rotation. It is thus important for the arcing horn to be wider than the 30 35 stationary contact so that while the arc is being deflected along the width of the extinction chamber, it can become "attached" to the arcing horn. Using an arcing horn can advantageously help in splitting the electric arc by facilitating entry of the arc into the extinction chamber. Specifically, the electric arc generated between the contacts under such circumstances tends to move from the stationary contact towards the 7 arcing horn and thus to come closer to the back of the extinction chamber. Another advantage associated with using an arcing horn is reducing the erosion of the stationary contact due to the arc as a result of limited 5 contact between the arc and the stationary contact. In an embodiment, the height of the arcing horn may be greater than or equal to the height of the stationary contact. In an embodiment, the movable contact may be 10 configured to move in rotation about an axis of rotation when the contacts are being separated, and a flux channeling element may be present inside the extinction chamber, the flux channeling element having a face situated beside the contact zone that, when the flux 15 channeling ·element ic observed in ·a. plane· ·perpendicular to the axis of rotation, presents the same shape as the path followed by the movable contact during separation of the contacts. Such a configuration is advantageous since in makes 20 it possible to conserve a constant distance between the flux channeling element and the movable contact while the contacts are separating, thereby further improving the attraction of the arc into the extinction chamber. In an embodiment, the device may further include a 25 flux channeling element present inside the extinction chamber, at least a portion of the flux channeling element being constituted by an arc switching element present facing the stationary contact, the width of the arc switching element being greater than the width of the 30 stationary contact. 35 In an embodiment, the flux channeling element may include an arc switching element together with an additional flux channeling element present in an electrically insulating channeling element support. Such configurations are advantageous since they make it possible to have simultaneously the effect of magnetic field lines generated by the magnet being "stretched'' 8 t·owards the inlet of the extinction chamber and assistance in causing the arc to enter into the extinction chamber because of using the arc switching element. 5 The device of the invention makes it possible to 10 15 extinguish an electric arc generated after passing DC or an alternating current (AC) between the contacts. Brief description of the drawings Other characteristics and advantages of the: invention appear from the following description of particular embodiments of the invention given as nonlimiting examples and with reference to the accompanying drawings, in which: ··Figure 1· isc:.iln exploded view .of•·an· arc extinction chamber of the invention; · Figure 2 shows the Figure 1 extinction chamber in the assembled state; · Figure 3 is a section view of the extinction 20 chamber of Figures 1 and 2, perpendicular on a plane to the height of the stack of splitter plates; · Figure 4 shows a circuit breaker device of the invention;-- · Figure 5 is a two-dimensional (2D) view of the 25 magnetic field lines created by the magnets in t.he extinction chamber of Figures 1 to 3; · Figures 6A and 6B show variant embodiments of extinction chambers of the invention; · Figures 7A to 70 show the use of an arcing horn in 30 a breaker device of the invention; and · Figures SA and 8B show variant embodiments of extinction chambers including a two-part flux channeling element. 35 Detailed description of embodiments Figure 1 is an exploded view of an arc extinction chamber l of the invention. The extinction chamber l 9 comprises a stack of electric arc splitter plates 2 mounted on a plate support 3. Mounting splitter plates 2 on the plate support 3 makes it possible to form an extinction chamber 1 that is rigid. The splitter plates 5 2 are made of mild steel, for example. By way of example, the plate support 3 may be made of vulcanized card. In a variant, the splitter plates 2 may be mounted directly on the box constituting the outer housing of the circuit breaker device. The extinction chamber 1 shown 10 in Figure 1. has a plurality of stacked splitter plates 2, e.g. at least three stacked splitter plates 2, e.g. at least five stacked splitter plates 2. The height h of the. stack of splitter plates 2 corresponds to the distance between the two splitter plates that are the 15 furthest apart. In the example shown, the· height•h of the stack of splitter plates 2 is measured perpendicularly to the splitter plates 2. The extinction chamber 1 has an inlet 10 and a back 11 situated remote from the inlet defined by the splitter plates 2. In 20 addition to the splitter plates 2, a permanent magnet 5 is present inside the extinction chamber 1. By way of example, the magnet 5 is made of NdFeB. As shown, the magnet 5 is present in an electrically insulating magnet support 7 that is present inside the extinction chamber 25 1. The magnet 5 may be in the form of a bar, as shown in Figure 1. By way of example, the bar may have a crosssection that is rectangular, square, or circular. As shown, the magnet 5 does not extend along the planes in which the splitter plates 2 extend, but along the height 30 h of the stack of splitter plates 2. In the example shown, the magnet 5 extends along a height ha, as measured along the height g of the stack of splitter plates 2, 35 that is greater than or equal to 50% of the height h of the stack of splitter plates 2. By way of example, the height ha of the magnet 5 is greater than or equal to 75% of the height g of the stack of splitter plates, the height ha of the magnet 5 being substantially equal to the ,, :-! 10 height .g of the stack of splitter plates, for example. Nevertheless, the height of the magnet is not limited to the configuration shown in Figure 1. Specifically, the magnet may present a height that is greater than the 5 height of the stack of splitter plates. In a variant, the magnet may present a height that is less than the height of the stack of splitter plates. For example, the magnet may present a height that is less than half the height of the stack of splitter plates, and under such 10 circumstances the magnet may be p~esent solely in the bottom portion of the extinction chamber. By way of example, and as shown, a single magnet 5 is present inside the extinction chamber 1, however it would not go beyond the ambit of the invention for a 15 plurality of magnets to be present inside the·extinction chamber 1. By way of example, the magnet support 7 is made of plastics material. As shown, a flux channeling element 6 is placed in contact with the magnet 5 and is likewise 20 housed in the magnet support 7. The magnet 5 and the flux channeling element 6 are electrically insulated by 25 30 the magnet support 7. By way of example, the flux channeling element 6 is made of mild steel. The flux channeling element may optionally have a laminated structure. The magnet support 7 includes engagement means 9, e.g. in the form of notches, that are to cooperate by engaging some or all of the splitter plates 2. The engagement of the magnet support 7 with the splitter plates 2 serves to hold the magnet 5 stationary relative to the splitter plates 2. Once the magnet support 7 is fastened to the splitter plates 2 via the engagement means 9, the magnet 5 is present inside the extinction chamber 1 beside the back of the extinction chamber 1 and in its central zone 35 Zc in the width direction of the extinction chamber 1, as shown in Figure 3. Figure 3 is a section view of the exb.nction chamber of Figures 1 and 2 on a plane I .I il H 11 perpendicular to the height of the stack of splitter plates 2. As shown, the splitter plates 2 are V-shaped when observed in a direction perpendicular to the planes in which they extend. In a variant, the splitter plates 5 may be of some other shape, such as a U-shape, when observed in a direction perpendicular to the planes in which they extend. Figure 3 marks the depth E of the extinction chamber 1 which corresponds to the distance between the inlet 10 of the extinction chamber 1 and the 10 back 11 of the extinction chamber 1,· as measured -· perpendicularly to the height ~ of the stack of splitter plates 2. There can also be seen the width L of the extinction chamber 1, where the width L is measured perpendicularly to the height ~ of the stack of splitter 15 ·plates 2 and···perpr~nd:i.cula:cly ta the depth·· E of .cthe· :. · extinction chamber 1. Unless specified to the contrary, the width L of the extinction chamber 1 corresponds to the inside width of the extinction chamber as measured between the ends 2a and 2b of the splitter plates 2. The 20 magnetization M of the magnet 5 (represented by arrow 15 in Figures 1 and 3) presents a non-zero component along an axis Y extending between the inlet 10 and the back 11 of the extinction chamber (also referred to as the depth axis Y of the extinction chamber 1). In particular, the 25 30 magnetization M may lie in the planes in which the splitter plates 2 extend. The magnetization M may be directed substantially solely along the depth axis Y of the extinction chamber 1. The magnetization M is shown as being directed towards the inlet 10 of the extinction chamber 1, however it would not go beyond the ambit of the invention for the magnetization to be directed towards the back 11 of the extinction chamber 1. As shown, the magnet 5 is present in a central zone Zc in the width direction of the extinction chamber 1. In other 35 words, the magnet 5 is present in a zone defined by planes Pa and P0 having respective equations xa = 0.25L and xb = 0.75L, where Lis the width of the extinction 12 chamber 1 and where xa and xb are measured a~ong the width L of the extinction chambe.r 1, taking one of the ends 2a or 2b of the splitter plates 2 as the origin. By way of example, the magnet may be present in a zone defined by 5 planes Pa and Pb having respective equations xa = 0.40L and xb = 0. 60L. In addition, the magnet 5 is situated beside the back 11 of the extinction chamber, i.e. it is closer to the back 11 of the extinction chamber 1 than is the inlet 10. 10 of the extinction chamber 1 .· In other words, the magnet 5 is present in a zone defined by planes P'a and P'b having respective equations Ya = 0.5p and Yb = p, where E designates the depth of the extinction chamber 1 and where Ya and Yb are measured along the depth of the 15 extinction chambe:r•. 1. and take· one.:· of the ends •2a: ro face (F) situated besi•dsr:·:the t:oi:1tacb zone (21) that, when the flux channeling element (6; 80) is observed in a plane perpendicular to the axis of rotation, presents the same shape as the path (C) followed by the movable contact (22) during separation of 20 the contacts (22; 25). 25 30 14. A device (20). according to any one of claims 10 to 13, characterized in that it further includes a flux ch~nneling element (80) present inside the extinction chamber ( 1) • element ( 80) element ( 82) the width L8 greater than at least a portion of the flux channeling being constituted by an arc switching present facing the stationary contact (25), of the arc switching element (82) being the width Lt of the stationary contact (25). 15. A device (20) according to claim 14, characterized in that the flux channeling element (80) includes an arc switching element (82) together with an additional flux channeling element (81) present in an electrically 35 insulating channeling element support (70).