US2218386A - Discharge device - Google Patents

Description

Oct. 15, 1940.

H. M. SMITH DIS CHARGE DEyICE Filed June 17, 1938 2 Shets-Sheet 1 Fig. I

Fig.2.

Inventor: Henry MSrnith,

y His Attorney.

Get. 15, 1940. M SMITH 2,218,386

' DISCHARGE DEVICE Filed June 17, 1938 2 Sheets$heet 2 Inventor Henry MSmith,

by 2&

torney.

Patented Oct. 15, 1940 UNITED STATES PATE T OFFICE 2,218,388 mscnaacs nnvrcs Henry M. Smith, Schenectady, N. I, ascignor to General Electric Company, a corporation of New York Application June 17, 1938, Serial N0. 214,258

4 Claims.

The present invention relates to improvements in pool type discharge devices.

It is a primary object of the invention to provide improved means for initiating a discharge 6 between an anode and a pool type cathode, such v sectional view of a discharge device suitably embodying the invention; Fig, 2 comprises an enlarged perspective view of one element of the device of Fig. 1; Fig. 3 illustrates a magnified fragmentary section of the said element; Fig. 4 illustrates a further mode of application of the 20 invention and Fig. 5 illustrates a still further modification thereof. v

Referring particularly to Fig. 1, I have shown a discharge device comprising a. sealed metal envelope I enclosing an anode l. l and a pool type 25 cathode l2. The cathode is shown as being in direct contact with the envelope. The anode ll is'insulatingly supported from the envelope by means of a connection which comprises an insulating portion l3 and a lead-in conductor It. In operation, potential is impressed between the anode and the cathode by means of a transformer IS. A load I! is connected in series with the secondary winding of the transformer so as to be energized by the current which flows 35 through the discharge device.

Also within the envelope, there is provided an additional cathode which is independent of the main cathode I2 and which is illustrated as com.- prising a coiled filament 20. This filament may 40 comprise either a plain tungsten wire or a refractory metal core coated or otherwise treated with an effective electron emitter such as an alkaline earth material. Its lead-in-connections are insulated from the envelopeby means of in- 45 sulating seals 2| and 22, and it may be maintained at a temperature of thermionic emission by current supplied from a heating transformer 23.

The function of the filament" is to facilitate initiation of a main discharge between the anode and cathode when a favorable potential is applied between them. In its application to this use the filament is caused to establish a preliminary discharge which ionizes the-residual gas 55 or vapor within the envelope. This may be M- complished, for example, by connecting the central point of the filament heating transformer 23 to an appropriate point of the secondary of the transformer It as indicated at 26. With this connection, an ionizing discharge will occur be- 5 tween the filament 20 and the anode II whenever the latter attains a positive potential. A

, resistor 28 may be inserted in the filament circuit to limit the current therethrough.

It is known that with a circuit such as that shown the ionizing discharge between the filament l6 and the anode II will eventually transfer to the surface of the cathode l2. However, the transfer occurs in such an irregular and undependable manner as to make the phenomenon l5 referred to of little practical use. That is to say, with the combination of elements so far described the time required for the transfer to take place may vary within 'such wide limits as to render the device extremely erratic in its operation.

In accordance with my invention, this difficulty is overcome by the use of insulating par ticles which form a plurality of sharp junctions with the cathode material of the cathode surface. My observations indicate that the improved operation thus obtainable is the result of the accumulation. of charges on the insulating L particles and the ensuing creation of a cathode spot by the high electrostatic gradients so produced. The charges referred to are provided by ions collected from the preliminary discharge established by the filament 20 and are effective to cause substantial field emission from. the oathode material adjacent the boundaries of its contact with the particles. This emission is sufll- 3 cient to initiate an arc discharge from the cathode almost instantaneously after starting of the preliminary discharge.

Whether this is the true explanation or not, I have found that highly reliable and consistent operation is obtainable by an arrangement such material, I may embed them in a mass of conducting particles, such as is represented, for example, by the solid body 30 of Fig. 1. This body, which is shown on an enlarged scale in Fig. 2, may comprise for instance an agglomeration of partially sintered particles of carburized molybdenum or tungsten as described and claimed in U. S. Patent No. 2,128,861 granted August 30, 1938 to Lewi Tonks. These particles are preferably assembled in a porous aggregate so as to permit mercury to be drawn by capillarity from the cathode to the upper exposed surface of the body. This surface is studded with embedded particles of an insulating material such as glass, quartz, alumina or the like. The body 30 may be permitted to fioat on the surface of the cathode or may be fixed by attachment to a wall of the envelope.

This mode of combination of the metallic and insulating particles is indicated in Fig. 3 which represents a microscopically enlarged fragment of the body 30. In this figure, there is shown one glass particle 30' in its relation to a group of surrounding metal particles 30". The metal particles, being effectively wet by the mercury of the cathode, serve to bring such mercury into intimate association with the edge of the glass particle 30. However, since the glass itself is not wet by the mercury there is little or no tendency for its surface to be submerged. It is, therefore, possible for the glass to become charged by the accumulation of positive ions derived from the preliminary discharge. Such charges, indicated imaginatively at a, operate on the adjacent edge of the mercury film 3| to produce field emission therefrom and to initiate a cathode spot as above suggested.

A device of the character described may be used as a single anode rectifier which is automatically rendered conductive at the beginning of each operating cycle by the functioning of the filament 20. Controlled operation may be accomplished in one way by controlling the application of potential to this filament. More sensitive control can be effected by the use of an electrostatic control member governing the initiation of the auxiliary discharge. 3

This latter feature is illustrated in Fig. 4 in which parts corresponding to elements described in connection with Fig. 1 are similarlynumbered. Here a control electrode or grid 35 is shown between the filament 20 and the anode II. In operation, this may be biased to a negative potential sufficient to prevent the initiation of a discharge between the filament 20 and the anode II. By this means, the firing of the tube can'be held off until the negative bias ofthe grid 35 is overcome by a pulse of positive potential. Upon the application of such a potential, the preliminary discharge will start so as to permit the establishment of the main discharge.

An additional grid 36 may be provided between the filament 20 and the pool l2 to prevent current flow between them during that portion of the operating cycle when the pool attains a positive potential. This may beaccomplished, for example,by connecting the grid 36 to the anode ll so as to cause it to assume a negative bias whenever the anode becomes negative, It will thus function to avoid the occurrence of an ionizing discharge during the reverse half-cycle and to minimize the danger of arc-back.

In Fig. 5, I have shown a further embodiment of the invention as applied in connection with a device which is adapted to be controllably conductive in both directions. This device comprises an arcuate envelope 40 which is closed at its ends by metallic cup-like receptacles 4| and 42. Each of thesecontains a pool of cathode material (indicated at 43 and 44) having associe ated therewith a composite body similar to the body 30 described in connection with Fig. 1. These bodies are designated by the numerals 46 and 41. At each end of the envelope there is provided an electron emissive filament shown at 48 and 49 respectively. The two filaments are connected in parallel to a heating transformer so as to be maintained at a temperature of effective electron emission.

In the device just described, each of the pools 43 and 44 can be made to serve alternately as anode and cathode. However, in order to shorten the discharge path, it is preferred to provide an auxiliary electrode element in proximity to each pool surface for receiving the discharge therefrom. Thus, in the arrangement shown, the auxiliary elements comprise bodies 53 and 54 of graphite or the like which are each conductively connected with one of the cathode pools by means of metallic conductors 56 and 51.

In the operation of the device, the filament 48 functions to start a preliminary ionizing discharge to the electrode body 53 whenever such body becomes positive with respect to the pool 46. The filament 49 performs a similar function in connection with electrode body 54. A circuit by which the desired relationship can be obtained is indicated diagrammatically in Fig. 5. It comprises a source of potential 60 and a resistance 6| connected between the cathode pools 43 and 44. The resistance 6| carries relatively little current since it serves mainly as a potential dividing means. To this end its resistance may be relatively high, say on the order of several thousand ohms.

In series with the supply source 60 and the discharge device there is provided a transformer 63 arranged to supply power to a load device 64. With a circuit such as that illustrated, this load device may comprise an element whose operation is to be controlled by supplying an adjustable asymmetrical current thereto. One example of such a device is a variable speed alternating current motor.

A variable connection 66 is provided between the resistor BI and the mid-point of the filament transformer 5|. With this connection tapped to the midpoint of the resistor 6|, as shown in the drawings, it is clear that the potential of each of the filaments 48 and 49 will be maintained midway between the potentials of the pools 43 and 44. Consequently when the pool 44 becomes positive with respect to the pool 43, the filament 48 will act to produce a discharge to the electrode body 53, which is at the same potential as the pool 44. In accordance with the principles previously described, this discharge will immediately be transferred to the pool 43,'the body 46 serving to increase the reliability and rapidity of the transfer, I As soon as the potential reverses and reaches a point at which an appreciable potential difference exists between the filament 49 and the electrode body 54, a discharge in the other direction will take place.

Under the conditions so far described the discharges through the device 40 will be symmetrical, that is, the period of current conduction in one direction will be equal in duration to the period of conduction in the other direction. However, if the connection 66 is moved to the extreme left, both filaments will be brought to the same potential as the pool 43. Consequently, under no circumstances can a discharge be produced unidirectionally, such passage occurring whenever the anode 53 becomespositive with respect .to the pool 43 and consequently with respect to the filament 48. On the other hand, if the connection 66 is moved to the extreme right, unidirectional conduction will take place in the other direction.

For positions of the connection 56 which are intermediate between the midpoint of the resistor GI and its extremities the device will neither be symmetrically conducting nor unidirectionally conducting. It will, on the contrary, conduct in both directions, such conduction being asymmetrical. In other words, current will be caused to flow in each direction through the device 40 but the period of flow in one direction will not be equal to the period of flow in the other direction. By varying the nature of the asymmetry by moving the conductor 66, appropriate regu-' lation of the load device may be obtained. It will be understood, of' course, that the particular control circuit shown is not essential to my invention and that other control arrangements may be interposed between the filaments 48 and 49 and the cathode pools.

I have so far described my invention primarily in connection with devices comprising pool type cathodes. It will be understood, however, that the combination of a discharge initiating filament and the use of exposed insulating particles in contact with the cathode surface may also be useful in connection with solid non-thermionic cathodes. That is to say, this combination may be used to initiate a discharge in the type of device which is conventionally referred to as a glow tube. It is also applicable either to power tubes or to lamps.

While I have referred to particular structural embodiments, various modifications may be made by those skilled in the art without departing from the invention. I, therefore, aim in the apenvelope enclosing an anode and a pool-type cathode, a rigid body in contact with the cathode surface and providing a number of contiguous metallic and insulating surfaces, the said metallic surfaces functioning to maintain the cathode material in contact with the said insulating surfaces, and means for producing an ionizing dis-' charge within the envelope, said last-named means serving in conjunction with the said rigid body to. facilitate the initiation of a main discharge between the anode and pool-type cathode.

2. An electric discharge device comprising an envelope enclosing an anode and a pool-type cathode, a rigid body in contact with the cathode surface and providing a number of contiguous metallic and insulating surfaces, the said metallic surfaces functioning to maintain the cathode material in contact with the said insulating surfaces, means for producing an ionizing discharge within the envelope, said last named means serving in conjuction with the said rigid body to facilitate the initiation of a main discharge between the anode and pool-type cathode, and auxiliary means including an electrostatic control member for positively controlling the initiation cathode.

4. An electric discharge device comprising an envelope enclosing an anode and a mercury cathode, a porous body comprising a metal capable of being wet by mercury and having insulated particles embedded therein, the said body being in contact with the mercury at its surface so that the said metal serves by capillary action to maintain the mercury in contact with the said insulating particles, and means for producing an ionizing discharge within the envelope, said lastnamed means serving in conjunction with the said rigid body to facilitate the initiation of a main discharge between the anode and cathode.

HENRY M. SMITH.

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