US2220089A - Gaseous discharge tube - Google Patents

Description

Nov. 5, 1940- M. J. DRUYVESTEYN ET AL 2,220,039

GASEOUS DISCHARGE TUBE Filed Feb. 4, 1938 By v Hiram/ v Patented Nov. 5, 1940 UNITED STATES 2,220,089 GASEOUS DISCHARGE TUBE M J 011ml y s eyn and Nicolaas Warmoltz,

Eindhoven, Netherlands, assignors, by mesne asv signments, to Trust Company,

Hartford National Bank and Hartford, Conn as trustee;

Application February 4, 1938, Serial No. 188,765 r In Germany FebruaryS, 1937 I 3 Claims.

This invention relates to gaseous discharge tubes, and more particularly to ionic relay tubes having a grid to control the discharge.

By gaseous discharge tube is means a tube whose filling comprises one or more gases or vapors, or a mixture thereof.

In such tubes the electrodes are so proportioned and interspaced with respect to the pressure of the gaseous filling, that the discharge can be interrupted without reducing the anode voltage to zero or to a small value. This effect is generally a function of the anode voltage applied to the tube and of the amount of anode current flowing at the time of interruption, and the tube is so designed that the positive ion sheaths occurring at adjacent portions of the negativelycharged control grid contact with each other and with the electrons of the discharge passing through the grid aperture in question. As a result, the passage of discharge current at this point is completely interrupted.

In the past such an interruption of the discharge could be insured only with very low discharge currents, and with grids having very small apertures. More particularly, as the thickness of the positive ion sheath isdetermined by the degree of ionization of the plasma in the apertures of the grid, the positive ion sheath becomes thinner with a high concentration of the discharge in the grid aperture, and as a result only discharge currents of very low intensity can be interrupted. This difiiculty is very pronounced .whenthe discharge concentrates upon a single aperture of the grid so that the. remaining aper tures are inoperative.

It is well known that when an arc-discharge issues from. an incandescent cathode in a gaseous filling at a comparatively low pressure, for instance from a few microns to several millimetres of mercury, there is formed in the immediate vicinity of the cathode a comparatively thin cathodic dark discharge space which absorbs substantially the entire voltage drop of the discharge. The thickness of this dark space in centimetres can be determined approximate y from the following formula:

,.nores the; influence of cathode curvature, this is permissible provided the thickness of the dark discharge space is very small relative to the radius of curvatureof the cathode surface, which is usually the casein practice.

From Formula 1 we may obtain the following simple Formula 2 for the maximum thickness (dmax) of the normal cathodic dark discharge space, when there are no other electrodes in the immediate vicinityof the cathode, since 17 volts may be inserted as the maximum value that may be expected for Vs. r

1.74 10'=. max Experiments have shown that this cathodic dark space must extend to the grid which is interposed between the cathode and anode, and

must completely fill the intermediate space in order that the discharge may be interrupted by this grid. This small'thickness of the cathodic dark space has caused considerable difficulties in the manufacture of' such tubes, because the small distance between the grid and cathodemade it very ,diflicult to prevent inadmissible heating of the grid, and it was also difficult to obtain and maintain a well-insulated arrangement.

We have found that a'very favorable grid action can be obtained, even when the grid is arranged beyond the range ofthe normal dark discharge space, and that under the influence of a neighboring electrode the cathode dark space can- Within certain limits-'-be drawn beyond its normal thickness and into the proximity of this electrode. For this purpose and in accordance with the invention, we so locate the grid that it separates thecathode from the anode, and that its distance from the cathode exceeds the above-defined maximum thickness (dmax) of the normal cathodic dark space, but is sufiiciently small to allow the cathodic dark space-which in this case will extend a greaterdistance from the cathodeto completely fill the portion of the discharge space between the grid and the cathode.

The above results may be obtained conveniently'by using a grid of wire mesh whose apertures have a maximum width of about 0.4 mm., andby using an argonfilling having a pressure during operation of not more than 0.7 mm. of mercury, or a filling of another rare gas, vapor or mixture thereof at such pressure that of the atoms isobtained.

A particularly advantageous electrode arrangement is obtained when using a grid-cathode distance of 2 to 4 mm.

From Formula 2 above, we find the following an aqual free path cathodic dark space as a function of the discharge current density 2' at the cathode surface:

2,220,089 values for the maximum thickness (dmax) of the of argon at a pressure of 0.3 mm. of mercury during operation and with a grid mesh of 230.

T bl I Table II G d th Discharge ri -ca d distance mm. curl-en ima 333m 7 J (ml) W1th tubes having a grid-cathode distance of From Table I it appears that (1...... does not reach 1 mm. even with the lowest surface loads of the cathode. Thus the increase that may be obtained according -to the invention-in the gridcathode distance is of the greatest importance because this increase enables one to preventexcess heating of the grid and insulation difiiculties. Notwithstanding this increase in grid-cathode distance, the space between the grid and the cathode can be completely filled up by the cathodic dark space. Furthermore, concentration of the discharge upon a part of the grid or cathode surface-which is possible with the usual construction and efiects interruption of the discharge-is eliminated, and as a result within certain limits the discharge can be interrupted independently of the anode voltage or anode current. a

In order that the invention may be more clearly understood and readily carried into efiect, we shall describe the same in more detail with reference to the accompanying drawing, in which the single figure is a sectionized veiw of an ionic relay tube according to the invention.

The tube illustrated in the drawingcomprises a glass envelope l havinga pinch 6. Supported from the pinch 6 by a lead-support 8 is an indirectly-heated cathode body 2 whose outer surface is provided with a suitable electron-emissive substance, and a heating element 3 having leads 9 and I0. Insulated from cathode 2 by means of two rings 1 of insulating material such as magnesia and disposed a short distance therefrom, is a cylindrical grid 4 of wire mesh provided with a lead ll. Located at a relatively large distance from the grid 4 and supported from pinch 6 by lead supports l2 are two plate-shaped anodes 5.

In the portion of the discharge space between the cathode 2 and the grid 4 there are no points at which the distance between the grid and cathode exceeds the values which are admissible ac cording to the invention. If a larger distance did exist, even at a single point, so that the cathodic dark space did not extend to the grid, it would no longer be possible to insure an accurate extinguishing effect under all conditions.

Experiments were made with tubes of the type illustrated, in which the anodes were electrically connected to a suitable current supply source. Cathode 2 had an active length of 23 mm., a diameter of 4 mm., and an active surface of 2.9 sq. ems, and grid 4' consisted of a metal 'wire mesh. We have made several tubes accord- 1.5 mm. and a grid mesh of 230 1., the maximum current that could be interrupted varied with the pressure of the filling as follows:

With the same grid-cathode distance as in Table III, and with an argon filling at a pres.- sure of 0.1 mm. of mercury, the influence of the mesh-width was as follows:

Table IV Current Mesh width (microns) (mm j All the above results were obtained with the thus. completing the main discharge circuit trav- :"1

ersing the tube. A second D. C.-supply I 6 is connected, with its positive terminal to the oathode 2, its negative terminal being connected, through a limiting resistance 11, to a switching key 18, the condenser l9 being permanently kept at the potential of the D. C.-supply l6. As soon as the key is depressed, the condenser communicates a negative impulse to the grid 4, thus interrupting the main discharge from the anodes 5, against the full pressure of .the D. C.-supply I 3. The lead-in conductor 9 of the heater 3 is directly connected to the cathode lead, whereas the lead-in conductor I0 is connected, through the limiting resistance 20, to the positive terminal of the D. C.-supply l3.

The experiments described above have been "executed with a voltage of the D. C.-supply l3 of about hundred volts and it proved possible to reliably interrupt the are by means of a condenser |9 of 2 P, the voltage of the D. C.-supply- Hg and a mesh-width of about 230 microns, an

ode currents of 500 mA. can be interrupted in a reliable manner.

While we have described our invention in connection with specific examples and applications, we do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What we claim is:

1. An ionic relay tube comprising an envelope, an anode and a cathode within said envelope and spaced apart to form a discharge space, a gaseous filling, and an apertured extinguishing grid adapted to be negatively charged between said anode and cathode and'disposed from the cathode a distance materially greater than the maximum 7 thickness dmax of the normal cathodic dark space, the distance dmax equaling 1.74X l0- i cms. in which i is the current density in amperes per sq. cm. of the discharge at the cathode surface, the cathodic dark space of the tube extending with its outer limits reaching the openings of said grid at the values of operating current density of the tube and completely filling the portion of the discharge space between said cathode and grid.

2. An ionic relay tube comprising an envelope, an anode and a cathode within said envelope and spaced apart to form a discharge space, a

filling of argon having a maximum pressure during operation of 0.7 mm. of Hg, and an extinguishing gridadapted to be negatively charged disposed between said anode and cathode and comprising wire wesh provided with apertures having a width less than 0.4 mm., said grid being spaced from said cathode a distance greater than the maximum thickness dmax of the normal cathodic dark space, the distance dmax equaling cms. in which i is the current density of the discharge at the surface of the cathode in amperes per sq. cm., the cathodic dark space of the tube extending into the openings in the wire mesh at the values of current density for which the tube is designed.

3. An ionic relay tube comprising an envelope, an anode and a cathode within said envelope and spaced apart to form a' discharge space, a gaseous filling within said envelope, and an apertured extinguishing electrode adapted to be negatively charged and separating said anode and cathode and spaced from said cathode by a distance of from 2 to 4 mm., the cathodic dark space of the tube extending with its outer limit reaching the apertures of said grid at the values of current density for which said tube is designed.

MARI JOHAN DRUYVESTEYN. NICOLAAS WARMOLTZ.

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