COLD CATHODE GAS TUBES FOR TELEPHOXE SWITCHING SYSTEMS 765 



tion factor M is increased with increasing overvoltage. The multipHca- 

 tion factor M inchides electrons released at the cathode by slow moving 

 metastable gas atoms as well as those released by the faster positive 

 ions. At very low overvoltages, these slow components must be included 

 before the current can build up.^ At higher overvoltages enough positive 

 ions are produced so that M is greater than unity without waiting for the 

 slow components. Thus the effective time per multiplication cycle is 

 reduced with increasing overvoltage. Since the number of cycles and the 

 time per cycle are both decreased the formative delay decreases rapidly 

 with increasing overvoltage. A typical formative delay for a neon filled, 

 molybdenum cathode switching tube at 5 volts overvoltage might be 

 of the order of 100 microseconds. 



APPLICATION TO A TALKING-PATH SWITCHING DIODE 



The principles discussed above have been applied in the development 

 of a cold-cathode gas diode for use as a switch in series with the speech 

 path in an electronic switching system. The objectives were a switching 

 voltage gain as high as possible, a breakdown time of less than a few 

 hundred microseconds, and a low transmission impedance for audio- 

 frequency signals. 



A sketch of one version of the resulting tube is sho^vn in Fig. 5. The 

 cathode is a molybdenum rod which has a small hollow cathode portion 

 in the upper end. The anode is a small molybdenum wire placed near the 

 minimum breakdown distance and slightly to one side of the opening 

 in the end of the cathode. A barium getter is flashed to one side of the 

 bulb wall and a small tungsten wire spring is arranged to make electrical 

 contact with the getter flash. A neon filling gas at a pressure near 100 mm 

 Hg is used. 



The cathode geometry has several interesting properties. It was 

 found that the shape of a cylindrical hollow cathode is unstable at very 

 high current densities and that it will rapidly grow into a spherical 

 cavity with a small orifice.* Typical dimensions are a sphere diameter 

 of 0.0:^0 inch and an orifice diameter of 0.008 inch. At an operating cur- 

 rent of 10 milliamperes, the current density in the orifice is of the order 

 of 50 amp/cm^. Once the sphere has stabilized it will operate many 

 thousands of hours with relatively small changes in shape. The trans- 

 mission properties of the stabilized spherical cavity cathode are similar 

 to the earlier negative resistance hollow cathode tubes.* Typical im- 



* This cathode was developed by A. D. White of Bell Telephone Laboratories 

 and will be described more completely by him in a forthcoming publication. 



