254 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



Mercury-arc rectifiers have been used for rectification for many 

 years. They operate upon the principle of a hot spot upon a mercury- 

 pool cathode maintained by positive-ion bombardment. The arc is 

 started by means of an auxiliary electrode. During recent years 

 mercury-arc rectifiers have been built in large capacities for supplying 

 direct current for electric railway operation. These rectifiers have 

 been built in large steel tanks and use 3, 6, or 12 phases with a corres- 

 ponding number of electrodes for anodes. For starting the main arc 

 an auxiliary or exciting electrode must be in operation all of the time. 

 The ionization produced by this auxiliary electrode produces condi- 

 tions favorable for an arc-back or a reversal of the rectified current. 

 Hence the anodes must be very carefully shielded and the arc path 

 must be relatively long. 



Quite recently a new method for starting the mercury arc in the 

 rectifier has been invented by Slepian and Ludwig. The device using 

 this new principle is called the ignitron. The ignitron is a mercury- 

 arc rectifier having a third electrode or ignitor which consists of a rod 

 of suitable refractory material which projects through the side of the 

 tube and has a point dipping into the mercury pool. The ignitor 

 electrode serves to fire the arc at will just as the grid of the thyratron 

 may start the gaseous conduction. If the ignitor electrode be made 

 positive with respect to the cathode pool and a suitable critical current 

 be permitted for a few microseconds, a tiny spark occurs between the 

 ignitor and mercury. If the anode potential is positive and of suitable 

 value, this tiny spark expands into an arc which is transferred to the 

 anode through the ionization process previously considered. The arc 

 forms a very low resistance path for the passage of current. 



The ignitor principle may be used for a single current surge of half- 

 wave rectification or it may be used for several waves, or for continuous 

 operation. The ignition process must be repeated each time the 

 alternating anode potential becomes positive. The ignitor electrode 

 may be fired from the same A. C. source of potential as the anode. Also 

 the phase relation of the ignitor voltage to anode voltage may be 

 varied to give only partial or even zero time of rectification. Thus 

 the ignitron has the same control characteristics as the thyratron. 

 The ignitron is inherently suited for larger current values of rectified 

 current than the thyratron. The advantages of the ignitron over 

 the mercury-pool rectifier are long life of mercury-pool type of cathode, 

 and low voltage drop in arc since cathode and anode may be very 

 close together. The latter advantage results because no arc-back can 

 occur on reverse cycle since firing occurs only at desired time (and 

 not continuously due to auxiliary electrode). The applications of the 

 ignitron include simple continuous rectification, rectification with 

 control, motor speed control, welding, and circuit breaking with 

 quicker response than mechanical devices. 



