338 ALTERNATL\<; CCKRENTS 



zero point and the resulting wave is similar to that shown in 

 Fig. 310(c), being more or less pulsating in character. 



The direct current leaves the cathode, enters the positive 

 terminal of the battery to be charged (or other translating 

 device) and flows to the neutral of the auto-transformer. 



The operation of the auto-transformer is as follows: Assume 

 that at some particular instant, terminal b of the transformer 

 secondary, Fig. 309, is positive and terminal a negative. Current 

 obviously attempts to pass from 6 to a through some external 

 circuit. One path is by way of the anode At, the tube, the 

 cathode and through the battery to the neutral N of the auto- 

 transformer. As some of this current must return to terminal a 

 of the transformer secondary, it attempts to pass through the 

 winding Nd of the auto-transformer A part of the current does 

 pass through this winding and in so doing creates a flux in the 

 core of the auto-transformer which induces an emf . in the winding 

 Nc. The direction of this emf. is such as to cause the remainder 

 of the current to flow from N to c. This current flows through 

 the local circuit NcA 2 . This, it will be remembered, is the prin- 

 ciple of the auto-transformer (see page 206, Par. 90). 



The anode A 3 is for starting purposes only. When the tube 

 is tilted, a conducting stream of mercury is established between 

 At and the cathode. The resulting current flow vaporizes some 

 of this mercury and so establishes the arc. A ballast resistance 

 R is necessary in order to limit the current at starting, since there 

 is then a metallic path of low resistance between A 3 and the 

 cathode. 



For low-voltage circuits, this type of rectifier has not as yet 

 been developed in large capacities. Figure 311 shows the front 

 and rear views of a complete rectifier panel such as would be used 

 for charging vehicle batteries. 



4. The Tungar. 1 The tungar is based on the following prin- 

 ciple: An incandescent filament emits minute negative charges 

 called electrons. When the discharge of these electrons occurs 

 in an electrostatic field, the electrons attain considerable velocity. 

 If a gas is present, these electrons collide with the gas atoms and 

 ionize them. That is, when an electron collides with an atom of 



^or complete description see "The Tungar Rectifier," by R. E. Russell, 

 General Electric Review, 1917, page 209. 



