RECTIFIERS: THE SYNCHRONOUS CONVERTER 341 



therefore, from the graphite to the filament, and the device acts 

 !< -tifier. A very small and almost negligible part of the 

 current is due to the electrons themselves, which act as carriers 

 of negative electricity from filament to anode. 



Figure 312 (6) shows the connections for one commercial type 

 of low-voltage tungar, the switches and cut-outs being omitted. 

 Both the current to be rectified and the current for heating the 

 filament are supplied by the transformer secondary, the filament 

 being connected between a 2.5-volt tap and one end of the sec- 

 ondary. Current regulation may be obtained by adjusting the 

 ivsi>taiic<- and the reactance. Where electrical connection be- 

 i load and primary mains is permissible, an auto-transformer 

 with taps may be used. 



The devices shown in Fig. 312 eliminate the negative half of 

 each wave, but this is not a serious disadvantage when ordinary 

 batteries are being charged. However., a two-bull) rectifier sup- 

 plying a continuous, pulsating current is also manufactured. The 

 efficiency of the tungar rectifier is from 35 per cent, in the smaller 

 sizes to 75 per cent, in the larger sizes. The capacities at 

 tit are not much in exceefi of 7.~>o watts. 



5. Electrolytic Rectifiers. Electrolytic rectifiers are based on 

 the following piinciple: If a 

 l<-ad plate and an aluminum 

 plate be immersed in a sndium- 

 or ammonium-phosphate solu- 

 tion, current can pass from the 

 solution to the aluminum. As 

 soon as the current attempts 

 and pass from the 

 aluminum to the solution, a 



thin insulating film of alumi- 



num oxide \* instantly formed 



OV.T the aluminum plate. ;md acts as an insulator up to about 

 150 volts. This prevents the current flowing from aluminum 

 to solution, and -uch a device may he n>-d. fch< U a redi- 



ilOWi such a -im pie rert ilii-r. liiving a continuous 

 pulsatiim current like thai >lnwn in Fiu r . Mid 



'i rectifier- in of low efhcieni cent, and lower, and 



i -mall capacity. They are u>ed primarily fur charging 



