336 



ALTERNATING CURRENTS 



Transformer 



i ^ QJ?J19JIQJIQ-Q AQ-QQ-PJL' ' 



, nnns 



mize sparking. It makes no difference how the battery is con- 

 nected, as the direction of excitation of the vibrating magnet 

 causes the current always to flow into the positive battery 

 terminal. 



This type of rectifier is designed for 8 amp. at from 8 to 10 



volts. Owing to difficulties, 

 due primarily to wave-form, 

 it has not been entirely 

 satisfactory in practice. 



3. The Mercury-arc Rec- 

 tifier. The mercury-arc 

 rectifier has already been 

 mentioned in connection 

 with the constant-current 

 transformer (see page 217). 

 The principle is the valve 

 action of mercury vapor. 

 In order to obtain the best 

 operation, the tube contain- 

 ing this vapor must be ex- 

 hausted to a very high 

 vacuum. Figure 309 shows 

 a mercury-arc rectifier tube 

 having four terminals. The 

 lower terminal is the cathode, 

 to which the current goes from 

 the tube. The two terminals, 

 A!, A 2, are the anodes from 

 which the current enters the 

 tube. A 3 is a starting anode, 

 by means of which the mer- 

 cury arc is established. Cur- 

 rent then enters the tube from either anode, A i, A 2 , depending upon 

 which side of the transformer secondary, ab, is positive. When 

 the current attempts to reverse its direction, however, the 

 mercury vapor acts as a valve and prevents any current entering 

 the tube at the cathode. If only one anode were used, the nega- 

 tive half of the alternating-current wave would be eliminated in 

 each cycle and the resultant wave would appear as shown in 



Auto-transformer and 

 Inductive Reactance 



FIG. 309. Mercury-arc rectifier for low 

 voltages. 



