452 



ENGINEERING ON THE FARM 



the knowledge of the others. ■ Those which are in most 

 common use are the split alternating-current system, the 

 harmonic system, and the polarity system. 



In the split alternating-current system one bell is con- 

 nected between each branch of a metallic circuit and the 



No. 6-A Ringer 



P- 1 10864 > 



ARMATURE 

 ANDCLAPPEft 



P-IQ77II 

 PIVOT SCREW 



P- 1 077 10 



PIVOT -SCREW ' 



P-157 

 lock kurs 



P-20093 

 ADJUSTING SCREW 



P-tQ77Q9 

 VOKfe 



GONGS > 



N?» 

 v NICKEL FINISH 



NP 25-A 

 BlACK FINISH 



P-13695 

 * 5C*ew 



P-38974 



WASHEft 



P-39033 

 SCREW 



P-133720 



COlLS-(SOO OHMS EACH) 



fP-43763 



BIAS NS SPRING 



After Western Electric Co. 



Fig. 345. A polarized or biased ringer 



earth. It is thus limited to two-party service on metallic lines. 

 In the harmonic system the bells used are so constructed 

 that they will respond to definite frequencies. Four fre- 

 quencies are customarily employed, and each bell used is 

 so designed that it will respond to only one of the four 

 frequencies. With this system four stations can be handled 

 selectively if the bells are bridged across the circuit, and if 

 they are connected between the branches of a metallic cir- 

 cuit and the earth, one for each frequency on each side, 

 eight stations can be handled selectively. Since it is neces- 

 sary to have four frequencies of alternating current which 

 are not subject to variation, it is practically imperative that 



