SOME ASPECTS OF NOISE INDUCTION 495 



and earth in the demonstration apparatus. As before, the effect of the 

 unbalance on the noise to ground is negligible, but it may cause a 

 material increase in the noise-metallic. 



While a 2-wire metallic telephone circuit has been used in the dis- 

 cussions, the same principles apply to a phantom circuit. In con- 

 sidering the effects of unbalances, transpositions, etc., on phantom 

 circuits, the two wires composing each of the side circuits from which 

 the phantom is derived may be considered as being in parallel and 

 treated as if they were single conductors. With this method of treat- 

 ment, the discussions of a 2-wire circuit can also be applied to a 

 phantom circuit, bearing in mind, among other things, that with four 

 wires to treat with instead of two, an unbalance in any of the four wires 

 will react on the phantom circuit as well as on the side circuit of which 

 it is a part. 



While for simplification the demonstration has been confined to the 

 effects of unbalances in the line conductors, it is evident that similar 

 effects can result from the equivalent series or shunt unbalances in 

 terminal equipment in central offices, in subscribers' sets, cables, etc. 



Interconnection of Balanced and Unbalanced Telephone Circuits 

 One of the factors which is of interest in connection with noise on 

 telephone circuits is that which is concerned with the phenomena which 

 occur when a well balanced and a poorly balanced telephone circuit 

 are connected together. It was pointed out previously that a well 

 balanced and transposed telephone circuit may be relatively quiet even 

 if it is exposed to induction. Also, if a poorly balanced circuit is not 

 exposed to induction, it may be quiet. If, however, the exposed, well 

 balanced circuit and the unexposed, poorly balanced circuit are con- 

 nected together either at some point along the line or through a cord 

 circuit not containing an isolating repeating coil, the overall connection 

 may be noisy since the interconnection in effect unbalances the other- 

 wise well balanced circuit. 



To demonstrate this the conditions shown in Fig. 19 are set up. 

 The metallic portion of the circuit at the left of the diagram is exposed 

 to the 3-phase power line but is well transposed and balanced. The 

 grounded circuit, shown at the right of the diagram, is not noticeably 

 exposed. 



The noise heard when the loud speaker is connected to the metallic 

 circuit (although it is exposed) is relatively low. Likewise, the noise 

 on the grounded circuit is relatively low. When, however, the 

 grounded circuit is connected to the metallic circuit the noise on the 

 overall circuit immediately rises because of the unbalancing effect of 

 the grounded circuit. 



