328 BELL SYSTEM TECHNICAL JOURNAL 



Class III Unbalances — Between Circuits in Different Quads 



Unbalances between two phantoms, or between pairs not in same quad, or between a 

 phantom and a pair not in same quad, in each case = Ci_i+C2_3 — (Ci_3+C2_4) in 



which, for 



(a) Phantom to Phantom, 1 represents the two wires connected in parallel to form one 

 pair of a quad, 2 represents the two wires of the quad, and 3 and 4 represent 

 similarly the pairs of another quad. 



{b) Pair to Pair, 1 and 2 represent the two wires of a pair and 3 and 4 the two wires 

 of another pair not in the same quad. 



(c) Phantom to Pair, 1 and 2 represent a phantom as in (a) and 3 and 4 a pair as in {b). 



The type of quad now most commonly used in toll cables in this 

 country is known as the multiple twin type and consists when com- 

 pleted of two twisted pairs which are again twisted around each other. 

 Differently colored wrappings of cotton around the several pairs hold 

 the two wires of the pair together and afford means of identifying 

 various types of quad and pair as used, for example, in the segregation 

 of the circuits operating in different directions in the so-called four- 

 wire circuits. 



A type of quad construction different from that described above and 

 commonly known as the "spiral four" type of quad has been used more 

 extensively abroad than here. In this construction four wires are 

 twisted together in such a way that at every position each wire occupies 

 approximately a corner of a square and the two diagonally opposite 

 conductors are used to form a pair. 



This construction has the merit of very low mutual capacitance of 

 the pairs, but the disadvantage of very high mutual capacitance of 

 the phantom. It has also been found more difficult with this con- 

 struction to obtain sufficiently good balance to give satisfactory loaded 

 phantom circuits. This type of quad has, therefore, in some cases 

 been used without utilizing the phantom circuits. The loss of these 

 phantom circuits is less than it might seem at first sight because, on 

 account of the inherently lower pair capacitance for a given space per 

 pair, more wires can be placed in the same space for a given capacitance 

 than with other types of construction. 



Another characteristic which under certain conditions is important 

 is the alternating current conductance or leakance. The leakance 

 which is measured in micromhos is that property which determines, 

 under given conditions of potential and frequency, the losses in the 

 insulation. These losses become of greater importance when the cable 

 is loaded than when non-loaded and also of relatively greater impor- 

 tance when the conductors are large because then the dielectric losses 

 become relatively greater in comparison with the lower losses in the 

 decreased resistance of the conductor. For this reason many of the 



