Transposition insulators were developed to allow the capability of attaching two telephone wires to the same insulator. This provided the ability to "transpose" the line pairs of each circuit (Swap the position of the pairs). Swapping the line position provided the same affect as twisted pair wire. The induction induced noise would be reduced by reversing the wires every half mile or so of line.
The need to insulate both lines from ground and each other provided some very unique designs. There are both single piece and two piece transpositions. Due to the fact that only one transposition was required for 10 to 20 poles, there were far fewer of these used than the normal pin insulators making them somewhat more scarce.
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