A device used in vernier potentiometers to reduce the effect of contact resistance. The device consists essentially of two or more sets of slide wires or resistance coils in cascade, each coil acting as a decade voltage divider for the preceding one (see diagram). If the total resistance of the first coil is 11R, the resistance of the second coil is made 2R and it is connected to the first by a pair of sliding contacts that move together and shunt 2R of the first coil. The total resistance of the shunt and the shunted portion of the first coil is R, thus the first coil is effectively divided into 10 equal resistances. The second coil therefore acts as a vernier scale.

When a voltage V is required accurately the approximate voltage is set up by adjusting the contacts on the first coil, fine adjustment being achieved by a sliding contact on the second coil. A small error on the positioning of the contact on the second coil has much less effect on the total voltage tapped than the equivalent error on a single coil, thus reducing the effect of the contact resistance of the slider. Further subdivisions can be effected in a similar manner, by subdividing the second coil into 11 parts and bridging with a further coil of total resistance 4R/11, and so on.

Kelvin–Varley slide