Abstract: An elongate bar of electrically conductive material and having a generally U-shaped cross-section carries on the inner side of one of its legs a support with a resistance layer facing towards the other leg. A slider is guided between the two legs of the bar and has a contact spring, one contact tongue of the spring bearing on the resistance layer and another contact tongue bearing on the opposite, second leg of the bar forming a collector element. A nose formed on the slider engages a groove which exists between the resistance element and the bent free end of the one leg of the elongate bar, to guide the slider. A cover plate overlies the opening of the U-shaped cross-section of the elongate bar. An operating knob is disposed outside the cover plate and extends through a slot in the cover plate operatively to engage the slider.