A potentiometer includes a rotatably mounted lever which acts as a follower member adapted to lie in contact with a moveable finger, the movement of which the lever is to follow, the lever being biassed at all times towards the finger by resilient return means.The lever is in the form of a fork having two arms whereby it is adapted to embrace the finger. Whichever one of these arms is in the trailing position with respect to the direction of rotation in which the lever is biassed by the return means is longer than the other arm. In the embodiment described, the longer arm is an active arm, the shorter arm being a security arm.