Abstract: In order to indicate the gap between the work rolls of a rolling mill, a pair of shoe structures are urged into engagement with the necks of the respective work rolls, so as to engage that part of the upper roll neck which is vertically beneath the roll axis and that part of the lower roll neck which is vertically above the roll axis and to be movable with the rolls in the vertical direction and an electrical transducer is carried by one of the shoe structures at a position away from the roll axis and a movable part of the transducer engages the other shoe structure so that the electrical output of the transducer varies in response to a variation in the gap between the rolls.