A probe (10) has internal circuitry capable of operating in a plurality of different modes, for example a variety of filter modes and turn off modes. A switch on the probe sends an input to a controller which is arranged to preset the mode in response to operation of the switch. The switch may be a push button (32) which is operated externally of the probe (10) by means of an actuator in the form of a plunger (34). The probe has a work-piece-contacting stylus (11) mounted on a seat from which it is deflectable. The stylus and the seat may comprise the switch. An indicator, eg an LED (44), indicates the mode in response to an output from the controller. The controller outputs a plurality of indications of different modes in a sequence and a mode is selected by operating the switch one the desired mode is indicated.