sealed off by the magnetic window 44 thus eliminating the aforementioned problems of exposed wiring and pressure seals needed in the potentiometer type devices the present invention also eliminates the internal sliding electrical parts that suffer wear from friction encountered in the potentiometer device there also is no requirement for a pressure equalization system using bellows and hydraulics such as utilized in the potentiometer calipers the only exposure to the well fluid in this invention is the initiator magnet and the mechanical linkage connecting it to the caliper arms the mechanical linkage assembly 30 is reliable and uncomplicated and serves to remove a complex non linearity component of movement in the caliper system the selection of the proper curve for channel 40 depends upon physical characteristics of the caliper arm assemblies such as the various lengths of the various arms the physical dimensions of the contact pads and the locations of the pivot pins connecting the arms and pads to each other and to the body although a specific preferred embodiment of the present invention has been described in the detailed description above the description is not intended to limit the invention to the particular forms or embodiments disclosed therein since they are to.