A device for the non-invasive sensing of the length of an implantable medical device includes animplantable medical device having first and second portions moveable relative to one another and a layer of resistive material disposed on one of the first and second portions. A contact is disposed on the other of the first and second portions, the contact being in sliding contact with the layer of resistive material upon relative movement between the first and second portions. A circuit is configured to measure the electrical resistance along a path including a variable length region of the layer of resistive material and the contact. The electrical resistance can then be converted into a length.