Length measuring device

A length measuring device comprising:

• a vernier caliper,
• a linear guide comprising a hollow profile, on which the vernier caliper is displaceably mounted externally so that the vernier caliper is capable of being brought into contact with an object to be measured in terms of an object length,
• an inner slide, which is displaceably mounted inside the hollow profile,
• a magnet arrangement which couples the vernier caliper and inner slide magnetically, so that the inner slide follows every movement of the vernier caliper along the hollow profile,
• a measuring means for measuring a position of the inner slide along the hollow profile, and
• a display, visible in a surrounding space of the hollow profile, of a measured object length determined by the measuring means in accordance with the measured position of the inner slide,
• wherein the hollow profile is provided with an end wall on at least one end,
• wherein the inner slide substantially covers a cross section of the hollow profile in order to form an acoustic resonator in an interior of the hollow profile between the end wall and the inner slide, and
• wherein the measuring means includes a loudspeaker and a microphone in an interior of the acoustic resonator and a control and evaluation unit connected to the loudspeaker and the microphone,
• said control and evaluation unit configured to cause the loudspeaker to issue an acoustic pulse, to record a pulse response of the acoustic resonator with the microphone, to determine from the pulse response of the acoustic resonator a basic frequency of the acoustic resonator, and to determine a length of the resonator and therefore the position of the inner slide along the hollow profile from the basic frequency of the acoustic resonator.