Firestone Pulse-Echo Reflectoscope

Everything else in this batch shines something THROUGH the patient and catches what comes out the far side. This one stands on one side and listens for the echo, and it arrived in medicine sideways - from a man trying to find cracks inside metal castings. **US 2,280,226**, "Flaw detecting device and measuring instrument", Floyd A. Firestone, filed 27. May 1940 and granted 21. April 1942. Its first claim is a complete description of every ultrasound machine ever built: *"means for transmitting a supersonic wave train into said part, and means for determining the time interval between the sending out of said wave train and the arrival of said wave train at some point of the part"*. Send a pulse. Time the echo. That is all of it. The drawing on this blueprint is his Figure 1 and it is worth studying, because it is an A-scan - the display every ultrasound machine still produces internally - drawn in 1940. A pulse oscillator gates a high-frequency oscillator; the burst goes into the part; echoes come back from the front face, from the flaw, and from the back wall; they are amplified, rectified, and painted against a linear sweep so that **horizontal distance on the screen is depth into the object.** It took a decade and a half for the medical use to follow. John Wild and John Reid in the United States were working on tissue echoes by the early 1950s, and Ian Donald in Glasgow brought obstetric scanning into practice in the late 1950s and 60s using industrial flaw detectors of exactly Firestone's type - the machines were already sitting in the shipyards on the Clyde. **You will build an A-scan and measure with it.** You will find the speed of sound in a material by timing an echo through a known thickness, then use that number to locate something you cannot see. And you will meet the trade that decides every probe ever made: high frequency gives fine detail and cannot get deep; low frequency reaches but cannot resolve. A sibling worth reading beside this one is the ultrasonic parking sensor already in the catalogue. Same physics, one target, in air, where the speed is 343 m/s and there is no coupling problem. Here the medium is dense, the echoes are many, and getting the sound IN at all is most of the difficulty.