Above, a heavy bar clamp I picked up at a tool sale several years ago. The bar is 40 inches long, so the clamp's capacity is 36 inches. The bar is also 1-3/4 inches wide and 3/8 inch thick, the heaviest bar I've ever encountered.
The wooden handle on the screw was gone, so I made that up. More seriously, the mechanism for locking the movable jaw was missing, so the previous owner just used a wooden wedge. Not good enough for me. I was able to find the original U.S. patent (thanks, Google patents!) issued in 1907 to James Taylor of Bloomfield, New Jersey (who was awarded patents for a number of different clamps) and to scale the drawing to fabricate the missing piece (Part 17 on the second patent drawing below):
Rather than using a straight slot, I cut a dovetail (Item 19 above) on my Bridgeport knee mill, but had to grind an old flat file to make the hardened, toothed insert (Fig. 7 above). Easier said than done, and I didn't get it quite right (I am an amateur, after all), but the clamp still works wonderfully. The spring on the steel dowel keeps the toothed insert firmly in position against the bar, locking the movable jaw in place. To release and move the jaw, you just put a finger in the exposed hole and pull upwards and backwards, releasing the spring pressure and eliminating the wedge effect.
I've used it several times now to tighten wooden parts that resisted the flimsier modern bar clamps in my shop. It's also so cool to be using a tool that's perhaps a century old and still doing a better job than most of its modern counterparts. So, thanks James Taylor, and I also have to say that I like your namesake's music. Rockabye sweet baby James.