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c. 1885


sigsig Signed on the brass turntable itself: 'W.H. BULLOCH, CHICAGO, ILL.' and also 'PAT'D 1880'. The device has two brass stage clips, and two L-shaped slide retainers to grasp diagonally opposite corners of a slide.

turntableConcentric rings are engraved on the table to assist in centering. There are also two lines, indicating how to attach the slide at an an angle between the two L-shaped pieces. The unusual mechanism allows one to center a slide easily.

turntableturntableThe L-shaped slide holders each attach to a small rectanglar piece that runs in a groove on opposite sides of the top plate. Each of these rectangular pieces are fastened to a small curved plate with teeth that ride in an archimedean spiral* machined into the top surface of the bottom plate. As the bottom plate is turned counterclockwise, and one holds the top place stationary, the spiral screw thread guides both L-shaped pieces towards the center of the table equally. In this way the slide is both held in place and automatically centered. This mechanism works on the same principle used in the chuck of a metal-working lathe.

turntableUnder the plates there is a knob which when turned clockwise, tightens the two plates against each other. Below that knob is another that regulates tension on the turntable rotation, and when fully tightened, holds the turntable in place.

This device is described in the U.S. Patent of April 20 1880. Although the device provides a centering mechanism, it is still requires proper adjustment of the knobs to work properly. In my own opinion, the best centering mechanism for a ringing table was the one developed by Ezra Griffith which automatically centers the slide with a sprung mechanism that is much easier to use. It is featured as part of the removeable foot of the Griffith Club microscope, and was apparently also sold as a stand-alone unit.

*Although the patent refers to a volute spiral, this is technically incorrect as a volute spiral has steadily increasing space between the coils as they move outward, whereas in this turntable, as in lathe chucks, the spacing remains constant, a form described more accurately as an archimedean spiral. The author is grateful to Paul Ferraglio for this information.