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In old scientific apparatus for electrical experiments, and more particularly in instruments such as galvanometers and torsion balances, we often need to hold one end of a thread in place. Examples include the following (particularly large photos are given as links):

Galvanometer (courtesy of museo galileo, here):

Nobili differential galvanometer

In one description, from museo galileo, here:

Nobili astatic galvanometer

A pair of astatic needles is suspended from the fine silk wire attached to a torsion mechanism at the top of the pillar. The needles can be raised or lowered by means of a complex screw-operated scissors-type mechanism.

In Coulomb's torsion balance, we get the following drawing, and description:

Coulomb balance

The top, n° 1, bears a knob b, the pointer io, and the clasp of suspension, q ... The clasp q (figure 2, n° 1) has approximately the shape of the extremity of a solid mechanical pencil clamp [porte-crayon], which can be tightened by means of the annulus q. The clasp of this porte-crayon holds the end of a filament of very thin silver. The other end of the filament of silver is fixed (fig. 3) at P, by the clasp of a cylinder Po of copper or of iron, whose diameter is but a [ligne], and whose end P is split and forms a clasp which is tightened by means of the collar Φ.

I've been reviewing these sorts of mechanisms for a while, and Coulomb's description (the clasp of a mechanical pencil) is the closest I've found to a proper description. But with mechanical pencils, as I know them, the clasp (e.g.a two-part split piece of plastic in a modern one) supports a fixed diameter of lead, typically much thicker than the width of a silk thread. As well, it's not clear from my (limited) research what mechanisms were used for "porte-crayons" in that time period.

I'm looking to recreate some of these instruments in some way, e.g. with modern methods of fabrication and using modern mechanisms available. But these thread-holders are giving me a fair bit of trouble. Is there any resource available where I can get mechanical drawings of such mechanisms, or is there a common "household item" which uses these sorts of clasps that I could look at? Does anyone have any ideas about how these would have worked?

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  • $\begingroup$ Push thread through a hole and tie a stop knot or swage on a lead stop bead. I reckon the clamping at the tip only has to be sufficient to restrain torsion and generally fix the length of the thread. It doesn't have to bear the tensile stress. Here's a modern collet for comparison - amazon.com/Hardinge-Smooth-Collet-Internal-Thread/dp/B007SXTD8W $\endgroup$ – Phil Sweet Feb 5 at 23:51
  • $\begingroup$ possibly something you already have - amazon.com/X-ACTO-2-Knife-Safety-Cap/dp/B000V1QV7O/… $\endgroup$ – Phil Sweet Feb 6 at 0:15
  • $\begingroup$ Love the xacto blade idea! I ended up using a part from a nice-ish ($5) mechanical pencil, and fixing the thread in the side of the collet instead of the center (the center is lead sized of course). Ive also come across some brass binding posts which seem similar in shape to the things shown, I think $\endgroup$ – Sam Gallagher Feb 6 at 5:04

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