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I'm measuring the distance between the two cords of a bifilar pendulum and its currently the lowest significant figure data I have. I considered wrapping a string around the two cords and halving the length of that, but the string needs to be taught to get a good measurement which then moves the cords together.

I'm aware that you can get a vernier caliper to measure lengths less that 15 cm, but that is too short for my needs.

EDIT:

It seems to me that I'm coming across as a bit idealistic but here's why getting four significant figures:

From a comment:

You have the treads of finite width

I've used a type of nylon fishing wire that claims to be 0.4mm in diameter which isn't stretchy.

how parallel are the threads

I was planning to measure the distances between the threads at the top and the bottom of the pendulum and then adjust until parallel

how are the threads attached to the rod and the points of suspension, is there a knot at the end of the threads, etc

I've made a rig out of some nuts, a threaded rod and two washers that gives a distinct point where the filament sways from.

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    $\begingroup$ If you are doing a standard bifilar experiment in the laboratory then a metre rule giving a measurement to about one millimetre should suffice? Think about it. You have the treads of finite width, how parallel are the threads, how are the threads attached to the rod and the points of suspension, is there a knot at the end of the threads, etc. All these factors might well contribute to errors which are larger than the accuracy of your measuring device? $\endgroup$
    – Farcher
    Nov 16, 2019 at 11:08
  • $\begingroup$ What are you using the bifilar pendulum for? $\endgroup$
    – sammy gerbil
    Jan 3, 2020 at 7:36
  • $\begingroup$ @sammygerbil I'm a high school physics student and I'm working on a project where I experimentally find an equation that gives me the time period of a bifilar pendulum. I've completed almost all of the experiments now by just using 3 sig fig data, but your welcome to tell me how to improve my method. $\endgroup$
    – Krish
    Jan 3, 2020 at 16:45
  • $\begingroup$ @Krish I think 3 sf accuracy is probably the best you can do. I was going to suggest using a travelling microscope which has a vernier scale so you can measure to 0.1mm. Another option is an optical lever which means creating a shadow of the apparatus on a distant wall using a point source then making measurements on the shadow and applying geometry (similar triangles) to work out the distance between the threads. But if the threads are fine it will be very difficult to see their shadow. $\endgroup$
    – sammy gerbil
    Jan 3, 2020 at 16:55
  • $\begingroup$ @sammygerbil The travelling microscope is a valid answer, a little out of my budget, but valid nonetheless. I have doubts about the optical lever though. Surely that just shifts the need for accurate measurement to the distance between the light source and the bifilar pendulum cords? Not to mention that the pendulum itself sways, shifting the shadow with it $\endgroup$
    – Krish
    Jan 3, 2020 at 17:04

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