Linear tracking turntables, whatever happened?

Moment of Inertia is denoted by I and is the angular analogue of mass. It is defined as the sum of (mass of molecule x distance to pivot squared). This is approximated by an integral (calculus), which is easier to calculate.

Thus, for a beam of just sufficient length and just sufficient strength to support the counterweight, the greatest Moment of Inertia I will be obtained by positioning the beam extending from the pivot parallel to an extension of the arm wand.

The obvious tradeoff is the amount of material required to position the beam that way.

There are other tradeoffs - one wants adjustability and rigidity and resonance control in the beam. Thus a system of a light but rigid tubing (say magnesium), with a lump of tungsten on the end, threading into a nut on the end of the arm, would give you the highest I/m ratio, but may be compromised with respect to resonance. I should try that on the Trans-Fi.

@ct0517 - another good question which will require a little exploration by me. Just when I thought I knew how to set this thing up after all these years! Your learning is valuable and obviously transferable--
Just literally removed my regular cartridge- have a rebuilt vintage cartridge arriving today as a stop gap (of all things, one of those old Monster Genesis 1000s). So, I can at least mess around with the arm a bit even though the cartridge I'll be using for now is far different in mass (and other things) from the Airtight that is going off for a rebuild. 

https://flic.kr/p/S8x8RW Dunno if this will work but here’s a shot of where the counterweight threaded tube mounts on the back of the air-bearing housing- looks like it is a threaded hole. Period. Thanks, @ct0517 if you click on the image link, and then click the image itself once it opens, it will enlarge. 

@whart 
Yeah looking at it on my phone.it appears once it threads in that's it because it ties into the Armtube. In the ET design there is a long air bearing spindle with the Armtube and end cap at opposite ends.
Can be adjusted independently.

@ct0517 ,

Regarding your post above...that the ET user should set the record height level with the inscribed line in the center of the manifold.....although I’ve owned mine for 30 years, and I set mine up (eyed) for the record height to be in the center of the spindle (looking straight into the spindle), I never remember reading in the manual any indication of that "inscribed line", therefore, I never noticed it. I just uncovered mine just to check and it is correct. (These types of things would be great to read in the manual.) I think it could use a re-write.

Thanks for that post.