How to measure tonearm effective mass

Chuck, I AM glad that the question brought responses from some of the most knowledgeable people around this part of the hobby.
Viridian, The Catch-22 is the bit about needing a cartridge of "known" compliance. I guess one can figure that out by starting with another tonearm of "known" effective mass, or else one can start with manufacturer data, which I am not at all sure I trust. But thanks. As far as that goes, there was a neat idea from John Ellison on VA that is germaine to this issue. For a case where we know either the effective mass or the compliance of the cartridge, you simply drop the stylus onto an LP whilst recording the harmonic motion of the bouncing cantilever. For that he uses his computer and appropriate software. Once you freeze the decaying sine wave that describes the signal coming from the bouncing of the cantilever, you can make measurements that will get you back to the unknown parameter.
Mark, Did you make reference to the John Ellison method? I will take a look.

Mark, I see you had posted your own novel method on VA. Looks a bit cheaper to try, compared to John's which requires one to buy the requisite computer software. Thanks.

Hey Chuck. Are you THE Chuck, formerly of Northern Virginia, to whom I spoke at RMAF?

The confounding reports of apparent tonearm/cartridge mismatches that sound great have me thinking about the quality of the data for compliance and tonearm effective mass that we start with. Also, for some of the newer/newest tonearms, there are NO data coming from the maker. To wit, I asked a dealer about the effective mass of the Talea. Turns out an exact number is not available.

Of course also, Raul and others have correctly (probably) pointed out that the compliance/mass issue is not at all the sole determinant of a good tonearm/cartridge marriage. I suspect that is a very important truth.

Funny that only one pivoted tonearm I can think of actually has capacity to alter its effective mass, and that is the MA707 or the CF1 (or both) made by Micro Seiki. They used the simple device of a weight that slides fore and aft on the arm tube. You'd think that idea would have been widely adopted. Maybe they had a patent.

Dan_ed, Good point about the tonearms that can accommodate two counter-wts, but as Dave mentioned, the range of adjustment of effective mass afforded by that arrangement would seem to be quite limited.

Mark, Despite the setback you described, do you continue to pursue a Dynavector-like tonearm design where the vertical effective mass could be adjusted? I wonder whether your final solution to that problem would be retro-fittable to my DV505, eventually. Actually, what I do now is just change headshells as and when needed to change EM. That obvious ploy does not allow "on the fly" adjustment to max out the performance of a given cartridge, however.

James, This is/was a thread about tonearm effective mass. I think you want to post on the thread about "Garrard" motor controllers. It's possible that Mark intends to sell his Garrard/Lenco controller.