I would think that there must be something holding the arm in place in the cup other than the ball floating in the oil pool. I would think it was either sitting in the bottom, held by the weight of the arm or the string would go right through, which obviously it does not. the description above makes it sound like the arm is on a pendulum, with the ball just touching the surface of the oil to slow the pendulum action down.
I would hope the ball is actually sitting in the cup and held in place by the weight of the tonearm and the string just provides lateral support.
I would hope the ball is actually sitting in the cup and held in place by the weight of the tonearm and the string just provides lateral support.