I use a dial indicator kit that includes the indicator, one of these
arms, and a modified pair of vise grips that the arm screws into. Each
"link" of the arm has a ball on on end and a socket on the other. A
cable runs through the arm. When you adjust a lever on one end, a cam
tightens the cable and secures the arm. I use it with the visegrip
clamped on my toolholder and the indicator touching various points on
the lathe headstock and chuck to measure runout.