TOWA Gauge for M Bobbins
To measure the tension of your bobbin, place your bobbin inside the bobbin case with the thread unwinding in the correct direction, then insert it into the TOWA gauge. Pull your bobbin thread and guide it underneath the bottom wheel, over the top of the top wheel and then inside the catch. As you pull the thread to the side, you will see the top wheel move down while the numbers increase. The higher the number, the tighter the tension. To loosen the tension, turn the bobbin case screw in small increments counter-clockwise. From Japan Large bobbin M class machines.