inserted in its place eliminating the need for rewinding the bobbin the disc 28 will usually have a diameter similar to the diameter of the new bobbin less coil and an axial bore 34 traversing its center to allow the disc to be mounted on the bobbin case mounting axis the disc has an inner face 41 fig2 facing the end face of the coil 26 when fastened thereto and an outer face 36 fig3 facing away from the coil disc 28 is preferably exceedingly thin with a thickness 35 in the range of 0 15 0 25 mm so as to be insertable into the bobbin case without interferring with the rotation of the coil due to its thinness the disc will usually be made from a rigid material such as a metal preferably steel so as to impart strength to the disc however a disposable disc made from a lightweight inexpensive material such as cardboard is also contemplated fig3 illustrates the outer or indicating face 36 of the disc 28 which is coated with alternate reflecting 38a and non reflecting 38b circumferentially spaced strips extending radially from the center of the disc the strips can be of different colors e g black and white or of different optically sensible materials the only condition is that one of the strips be sensed by the optical sensor 20 fig1 during the rotation of the disc as described in our aforementioned patent specifications in relation to the outer face of the end wall of the bobbin an alternate embodiment is illustrated in fig5 wherein the indicating face 36 of the disc has circumferentially spaced holes 39 cut out of a reflecting background 40 the opposite face 41 of the disc 28 illustrated in.