The ornamental design for a sensor for a knuckle, as shown and described.
FIG. 1 is a perspective view of a portion of a knuckle with a sensor retained therein showing my new design;
FIG. 2 is a side plan view of FIG. 1;
FIG. 3 is a plan view taken along line 3—3 of FIG. 2;
FIG. 4 is a top view of FIG. 2;
FIG. 5 is a bottom view of FIG. 2; and,
FIG. 6 is a plan view taken along line 6—6 of FIG. 2.
The broken line showing of portions of the knuckle and an associated electrical cord is included for illustrative purposes only and forms no part of the claimed design.