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The rollerball at the tip of this pen contains a number of magnetic points, spaced equidistantly around the surface. A sensor in the pen picks up every time one of the points goes past and passes the signal up the pen to the odometer set in the side of the pen.
The odometer can be scaled to show different
values for the same unit distance - and as well as being used to measure distances (by tracing over, or ruling a straight line) it can also be made to perform simple arithmetic by scribbling from left to right the appropriate number of times.
(?) Digital Map Distance Measure
Similar, but wheel instead of ball point [csea, Mar 26 2007]
||Could also be fitted to roller ball type
arm-pit deodorants. Not sure how
useful the measurements would be, but
I have never been deterred by that. +
||Who's been using my roll-on?!