Product: Pen
Birodometer   (+11, -2)  [vote for, against]

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.
-- zen_tom, Mar 26 2007

(?) Digital Map Distance Measure http://www.tesora.c...ampaign=xmldatafeed
Similar, but wheel instead of ball point [csea, Mar 26 2007]

Like this? [link]
-- 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. +
-- xenzag, Mar 26 2007

Who's been using my roll-on?!
-- nuclear hobo, Mar 26 2007

random, halfbakery