The slide rule that keeps you from getting pregnant

This contraption is supposed to help you follow the "rhythm method" of contraception. It looks like a combination of the slide rule, the old printer/editor's proportion wheel (which I used in the days before everything photographic went to computers and we used to use real film and real stat cameras to make halftones).

BoingBoing found it via Harvard Magazine (and do check out both links).

For reference's sake, here is a slide rule (and do visit the Web site devoted to slide rules):

And also take a look at a fascinating discussion about "Engineering Before Computers," with a bigger version of the slide-rule image above (you have to right-click and view it in a separate window to see it in all its geeky splendor). There are more than a few cool devices there.

And here is the proportion wheel we all used in the pre-Photoshop days of journalism:

And while we're in a geeky mood, here's Steve Wozniak's beloved HP calculator, which he sold for $250 to help finance the first Apple printed-circuit board in the '70s (OK, it's not Woz's exact HP35 ... just a random HP35, but you get the idea, right?):

Thanks to InsideDGW for the image and HP for more info on the HP35 than you'll ever need.