MIMF

Do heavier strings put more tension on a guitar and, if so, why? Also, if so, is it a significant amout between "light" gauge and "medium" gauge? What should I expect to observe as a result, if any?

Well, mathematically the frequency of a vibrating string is (1/(2 * length)) * sqrt (tension/mass). So if you increase the mass (weight) of the string, the tension must go up in order to keep it at the same pitch.

Here's a nice doc from D'Addario which shows the difference in tensions for all their different string sets: http://www.daddario.com/upload/tension_chart_13934.pdf

The D'Addario pdf's are very helpful, particularly when building mixed string sets (for altered tunings, for example). There are a couple of good on line string tension calculators, but I find this page very helpful for evaluating gross changes in string gauges and tunings

http://theunofficialmartinguitarforum.y ... ng-Tension

One of the things that I find interesting is that lights at concert, mediums at D# and "heavies" at D all have about the same tension. I also find the section on 12 strings very helpful (I own 3 of the things and play in all kinds of altered tunings).

What should you expect in going from lights to mediums? About 12 percent increase in tension - will be a little harder to fret and bend, might have a little more volume because of the added drive to the top. Some necks might see a hair more relief, but usually not with one step in gauge. Nut slots might be a tiny bit tighter - again, most guitars can take one step. On a lightly braced guitar the top might rotate a smige more - there could be a long term change in neck angle and top belly (one of the reasons people argue that 12 strings should be strung lightly and/or down tuned). If you were on the ragged edge of buzzing with lights it might clear up with mediums. If a guitar was specifically designed for lights it is possible that the warranty would be affected.

Your fingers will probably tell the difference, your guitar may or may not.