A friend of mine asked me this question a couple of days ago, and so far, I don't have a good answer.
Wikipedia tells us that, in real life, industrial revolution "included going from hand production methods to machines, new chemical manufacturing and iron production processes, improved efficiency of water power, the increasing use of steam power, the development of machine tools and the rise of the factory system."
Now, suppose you are a 14-th century England with no magic, and you suddenly got a bunch of wizards(let's say 10 of them, each at lv 12). Just your ordinary wizards without any particularily unusual spells in their spellbooks. What can they do to initiate their own industrial revolution?
Well, for starters, they can just make pure iron. Wall of Iron spell makes a 5ft sq/level , 1/4 inch/level thick wall of, well, iron. That is 16.7 metric tons(5*5 square=25 square feet=2.32sq meters; 3 inch=0.0762 meters;density of iron=7.87 tons/cubic meter; 2.32*0.0762*12*7870=16.7 metric tons) of iron for your average 12 lv wizard, per one cast of the spell. And they can cast 2 of those per day(3 with a high INT score), which brings them to 33.4 tons per day minimum. Internet tells me that british iron production in 1700 was 12,000 metric tonnes a year, or 32 tons per day, so 10 wizards can outproduce a country without even trying much.
Secondly, they can eschew machine tools. Who needs machine tools when you have Fabricate? Transmute all that iron you just made into whatever is made of iron. Pots, nails, I-beams...Sky is the limit, really. And it only takes a minute to turn all that iron you produced with your wall of iron into finished products.
Next, why don't we ruin agricultural sector while we are at it? For example, we can make a tractor. Be it a construct, an actual vehicle with a Wondrous Item for an engine or something else, point is, Wizard can make it happen. And it probably wouldn't cost a stupendous ammount of money to do so. Now your agriculture is incredibly efficient, and you don't even have to waste money on gas.
Did I already mention a Wondrous item in place of the engine? I am pretty sure that there are a thousand ways to make Perpetuum Mobile with magic for less than 10000 gold. Make a permanent shocking grasp spell to get a perfect electricity generator for example, it will cost you ~1k gold. Same with more "mechanical" powersources.
I might well be missing some other potential exploits, but the general idea is clear-if you just add Wizards into a medievil setting, things aren't going to stay the same for long. Something is going to change radically, to the point where you won't be able to recognise the world.
Now, suppose you wanted to keep the tech level stable, AND have wizards. For that, you would need some counterbalance to magic, something that would slow down the progress of society back to normal levels. What do you think that might be?
