I’m putting this here, since I doubt that there’s a factual answer to this. By “most important” I mean the one that had the biggest impact on the 19th Century. The automobile was invented then, but it didn’t really begin to have a significant impact until the 20th. As a WAG, I’d say the locomotive, but I’m not enough of an expert about the 19th Century to know for sure.
Seems like it would be hard to beat the steam engine. It had an enormous impact on the economy, both in America and in international trade.
- For America (and England, W. Africa, France, Egypt and India to lesser extents) I’d go for the cotton gin. It was invented in the last decade of the 18th century, but it didn’t catch fire until the 19th and it revolutionized the agriculture of the Deep South. Without it, slavery would probably not have died by 1860 but I doubt it would have been nearly as widespread.
Next up would be
Photography (the famous 1820s Niepce “First Photo” [yes, purists, I know there are arguments as to whether it’s the first, but it’s generally given credit]) as in addition to preserving so many images and making appearance such an issue for politicians and others it of course set into motion movies/television/etc., and unlike voice recordings it came along early enough in the century to have an impact on average lives.
The telegraph- for the first time information traveled faster than humans could and ticked off the “Faster! Faster! Faster!” set.
Then there’s the railroad, the newer and bigger printing machinery, revolvers and reliable repeating rifles, barbed wire, so many others, but I’ll go with the above as first, second, and third in that order.
I’ll second the steam engine. It provided the power to start the Industrial Revolution.
The steam engine was an 18th century invention. (Thomas Newcomen, 1712).
I’d suggest the Bessemer process which revolutionised the making of steel.
The electric generator and motor.
There are a lot of possibilities (I think the Telegraph, and the Telephone has a good claim, as do dynamos), but I think that Electric Light Sources, especially the Edison and Swann filament Electric Light Bulb deserve a big place. Although other light sources were available, this opened up the possibilities further, gave better light, and made it much more transportable and safer.
Steam-Powered Printing Press.
Literacy and the modern newspaper.
Telegraph. Long-distance instantaneous communications changed the very way we think about the world.
The locomotive. It transformed the human race itself, by allowing people to easily travel beyond their towns, meet different people from different places, marry and breed.
I really don’t count the locomotive as a seperate invention - it’s an application and adaptation of the steam engine, but while there were a lot of technical challenges to make it work as we tend to think of it, I don’t think that any of it was quite the “Eureka!” moment that to me goes with a new invention.
I’m going to split my vote between the telegraph (without which the railroads were death factories, AIUI) and the Diesel cycle engine. I grant that the Diesel cycle engine wasn’t very important during the 19th centuries, but the footprint of the engine, today, is everywhere.
I’d go with the telegraph. It was a major invention and its effect was felt almost immediately in the 19th century. Nowadays, we’ve gotten used to instant communication and we’ve forgotten what a huge impact it was when it first happened.
Did you see the associated Sears ad on that page? I mean sure, it’s probably great to get a vibrator for $6, but on the other hand, it was probably hard to be conspicuous when the “portable” version was that big and weighed 3.5 pounds. I’m also disturbed that the ad was adjacent to an ad for “buffer and grinder attachments.”
I think that’s a misprint. It should have said “bumper and grinder attachments.”
Yes, but just as the internal combustion engine was developed in the 19th century but didn’t have much impact until the 20th, so the steam engine had little impact until it powered industrialization in the 19th century. Since I live in the West, however, I tend to think of those things that had the greatest impact here in the 1800s, and that would definitely be the steam engine mounted on a railroad locomotive. But for railroads, most of the West would have remained uninhabitable, perhaps for decades. (Some would argue that might not have been a bad thing.)
The assembly line.
The assembly line may indeed be a 19th century invention (although that’s disputed) but it was Henry Ford who put it to work in the 20th century and revolutionized manufacturing.