Good rulers support invention of useful things. But this question has two sides.

• Support of invention


• Prevention of fake inventing

Basis:

Many inventions arise as side-effects of searching for something else. The base of modern chemistry is from alchemy and its attempts to find the philosopher's stone and so on. Also, there is some research that doesn't have direct effects but still it is needed.

Theory:

Fantasy without the philosopher's stone (or something similar) may sound like nonsense. But let's say that ruler does not believe in the existence of the philosopher's stone (or invisibility, etc) and wants to invent something really useful (for example, how to improve public health and cure illnesses). Instead, the ruler resolves to pay such research very well.

And where there is money, there are also cheaters. So the ruler wants to check that all research s/he pays for are really related to the main goal.

Background/Inspiration:

This question is inspired by one old Czech movie and one concrete scene from it:

The alchemist is supposed to prepare a rejuvenation potion of but he creates anything else instead of it (for example, floor polish).

In one invention stage, (in that mentioned scene) he was supposed to find mandragora and prepare it but he cooks sausages with horseradish in a great pot instead.

Historically, this is what patents are for. A patent does three main things:

FIRST a patent grants the inventor a monopoly on making and selling their invention for some period of time, with legally established methods of enforcing the monopoly. They can charge license fees, or sue to stop infringement and obtain reparations.

SECOND a patent requires the inventor to disclose the invention in public records -- which means that, in order to gain his monopoly, he has to tell the world how the item is made. Disclosure does two important things: it ensures that the invention can be copied after the patent expires, and it allows anyone to replicate the patent for research purposes, which prevents patenting bogus inventions. If your invention works, but no one can replicate it, someone else can patent the actual way you make it -- and if it doesn't, no one will want to buy it.

THIRD a patent expires after some period (in the USA, that's 17 years, with a shorter renewal available in some cases). After that, anyone can make the patented item, or make improvements on it, without paying a license fee -- the patented item becomes open to competitive forces.

Patents aren't perfect -- one could argue there are some heinous issues with current American patent law -- but historically, they're widely held to be a major contributor to scientific and industrial progress, by making inventing both profitable and publicly available.

+1 @ZeisIkon, Patents are one arm of a two prong approach to getting a culture of invention going.

The other prong to get this going is expressed interest by the ruler. If the ruler likes painted duck decoys, people will get really good at painting duck decoys to get the ruler's attention.

If the ruler lets it be known to the land that they love new things and new ideas, people will bring new ideas to them to show off. If the ruler then does something that rewards the ones with truely useful or interesting inventions, the rest of the people will see this as a method of advancement and a way to catch the ruler's eye.

Nobles may search for clever people to present to the ruler and thus gain prestige that way.

Since the ruler cannot see every new idea, have festivals to show off new inventions. That way people without noble backing can see that they have a chance to get recognition.

Eventually there would have to be some kind of filter so regional events will have to be held with the best stuff being presented to the ruler.

Also, events can be held with a theme and/or to solve a particular problem. For example, figure out a way to provide more water to the capital or grow more food with less work. However, there still should be general events to promote creative thinking.

Those presenting their inventions at a contest can be given patents if the invention is unique and useful.

There are two ways of achieving this:

• Supporting the research


• Rewarding for inventions

Of course, any good ruler can do both.

For research support, the ruler can (in the order of practicality in medieval times) employ scientists directly, establish an academy, give grants to an independent institution, or give private grants to any researcher.

For rewarding the inventions, the ruler should establish a scientific panel for evaluating them, and then either employ the inventor directly, or give him (or her) a grant to implement the invention independently. Patent system (@Zeiss Ikon) also can be helpful.

Hand out prizes at the annual town fair.

This is, in fact, how craftspeople and inventors rolled out new things. Most medium to large towns in Medieval Europe, for example, had 1 or 2 fairs a year where people sold and bought goods, traders came back from trips abroad, and the entire community came together.

Your ruler could easily add in an invention contest. This could be open or it could be themed, depending on what the ruler wishes. There can also be categories. The ruler may wish to reward young people or new inventors as well, even if their work isn't all there yet.

Winners can get cash, but a better prize would be funding to polish the invention and manufacture it. Young people can get the prize of funding for secondary school and higher education. Or apprenticeships with master craftsworkers (which cost money).

An open public contest won't eliminate cheaters, but it greatly diminishes them. It's unlikely someone could get away with stealing someone else's work, in a town where everyone knows each other. It's also very hard to fake your way through an invention with the entire town watching and checking up on you.

For the ruler, this has the great advantage of opening the field. Normally inventions might come from universities or other education sites, from large organizations like a monastery, or from the wealthier merchants in the business community. They can all still compete, but allowing anyone to enter greatly increases the number of viable inventions.

If the ruler only rules one large town or city, with other places within commuting distance (meaning a day or two by wagon), have the contest there. If the ruler rules over a larger area, have multiple contests in the larger towns then have a grand fair at the end of the year (for example, hold the smaller contests at the usual fall harvest fairs then hold the final one either just before the winter snows get bad or very early in the spring before planting.

Run a peer-reviewed journal

Presumably, the king is funding research, but there's a lot of bogus science. The king probably doesn't understand alchemy, but the alchemists do. Require that everything published in the journal be verified by an independent alchemist or two. Give grants for successful publishing. The more consistently your invention works, the more money you get from the king.

Like patents, there are ways to game the system, but this at least weeds out the obvious liars.