The earliest mixers recorded are circa 1930 and seem to have appeared all over Europe around the same time

The normal mixer is an inverted gas cooker ring, a hole in the center for the air to pass through surrounded by small inlet holes that deliver the gas.

The mixer uses the pressure drop theory to deliver the gas i.e. the air pressure before the mixer is higher than that behind the mixer so a slight vacuum is formed which sucks the gas into the inlet manifold.

There are thousands of different mixers, each one designed specifically for a type of carburetor or throttle body. Some have more than one spray ring and some have central rings inside the pressure drop area, but most of them have no moving parts and that is why they have dominated the gas delivery systems from circa 1930 to 1997 when the 'mono point' came onto the market.

The mono point is a form of mixer and has a delivery cone that slides up and down a shaft delivering more or less gas, as the engine needs it. The mono point is set up on manifold pressure - the higher the vacuum the higher the gas volume and visa versa.

Another unusual mixer is the moving plate mixer fitted to big 4 choke Holly carburetors. This is a manual or semi automatic system. The reason for this strange mixer to come into existence is that when a mixer is fitted the air intake is restricted (to create the pressure drop) on the bigger engines this would make them run too rich on petrol (the emissions would go up and you would be wasting fuel), so the pressure drop plate is lifted up by a vacuum bag when the vehicle is running on petrol, and drops when the vehicle runs on gas.