A brass ring is a small grabbable ring that a dispenser presents to a carousel rider during the course of a ride. Usually there are a large number of iron rings and one brass one, or just a few. It takes some dexterity to grab a ring from the dispenser as the carousel rotates. The iron rings can be tossed at a target as an amusement. Typically, getting the brass ring gets the rider some sort of prize when presented to the operator. The prize often is a free repeat ride. The phrase to grab the brass ring is derived from this device.
Brass ring devices were developed during the heyday of the carousel in the U.S.--about 1880 to 1920. At one time, the riders on the outside row of horses were often given a little challenge, perhaps as a way to draw interest or build excitement, more often as an enticement to sit on the outside row of horses which frequently did not move up and down and were therefore less enticing by themselves. Most rings were iron, but one or two per ride were made of brass; if you managed to grab a brass ring, you could redeem the ring for a free ride. References to a literal brass ring go back into the 1890s.
Once the ride started moving, a metal arm was swung out for riders to try to grasp the ring from. As the wooden horses or other creatures circled around the center where the machinery and organ were housed, rings were fed to one end of a wooden arm that was suspended above the riders, who hoped that the timing of their horse's rise would coincide with their approach to the ring, which they would try to grab.
On some rides this held a single brass ring, which was difficult to grab since the outside edge of the carousel is always the fastest moving. Whoever managed to retrieve the brass ring, if anyone, could redeem it for a free ride. Another system had a dispenser of rings, most of which were steel and had no value, but one per ride was the brass one that won the prize. In this system, there was a target to throw the ring into (for example the Santa Cruz Beach Boardwalk Looff Carousel uses a clown target as the image shows, and Knoebels Grand Carousel uses a Lion), discouraging retention as a souvenir.
The term has been used as the title of at least one book.
The following carousels in the United States have operating brass ring dispensers/targets