The closer the kids sit to the fulcrum, the faster the movement. A mechanical metronome is sort of half of a teeter-totter. The moveable weight is one child, and the unmoveable counterweight is the other child. It is possible to get smaller ones, too, that do not take up a lot of room on the piano. This type is sensitive to being dropped or the weights in the moving arm otherwise disturbed.
Speed is adjusted by the dial.
Often these metronomes are quite large. These tend to be small, about the size of a credit card. They are battery-powered. This metronome is, however, excellent as a portable device. Cost is usually much less than the mechanical or electric types.