The standard settings are 40 - 208. Although some teacher despise them, a metronome is an important practice aid, in my opinion. It lifts from the student's shoulders the burden of maintaining a steady tempo during the learning stage of a piece so the student can concentrate on locating the notes accurately and playing them for the duration specified by the composer. Another use for a metronome is for a gradual increase in tempo. The increments are so small that almost without knowing it, the student increases the speed at which she can play the piece accurately.
Many teachers use the metronome as an adjunct to technical drills.
These are the wind-up kind and work by a spring mechanism. When the spring unwinds entirely, the metronome stops and must be re-wound. Speed is changed by moving a weight on a pendulum-type arm. The closer the kids sit to the fulcrum, the faster the movement. A mechanical metronome is sort of half of a teeter-totter.