Older type metronomes are cone-shaped clock mechanisms with the adjustable weight on the end of the pendulum rod. Pace is inscribed on a metal plate beneath the pendulum ( from the slowest, 40 beats per minute, to the fastest, 208 bpm). Pendulum makes constant and even swings from side to side, and the beats can be heard. When the weight is at the top the pendulum’s swings and beats are slower and they are faster if the weight is lowered.
Today, electronic metronomes are in use (they work on electricity or battery) in the form of a pocket calculator. They have no weight or pendulum. Speed of the beats on them is set by the circuit adjuster or even by simply selecting the desired speed (the number). Pressing certain keys speed is displayed on the small screen–display (see picture above, four metronomes in the background).
Taken from my book