A simple development of the single-stroke bell was the sprung bell. This had previously been used, mechanically actuated, for servant-call bells in large houses. Instead of working a clapper, the electromagnet shook the whole bell, which was mounted on a flexible spiral spring. The inertia of the heavy bell on the light spring would continue ringing for some seconds after the stroke. Although the sound would rapidly die away, the visible trembling of the bell could indicate which bell had been rung, amongst a panel of several.