Split-seconds chronograph
A chronograph with two superimposed seconds hands, one of which can be halted to read an intermediate time and then caught up to the other.
A single chronograph hand can time one event. Two horses crossing a line seconds apart, or two laps of a track, means either two instruments and two observers, or accepting that the first reading is lost when the hand is reset for the second. Joseph Thaddaeus Winnerl arrived at the answer in Paris by about 1838: two seconds hands on one axis, started together, one of which can be stopped, read, and returned instantly to the other.
The two hands sit on concentric arbors, the lower one the ordinary chronograph wheel, the upper the split wheel with its own heart-shaped cam. A lever ending in a small ruby roller rests in the notch of that cam, and it is the roller, not any gear tooth, that drags the split wheel round in step with the wheel beneath. Pressing the split pusher closes a pair of sprung pincers on the rim and stops it; the wheel below carries on, pushing the roller out of the notch and up the flank of the cam. Releasing the pusher lifts the pincers, the roller slides back down into the notch, and in doing so swings the stopped hand forward until it lies exactly beneath the running one.
The cost is friction. While the split hand is held, the train has to drag the roller round against the cam, which draws amplitude from the balance; hold it too long and the rate suffers. Better constructions add an isolator that lifts the roller clear the moment the pincers close, at the price of more parts and a more delicate adjustment.