The split control mechanism has been removed for easier viewing and understanding in the photo above. When the split mechanism is called upon, by pressing the split button on the case, pressure is applied to shaft "A" which in turn pulls hook "B" that rotates crown wheel "C" 1/16th of a turn, and jumper spring "D" holds the crown wheel stationary. Two arms, "E" and "F" form a set of scissor arms. These scissor arms will grab and stop the split-seconds wheel (you can see this very clearly in drawing #1) when called upon to record an intermediate event.
This action must be very swift in order to stop the seconds hand at a fraction of a second but also gentle enough not to unsettle the hand so it shows a false reading. It is, I can attest, a very fine skill to adjust the proper operation of a split chronograph. Stopping the split-seconds hand is the first operation that must be accomplished in the Double Split, but not the only. The two pins "G" and "H" will be responsible for the clutch actions that will be examined towards the end of this article.
I save the most interesting aspect of the A. Lange & Söhne Double Split for last. As mentioned a couple of pages ago, each set of chronograph recording hands, minutes and seconds, must be either coupled together (as in recording a single event) or decoupled (as when the split function is called to action for a secondary event). And, when the split function is called out of action, the recording hands (minutes and seconds) must again become instantly and perfectly synchronized with their counterparts. This latter point is extremely important.
In order to record two separate events, there needs to be a pair of clutch systems, one for minutes and one for seconds. In the photo above we can see these clutch mechanisms, "A" is for the minutes and "B" for the seconds recording. To give you a better perspective the clutches have been removed (out-takes, left) and shown from the top and the bottom. Their function will become clearer in the following drawings.
In the drawing above are the hammers "A" and heart cams "B" as mentioned earlier. In the upper drawing, the minutes- or seconds- recording hands are in synchronized mode, coupled since the hammer is in direct contact with the heart cam and held there semi-rigid via the force of the gold-colored spring. In the lower drawing, they are not synchronized, uncoupled since the hammer has been held apart from the heart cam. Think of it this way, in the upper drawing, you are strolling along with your friend, synchronized, but suddenly your friend needs to stop while you keep on moving (lower drawing). At a further point down the way you agree to get synchronized again with your friend. Magically, at the touch of a button, your friend is at your side again. When the hammer falls to the heart cam, there is no other possibility since the heart cam is eccentric and cannot avoid the path of least resistance.
Something more. All split chronographs suffer a liability when the split function is being used, the hammer is in constant contact, dragging, around the heart cam. The A. Lange & Söhne Double Split has avoided this drag factor by totally decoupling the hammers and heart cams while the split function is in use.