• Balance Wheel
• A Transistor acting as a switch
• No Mechanical contacts

Sometimes the coil is on the balance wheel and the magnets are on the base plates, but it is more common to find it the other way around.

Because a transistor can act as switch, these were the first battery-powered watchs that no longer needed a physical electrical contact. The days of worn-out, sparking contacts were over.

The voltage to steer the base of the transistor is generated by induction in the trigger coil; the visible coil in these watches is actually two coils side-by-side. As the fixed magnets on the balance pass over these coils, the trigger coil generates a voltage and the transistor completes the circuit on the impulse coil and the balance receives an impulse. When the balance swings back due to the spring, the trigger coil generates a reverse voltage and the transistor breaks the circuit to the impluse coil.

This system was invented, and patented, by Etablissements Leon Hatot (ATO) in the early 1950s. As a result, millions of watches carry the ATO or Lic. ATO mark either on the movemnent plates and / or on the dial. You can see this quite clearly on this Nivada dial.