The basic operating principle of the Bulle Clock, invented in the early 1900s by M. T. Favre-Bulle is identical to that of Alexander Bain's invented some 80 years earlier. Bain's sliding-bar contact is replaced by a silver pin and a fork lever with a contact on one side only, to give an impulse at every other swing. A solenoid with a high-resistance winding swings over a magnet with consequent poles (south-north-south). When current flows through the solenoid, the magnetic field thus produced interacts with that of the permanent magnet to give impulse to the pendulum. An isochronal spring is fitted to the pendulum to reduce the circular error which would result with changing amplitude as the cell voltage varies with temperature and use. Tens of thousands of these clocks were manufactured in France. Later models were housed in conventional clock cases as the novelty of the pendulum display wore off.