The Panel switching system was an early type of automatic telephone exchange for primarily urban service, first introduced in the Bell System in the 1920s and replaced by modern systems during the 1970s. It was developed by Western Electric Labs, later called Bell Labs, in the U.S. in parallel with the Rotary system at International Western Electric in Belgium before World War I which was used in Europe. Both systems had many features in common.The first Panel exchange was installed at the Mulberry Central Office in Newark, New Jersey. It was placed in service on January 16, 1915. It was a semi-automatic system using telephones without a dial. Operators answered calls and keyed the station number into the panel switch. The next installation was in the Waverly central office on June 12 of the same year, also in Newark.The Panel Machine Switching System was named for its tall panels covered with 500 rows of terminals. Each panel had an electric motor to drive its, usually sixty, selectors by electromagnetically controlled clutches. The selector was similar in effect to a stepping switch though it moved continuously rather than in steps. Each selector had five brushes, each of which could select from 100 terminals arranged in groups. Pulses were sent back from the selector to a register, which had received the dialed digits, rather than forward as in the Strowger switch step-by-step (SXS) system, hence the signaling was called revertive pulsing.