At the end of 1922, WWV's purpose shifted to broadcasting standard frequency signals. These were an important aid to broadcasting and amateur stations, because their equipment limitations at the time meant they had difficulty staying on their assigned frequencies. Testing began on January 29, 1923. Regularly scheduled operations began on March 6, 1923, initially consisting of seven transmitting frequencies ranging from 550 to 1500 kHz (wavelengths of 545 to 200 meters). The frequencies were accurate to "better than three-tenths of one percent". At first, the transmitter had to be manually switched from one frequency to the next, using a wavemeter. The first quartz resonators (that stabilized the frequency generating oscillators) were invented in the mid-1920s, and they greatly improved the accuracy of WWV's frequency broadcasts.