Unlike modern AM radio stations that transmit a continuous radio frequency, whose amplitude (power) is modulated by an audio signal, the first radio transmitters transmitted information by wireless telegraphy (radiotelegraphy), the transmitter was turned on and off (on-off keying) to produce different length pulses of unmodulated carrier wave signal, "dots" and "dashes", that spelled out text messages in Morse code. As a result, early radio receiving apparatus merely had to detect the presence or absence of the radio signal, not convert it to audio. The device that did this was called a detector. The coherer was the most successful of many detector devices that were tried in the early days of radio.