While working to develop AM transmitters, he realized that the radio wave detectors used in existing radio receivers were not suitable to receive AM signals. The radio transmitters of the time transmitted information by radiotelegraphy; the transmitter was turned on and off by the operator using a switch called a telegraph key producing pulses of radio waves, to transmit text data using Morse code. Thus receivers didn't have to extract an audio signal from the radio signal, but only detected the presence or absence of the radio frequency to produce "clicks" in the earphone representing the pulses of Morse code.