The radio-telephone systems developed by Lee DeForest and Adolphus Slaby used similar techniques -- a Poulsen arc-transmitter, plus an electrolytic detector, which had been independently discovered by W. Schlömilch in Germany and Reginald Fessenden in the United States. However, it was only with the development of vacuum-tube transmitters and receivers that radiotelephony would become practical.
The Literary Digest, June 15, 1907, page 949:
WIRELESS TELEPHONY AT LAST
IT is now definitely stated that wireless telephones are to be placed experimentally in some of the North-River ferry-boats in New York, and the reports of successful trials of the apparatus on shore make it certain that it is now possible to telephone without connecting wires, altho much remains to be done before the perfection of a practical commercial system. A contributor to Energy (Leipsic, Germany, April) tells us that during a recent lecture by Professor Slaby, in the Technical School at Charlottenburg, messages were exchanged between the school and the buildings of the Wireless Telegraphy Company at Berlin, with complete success. Says this writer:
"Just as in wireless telegraphy, ether-waves are used for the transmission of communications by wireless telephony. The present form of wireless telegraphy is that of vibrations arising from a spark flashing across the air, which open and close a circuit in the receiver, and thereby print the signs of the Morse alphabet on a slip of paper.
"However, the spark is not available for the transmission of waves caused by the voice and converted into an electric current by the microphone, because of the excessively fine modulations of the vibrations. The duration of the vibration of a spark is about one hundred-thousandth of a second, and, therefore, is not sufficient for reproducing the vibration of a voice, as, for instance, that of a soprano, which lasts one-thousandth of a second. For the transmission of the voice, a medium is necessary that continues to vibrate without interruption.
"Poulsen found this medium in the electric luminous arc. If a wire be conducted from the lower carbon of a burning arc-lamp down to the ground, and another wire be extended from the upper carbon into the air, as employed in wireless telegraphy, the wire in the air emits uniform ether vibrations. Their action is not manifested in jerks, as in the discharge, of sparks, but is uniform and constant. If a microphone, such as is in the speaking-apparatus of an ordinary telephone, be attached to the air-wire, and one speak into it, the vibrations emanating from the luminous arc of the air-wire are influenced in their intensity by the vibrations of the voice."
To receive the vibrations, an electrolytic cell devised by Schlömilch is used. Two very thin platinum wires in a vessel of dilute sulfuric acid are traversed by a very weak current which at the same time passes through a common telephone-receiver. The intensity of the current is affected whenever the electric vibrations strike one of the receiving wires, which extends upward from the cell. Variation of the vibrations takes place exactly as in the transmitter; consequently, the same sounds can be perceived in the receiver as were spoken into the microphone. The writer concludes:
"Altho wireless telephony represents a valuable addition to wireless telegraphy, nevertheless, when compared with the wire telephone generally in use to-day, it suffers a serious disadvantage. It does not allow of a rapid change from hearing to speaking. If one is listening to a wireless conversation, one must patiently wait until the man at the other end has finished, and then the system must be switched in order to reply. Therefore, for urgent cases, wireless telephones can not be regarded as serviceable media for the transmission of messages. It is hoped that in a short time these drawbacks will be alleviated."