Popular Mechanics, March, 1907, page 315-316:
 
Professor Clark

    Wireless telephony is an accomplished fact and now it only remains for its inventors to perfect their apparatus and adapt it to the commercial needs of the day. The world is no longer incredulous. The rapid advancement made in the use of wireless telegraphy has swept away the doubts of even the most conservative and business men are looking forward to the time when unrestricted verbal communication between New York and London, or Chicago and Berlin, may save them a trip across the ocean.
    Thomas E. Clark, of Detroit, has produced a wireless telephone system which seems to be the nucleus of the world-connecting system yet to come. Instead of the air, as in wireless telegraphy, he uses the earth as his medium of transmission, success depending, it is said, upon sustained vibrations. As yet Mr. Clark has tried his instrument with only two dry cell batteries and with apparatus designed for short distances. These experiments, however, have been attended with promising success. The sound was carried without loss in volume, change of pitch, or the disagreeable sonorous defects incident to the metallic circuit 'phones.
    Despite this wonderful achievement, the difficulties yet to be met are great. For instance, in order for one patron to call up another in a distant city, where hundreds of telephones were in use would necessitate that the sending instrument or transmitter be given the right degree of power to overcome both the distance and the varying accidental conditions which affect transmission and be attuned to set up vibrations in the particular 'phone desired, calling the attention of the owner and enabling him to hold converse over the wireless line. The field of operation, however, cannot be limited and may eventually include any two points on the earth's surface.