The most modern of modern gadgets, the wireless telephone,is actually not so modern after all. Within 6 years of the first American patent for a telephonic device (Alexander Graham Bell, 1876), another researcher discovered a method for sending voice through the air without connecting wires. His name was Amos Emerson Dolbear (1837-1910). From 1874 on Dolbear was chairman of the Physics Department at Tufts University. He worked throughout the 1870s on designing a working telephone (as did many other scientists in Europe and America), but was beaten to the patent office by Bell. Undaunted, Dolbear continued to develop his own ideas about electrical communication. One day in 1881 he was working in his lab when he made a startling, accidental discovery:
While at work at the single terminal receiver . . . the cord became detached from the line while I was unaware of it, and I still heard the speech from the transmitter plainly. Upon noticing this I began backing away from the end of the wire from the transmitter, letting the single cord hang free in the air. I could hear the talking in the most remote part of the room.
Wireless telephony was the next big step forward in communication. Telegraph and telephone systems required vast amounts of wire strung overhead on unsightly utility poles. The benefit, in time, money, and esthetics promised by wireless communication would be enormous. Dolbear applied for a patent on his discovery in 1882. Patent 350299 was granted to Dolbear in 1886, but nothing came of it. Wireless telephony did not become widespread till a century later, and not truly ubiquitous until after the turn of the 21st century. Why? Was there a conspiracy by the hard-wired phone company to suppress competing technology? Or was there a weakness, a fatal flaw in the Dolbear system? I decided to try to replicate his 1882 design, with features from later wireless phone designs by Nathan Stubblefield, Archie Collins, and Hugo Gernsback to uncover how well it worked--and how well it didn't.
Professor Dolbear's patent describes the parts of his 1882 wireless phone, but he doesn't really explain how it works. He writes vaguely about varying potentials in the ground connection, but simply put, he uses the earth instead of metal wire for his connections. This method is called earth conduction telephony. (As a side note, Dolbear's use of an isolated capacitor in the air made it impossible for Marconi to patent his early radio apparatus in the U.S. Dolbear's patent covers the use of aerial antennas! Marconi's company had to buy Dolbear's patent in order to market his wireless sets in America.)
Conduction technology goes back at least to Samuel Morse, who invented his telegraph in 1837. Crews stringing telegraph lines hither and yon faced obstacles at natural barriers like mountains, rivers, and ravines. Morse theorized his electrical signals would cross some distance if fed into the ground (or water) at one spot, and picked again on the other side. It worked, but only over short distances.
About 50 years later a Kentucky farmer and inventor named Nathan Stubblefield developed an earth conduction wireless phone. He demonstrated his wireless phone in Washington, D.C. and Philadelphia, trying to attract investors. Stubblefield used conventional phone components connected to long iron rods driven into the earth. Range was still short--effective range was measured in hundreds of yards, though some accounts say Stubblefield's device worked as far as a mile. The secret to Stubblefield's apparatus was power. More batteries meant more voltage and more range. That's why his 1902 model phone (see photo) has such a tall cabinet; it's stuffed with dry cell batteries. Notice too the radial switch operated by his son, Bernard. Moving the switch could add or subtract batteries from the circuit, suiting power to performance.
Stubblefield's dream of earth conduction telephony ended, partly because of the inate weaknesses of the technology, and partly due to the machinations of shady stock promoters. Stubblefield turned to an alternate method of wireless communication, electromagnetic induction.
Induction is the creation of electromotive force across a conductor when it is exposed to a varying magnetic field. In other words, an electric current in one coil of wire will induce a similar current in a nearby coil. Notice the fatal word "nearby." If you use a device to vary the current in one coil--say, telegraph key, or a microphone--the receiving coil will reproduce the variations, which can be converted into sound by using telephone-style headphones.
The strength of an electromagnetic field is inversely proportional to the square of the distance from the charged particle:
F = kQ/r^2
where F is force, k is a constant [9 x 10^9 Nm^2/C^2], Q is the charge of the particle [in coulombs], r is the distance from the charge. Plainly put, the field weakens drastically with distance. That's the reason we don't use wireless induction phones today.
Not that clever men didn't try to make it work! From Professor Dolbear to Archie Frederick Collins, various engineers and inventors tried to make induction phones practical. What follows are my modern attempts to replicate their efforts..