Many people consider Alexander Graham Bell’s 1876 telephone patent to have been the most valuable ever issued by the U. S. Patent Office. In his recent book, The Telephone Gambit: Chasing Alexander Graham Bell’s Secret, author Seth Shulman claims that Bell “stole the key idea behind the invention of the telephone,” having copied the idea for a liquid transmitter from a patent caveat filed by rival Elisha Gray, and that Gray’s idea unlocked the secrets of the telephone for Bell.

Putting aside whether Bell stole anything or not, Shulman’s theory contains a major technological error. While Bell used the liquid transmitter to confirm his theory about how a telephone worked, the device did not lead directly to the first commercial telephone. It was induction — the idea in Bell’s receiver — not the liquid transmitter that allowed for the first telephones to go into commercial operations in 1876 and 1877. Indeed, to understand the controversy, it’s helpful to examine the different approaches that Bell and Gray employed in building a working telephone.

The telephone is the classic analog communications device. The telephone transmitter converts each peak and trough of the human voice into an equivalent (or analog) signal, variations in the voice’s sound wave matching variations in the electrical current. The telephone receiver simply reverses the process: after having traveled along a wire, the current wave is transformed back into sound waves that we hear.

Both Bell and Gray understood the potential for developing communication systems around the idea of converting sound waves into electric current waves, although Bell probably understood this idea sooner and more fully than Gray, having written the first description of it on November 23, 1874. Both eagerly investigated this idea to create better telegraph devices. But understanding the underlying idea of a technology does not necessarily lead to invention. The process of invention centers on finding the most effective way to manifest the idea and put it to use. Bell and Gray took very different paths in trying to realize the grand idea of analog communications.

One way to convert sound waves into electric current waves lay in exploiting the relationship between electrical current and magnetism. In 1819 Hans Christian Oersted discovered that current flowing through a wire creates a magnetic field. In 1831, both Joseph Faraday and Joseph Henry independently discovered the inverse of Oersted’s discovery — that if one moved a conductor or wire through a magnetic field, one could create or, more precisely, induce, a current to flow through the wire. This discovery led not only to the development of electric generators and motors, but to the telephone.

Bell used induction extensively in his efforts to create a harmonic telegraph that would send several messages over a single telegraph wire, a potentially valuable innovation for telegraph companies. Bell assigned a distinct tone to each message, but they always interfered with each other and scrambled the communication. While pursuing this dream, Bell gained a fundamental insight. On June 2, 1875, the year before he made his famous first call, Bell was listening to a reed relay receiver while Watson was fiddling with the transmitter. As Watson plucked the transmitter’s reed, Bell heard not only the primary tone, but all of the overtones. He suddenly realized that he could send a sound wave as complicated as the human voice over the wire. The following day Bell constructed the “Gallows telephone,” an induction device that resembled a toppled hangman’s scaffold.

In this device, current from a battery flowed through a coil and created a magnetic field. Very close to the core of the coil, he placed a piece of iron, which was attached to the transmitter’s diaphragm. When the iron vibrated, it perturbed the magnetic field; the corresponding changes in magnetic field induced a varying current in the circuit. Although a beautiful scheme, the Gallows telephone only produced muffled sounds because the induced current was so small. 

Nevertheless, Bell wrote up the idea during the fall and winter of 1875-76 in a patent application. After some delay, he filed it on the same day as Gray’s caveat. (Gray had realized the potential of analog communications and had been tinkering with his own multiple-message telegraph; while in Washington filing other applications, he threw in a brief caveat outlining his ideas for voice transmission.)

Bell had not finished his search for a practical way to convert sound into electricity. Consequently, in February 1876, Bell investigated a second technique, which was to vary the resistance in the circuit. Georg Ohm had discovered the basic principle in 1827 when he described the relationship between voltage, resistance, and current. “Ohm’s Law” states that the current in an electric circuit is directly proportional to voltage, but inversely to the resistance; in other words, if the resistance in the circuit is increased, then the current is decreased. 

Bell took full advantage of Ohm’s Law to design the liquid transmitter that he used to make the famous call to his assistant, “Mr. Watson, come here, I want to see you,” on March 10, 1876. 

As one spoke into the mouthpiece, which had a thin membrane or diaphragm on the bottom, the sound waves caused the diaphragm to vibrate. A needle, located at the center of the diaphragm, was positioned to dip into a bowl full of water, slightly acidulated to increase conductivity. Based on his reading of contemporary electrical science, Bell knew that the current passing through the circuit would vary proportionately with how much of the needle’s surface area was in contact with the water. By carefully adjusting the needle, Bell succeeded in creating an electric current wave in the circuit that duplicated a voice’s sound waves and enabled him to speak over the wire to Watson, who was listening to a receiver down the hall. Notably, Watson’s receiver was the same induction device that Bell had been using for months.

The liquid transmitter demonstrated Bell’s grand idea for analog communications, proving that it was indeed possible to convert sound waves into electric current waves and then back again. Yet while functional, the liquid transmitter was extremely difficult to adjust, and temperamental because the water often spilled. Instead, now knowing that his fundamental idea was correct, Bell went back to his induction devices and perfected them. A few months after his call to Watson with the liquid transmitter, Bell was demonstrating successful versions of his induction telephone, which he exhibited at the Philadelphia Centennial and subsequently put into commercial service.

In contrast, Elisha Gray investigated the interplay of sound and electricity by using a third technique, the Page effect. In 1847, Charles Grafton Page observed that an electromagnet will make sounds when the current running through it is changed. The sounds are caused by actual changes in the length of the iron core of an electromagnet; those changes in turn are related to the magnetic properties of iron, specifically the elastic strain in microscopic magnetic domains. 

While Gray employed a Page-effect receiver in his multiple-message telegraph experiments, he found that it did not work well for voice transmission because, among other things, it doubled the sound frequency, which made the caller sound like a chipmunk. Consequently, in his famous caveat showing his version of a liquid transmitter (which Gray thought worked by varying the distance between the needle and an electrode in the bottom of the water container), he sketched an induction receiver based on the same idea that Bell had applied in his Gallows telephone. Moreover, while he did not follow up on his liquid transmitter design, Gray instead filed a patent application in late 1876 depicting his own version of an induction transmitter and receiver. Not surprisingly, his application was turned down because Bell had already patented this form of telephone.

Hence, when we look at the patent documents from that exciting moment in 1876, we may want to focus not on the much-contested liquid transmitter, but rather on the receivers because both Bell and Gray realized that the secret of cracking analog communications lay in induction. The invention of the telephone was a race to come up with the best way to realize the fundamental idea of analog communications. 

CAPTIONS

Alexander Graham Bell, above, and Elisha Gray, left, battled to invent a telegraph in the mid-1870s capable of sending multiple signals at the same time.
Bell’s telephone patent application, left, included diagrams of undulatory current, reed receivers, and a cone mouthpiece. Gray’s patent caveat, below, filed on the same day as Bell’s application, was based on a different underlying technique of capturing the sound waves of a person’s voice.

In 1877 a genteel audience in Salem, Massachusetts, watched in disbelief as Bell spoke to Thomas Watson in Boston, some 15 miles away. A re-porter on the scene dictated a report, the first newspaper article sent over a wire.