Overview:
This text, published in 1919, is a historical overview of the technology and principles behind telephony and telegraphy. It delves into the development of both manual and automatic telephone systems, discussing their advantages and drawbacks. The text highlights the early forms of telephone equipment, including the magneto and carbon transmitters and receivers, as well as the switchboards, hook switches, and other components that made early telephone systems possible. It also explores the fundamental principles of electrical signaling, including the telegraph sounder, vibrating bell, and polarized ringer.
The text provides a detailed look at the evolution of telephone line construction, including the transition from grounded lines to metallic circuits, and the use of open wires and cables. It discusses the properties of telephone lines, such as conductivity, inductance, capacity, and insulation, and the impact these have on transmission quality. The text also discusses the development of techniques to improve transmission quality, including the use of Pupin loading coils, and the challenges of creating telephone repeaters.
Key Findings:
- The early development of telephony was heavily influenced by the existing technology of telegraphy. Both manual and automatic switching concepts were adapted from telegraph systems.
- Carbon quickly became the preferred material for varying resistance in telephone transmitters. Its ability to change resistance under pressure made it superior to other materials.
- Electromagnetic principles were crucial for both transmitting and receiving sound. The magneto receiver and transmitter, and the various forms of relays and bells, all relied on electromagnetism.
- The development of automatic telephone systems was slow due to the difficulty in replacing manual switching with a reliable automated system.
- The introduction of common-battery systems offered significant advantages over local battery systems in terms of cost and efficiency.
Learning:
- The reader will gain a historical understanding of the development of telephony and telegraphy. This includes the key inventions, principles, and challenges that shaped the evolution of these technologies.
- The reader will learn about the various components of early telephone systems. This includes transmitters, receivers, switchboards, hook switches, and line signals.
- The reader will understand the importance of the physical properties of telephone lines, such as conductivity, inductance, and capacity. They will learn how these properties impact transmission quality and how they are mitigated through various techniques, such as loading coils.
- The reader will gain insight into the challenges and advancements in developing reliable telephone repeaters. This includes understanding the different types of repeaters and their limitations.
Historical Context:
This text was written in 1919, during a time of rapid technological advancement. The world was still recovering from World War I, and there was a growing demand for communication technologies, such as the telephone and telegraph. The invention of the wireless technologies, though still in its infancy, was also beginning to revolutionize long-distance communication. The development and refinement of telephony and telegraphy during this period laid the foundation for the modern communication systems we use today.
Facts:
- The telephone was invented in 1875 by Alexander Graham Bell. Bell, a teacher of deaf mutes, was experimenting with multiplex telegraphy when he observed the vibration of an iron reed near an electromagnet. This led to the invention of the first telephone.
- The first public use of the telephone was at the Centennial Exposition in Philadelphia in 1876. This demonstration sparked widespread interest in the new technology.
- The earliest telephones were magneto phones, which used a permanent magnet to produce a magnetic field. However, these were not very powerful.
- Carbon became the preferred material for varying resistance in transmitters. This allowed for much stronger signals to be transmitted.
- The induction coil was essential for transmitting sound over long distances. It acted as a step-up transformer, increasing the voltage of the signal and reducing losses.
- The first automatic switching equipment failed because it attempted to bring each line in the central office within connecting reach of each connecting mechanism.
- The most successful automatic telephone systems eventually adopted the trunking method. This involved extending the calling line link by link until it found the called line within a smaller group.
- The invention of the multiple switchboard was a major advancement in manual telephone systems. It allowed each operator to reach every line in the exchange, dramatically improving service.
- Telephone lines initially used iron wires, but copper wires quickly replaced them. Copper offered superior conductivity and resistance to rust.
- Telephone lines initially used grounded circuits, with the earth serving as one conductor. However, metallic circuits, with two wires, became the standard, offering improved reliability and reduced interference.
- The use of cables for telephone lines allowed for the use of smaller, more conductive wires and reduced the space required. However, cables limited the distance over which speech could be transmitted.
- Oliver Heaviside proposed increasing the inductance of telephone lines to counteract the harmful effects of capacity. This was a major step in improving long-distance transmission.
- Pupin loading coils were developed to improve long-distance transmission. By inserting inductances at specific points along the line, they counteracted the effects of capacity and reduced signal attenuation and distortion.
- The invention of the Shreeve repeater allowed for the transmission of voice currents over longer distances. It used a telephone receiver and a carbon transmitter to amplify the signal.
- The human ear can perceive sound waves at frequencies from 32 to 32,000 vibrations per second.
- The human voice typically uses fundamental frequencies between 85 and 160 vibrations per second for men, and 150 and 320 for women.
- The earliest electric signal was the telegraph sounder. It relied on the make and break of a circuit to produce an audible click.
- The magneto-bell was a major advancement in telephone signaling. It used a hand-cranked generator to produce alternating current, which operated a polarized ringer.
- The gravity cell was a type of battery that produced a constant current. It was used in early telegraph and telephone systems.
- The Fuller cell was another type of battery used in early telephone systems. It had a higher voltage output than the gravity cell but required more maintenance.
Statistics:
- Sound travels at a rate of about 1,090 feet per second in air.
- The pitch of the human voice ranges between 87 and 768 vibrations per second.
- An average man speaks mostly between the fundamental frequencies of 85 and 160 vibrations per second.
- Many female speaking voices use fundamental frequencies between 150 and 320 vibrations per second.
- The earliest incandescent lamps, invented by Edison, used a carbon resistance material.
- Independent telephone companies have installed nearly sixteen times as many telephones as the Bell organization during the same time period.
- At the beginning of 1910, there were 3,633,900 telephones in service by the Bell organization, and 3,911,400 independent telephones.
- One-twelfth of the population of the United States had a telephone in 1910.
- A copper wire with a diameter of .08 inch weighs approximately 100 pounds per mile.
- A telephone line composed of two wires of .036 inch diameter, wrapped in paper and twisted into a pair, has a mutual electrostatic capacity of approximately .08 microfarads per mile.
- Rubber and gutta-percha insulators provide capacities as great as twice that of dry paper.
- A good 2-3/4 inch by 6 inch dry cell will give approximately 30 ampere hours throughout its useful life.
- A gravity cell has a voltage of 1.08 volts and an internal resistance that varies from 1 to 6 ohms.
- The Fuller cell provides an electromotive force of 2.1 volts.
- The Lalande cell has a constant electromotive force of about two-thirds of a volt.
- A good 2-3/4 inch by 6 inch dry cell will give 18 to 20 amperes on a short circuit.
- The electromotive force of the chloride of silver cell is 1.03 volts.
- The electromotive force of a LeClanché cell is about 1.47 volts.
- A magneto generator can produce an electromotive force of up to 100 volts.
- A magneto generator typically produces a current of about 1,000 cycles per minute.
Terms:
- Acoustics: The science of sound.
- Timbre: The characteristic quality of sound that distinguishes it from other sounds of the same pitch and loudness.
- Harmonic: A sound wave whose frequency is a multiple of the fundamental frequency.
- Overtone: Another term for harmonic.
- Microphone: A device for converting sound waves into electrical signals.
- Induction Coil: A transformer used in telephone systems to increase the voltage of a signal.
- Condenser: A device that stores electrical energy in an electric field between two conductors.
- Dielectric: The insulating material separating the conductors in a condenser.
- Farad: The unit of capacitance.
- Mho: The unit of conductivity.
Examples:
- The Doppler effect: As an object emitting sound waves moves towards an observer, the pitch of the sound appears to rise, and as it moves away, the pitch appears to fall. This effect is used to illustrate the relationship between frequency and pitch.
- The phonograph: The phonograph uses a diaphragm to convert sound waves into mechanical vibrations, which are then recorded in a groove on a record. This demonstrates the ability of sound waves to move diaphragms.
- The Hughes microphone: This early microphone used loose contacts between two carbon blocks to vary the resistance of a circuit, making small sounds audible. This is a precursor to the modern carbon transmitter.
- The Blake transmitter: This single-contact transmitter used a carbon and a platinum electrode to vary resistance. It was known for clear articulation but lacked power.
- The White (solid-back) transmitter: This transmitter used a granular carbon chamber between two electrodes, one fixed and one attached to the diaphragm. It was much more powerful than the Blake transmitter.
- The Acousticon transmitter: Designed for use with hearing aids, this transmitter was highly sensitive and could pick up sounds from a distance.
- The gravity cell: This battery, also known as a blue-stone cell, uses copper and zinc electrodes and a solution of copper sulphate. It was a popular choice for providing a constant current.
- The Fuller cell: This battery uses zinc and carbon electrodes and a solution of bichromate of potash and sulphuric acid. It could produce a larger current than the gravity cell.
- The Lalande cell: This battery used zinc and copper oxide electrodes and a solution of caustic potash. It was designed for heavy-duty service and provided a constant current.
- The chloride of silver cell: This cell was widely used as a standard for testing purposes. Its small size and long life made it ideal for portable testing devices.
Conclusion:
This 1919 text provides a fascinating and detailed account of the development and technology of telephony and telegraphy during a pivotal time in the history of communication. The text highlights the evolution of both manual and automatic telephone systems, outlining the challenges and triumphs of early telephone engineers. The reader is introduced to the fundamental principles and components that powered early telephone systems, including the various types of transmitters, receivers, switchboards, and signal systems. The text also explores the essential properties of telephone lines and the techniques used to improve transmission quality, such as loading coils and repeaters.
While technology has advanced significantly since 1919, this text offers valuable insights into the early days of communication technology. It provides a historical foundation for understanding the principles and innovations that paved the way for the modern communication systems we rely on today.