Other articles in the series:
- History of the relay
- History of electronic computers
- History of the transistor
- Internet history
Π we learned how an American scientist and teacher traveled across Europe for the first time. While visiting London, he made a special visit to a man deeply respected by him, a mathematician . Together with Henry was his friend, Alexander Bach, and his new acquaintance, also an experimenter in the field of telegraph, . Babbage told the guests that he was going to show his calculating machine to a member of Parliament soon, but with even greater pleasure he shared with them the idea of ββhis new machine, "which will greatly exceed the capabilities of the first." Henry recorded the general details of this plan in his diary:
This machine is divided into two parts, one of which Mr. B. calls storage, and the second - a mill. The vault is filled with wheels with numbers painted on them. Periodically, levers pull them out and move them in the mill, where the necessary manipulations take place. At the end, this machine will be able to tabulate any formula of an algebraic nature.
The historian cannot help but feel a chill running down his spine from such random intersections in human lives. Here two threads of the history of computing machines crossed, one of which was nearing its end, and the other was just beginning.
After all, although Babbage's machine is often presented as the beginning of the history of modern mainframe computers, the connection between them is rather weak. His machine (which he never built) was the culmination of a dream of mechanical computing. This dream, first voiced by Leibniz, was inspired by the ever more complex watch movements created by craftsmen since the end of the Middle Ages. But no general-purpose computer has ever been built on pure mechanicsβthe task is far too complex.
But the electromagnetic relay, conceived by Henry and others, can be quite easily implemented in computing circuits, the complexity of which without it seems unimaginable. However, until this moment there were still decades, and such a development could not have been foreseen by Henry and his contemporaries. It was the progenitor of the myriad transistors that made possible today's digital world, so deeply intertwined with our modern lives. Relays filled the innards of early programmable computers that ruled for a short time until they were replaced by their purely electronic cousins.
The relay was invented several times independently in the 1830s. Its goals were diverse (its five inventors came up with at least three applications) - as were the examples of use. But it's convenient to think of it as a dual-use device. It can be used as a switch to control yet another electrical device (including, importantly, another relay), or as an amplifier to turn a weak signal into a strong one.
Switch
Joseph Henry combined in one man deep knowledge of natural philosophy, mechanics and interest in the problem of a mechanical telegraph. In the 1830s, such a set of qualities was, perhaps, only Wheatstone. By 1831, he had built a circuit 2,5 km long, capable of operating a bell, using the most powerful magnet in existence. Perhaps if he continued to work so actively on the telegraph, and showed the same perseverance as Morse demonstrated, then it was his name that would be inscribed in the textbooks.
But Henry, a teacher at the Albany Academy and later at the College of New Jersey (now Princeton University), built and improved electrical devices for research, teaching, and scientific demonstration purposes. He was not interested in turning a pedagogical tool into a messaging system.
Around 1835, he came up with a particularly ingenious demonstration using two circuits. Recall that Henry discovered that electricity has two dimensions - intensity and quantity (we call them voltage and current). He created circuits with intense batteries and magnets to transmit electromagnetism over long distances, and circuits with quantitative batteries and magnets to create electromagnetic forces of great power.
His new unit combined both properties. A powerful quantitative electromagnet could lift a load of hundreds of kilograms. An intense magnet at the end of a long loop was used to lift a small metal wire: a switch. Closing the intense circuit caused the magnet to lift the wire, which opened the switch and the quantitative circuit. The quantitative electromagnet would then suddenly drop its load with a deafening roar.
This relay - namely, this role was played by an intense magnet and its wire - was necessary to demonstrate the transformation of electrical energy into mechanical energy, as well as how a small force can control a large one. Lightly dipping the wire in acid to close the circuit resulted in a slight movement of the small switch, resulting in a catastrophic drop of metal, enough to crush anyone foolish enough to stand under it. For Henry, the relay was a tool for demonstrating scientific principles. It was an electric lever.
Henry was probably the first to connect two circuits in this way - in order to use the electromagnetism of one circuit to control the other. The second place, as far as we know, belongs to William Cook and Charles Wheatstone, although they had very different goals.
In March 1836, shortly after attending a demonstration in Heidelberg of a telegraph using a galvanic needle to transmit signals, Cook was inspired by the music box. Cook believed that the use of needles representing letters in a real telegraph would require several needles, and they would need several circuits. Cook, on the other hand, wanted the electromagnet to activate the mechanism, which can already be arbitrarily complex in demonstrating the desired letter.
He conceived a machine that looked like a music box, with a barrel surrounded by many pins. On one side of the barrel there should be a dial with letters. At each end of the telegraph line there should be such a box. The cocked spring should make the keg rotate, but most of the time it will be held in place by the stopper. When the telegraph key is pressed, the circuit is closed, which activates the electromagnets that open both constipation, and both machines rotate. When the desired letter is shown on the scale, the key is released, the locks fall into place and stop the movement of the kegs. Cook, unknowingly, recreated Ronald's two-decade-old chronometric model of the telegraph and the early experiments of the Chappe brothers with the telegraph (only they used sound rather than electricity to synchronize the scales).
Cook realized that the same mechanism could help solve the telegraph's long-standing problem of notifying the receiving party of a new message. To do this, you can use a second circuit with another electromagnet that would activate a mechanical bell. Closing the loop would retract the stopper and the bell would ring.
In March 1837, Cook began work with Wheatstone on the telegraph, and around this time they considered the need for a second circuit. Instead of setting up an independent circuit for the alert signal (and running miles of extra wire), wouldn't it be easier to use the main circuit to control the signal?
By then, Cooke and Wheatstone had returned to the needle design, and it was quite obvious that a small piece of wire could be connected to a needle so that when its end was attracted to an electromagnet, its tail would complete a second circuit. This circuit would actuate the signal. After a certain interval, during which the recipient of the message could wake up, turn off the signal and prepare a pencil and paper, the needle could already be used to transmit the message in the usual way.
In the course of two years on two continents, twice, for two different purposes, people realized that an electromagnet could be used as a switch to control another circuit. But one could also imagine a completely different way of interaction between the two circuits.
Amplifier
By the fall of 1837, Samuel Morse was confident that his idea for an electric telegraph could be made to work. Using an intense battery and Henry's magnet, he sent messages over a distance of half a kilometer. But in order to prove to Congress the possibility of transmitting messages by his telegraph across the entire continent, he needed much more. It was clear that no matter how powerful the batteries were, at some point the loop would become too long to transmit a legible signal to the other end. But Morse realized that, despite the strong drop in power with distance, an electromagnet could open and close another circuit, powered by its own battery, which in turn could transmit a signal further. The process can be repeated any number of times and cover distances of any length. Therefore, these intermediate magnets were called "relay" - like postal stations for changing horses. They received an electrical message from a weakening partner and carried it further with renewed vigor.
It is impossible to establish whether this idea was inspired by the work of Henry, but Morse was certainly the first to use a relay for such a purpose. For him, the relay was not a switch, but an amplifier capable of turning a weak signal into a strong one.
On the other side of the Atlantic around the same time , a London pharmacist, came up with a similar idea. He probably became interested in the telegraph around 1835. By early 1837 he was regularly experimenting with a XNUMX-kilometer circuit in Regent's Park, northwest London.
Shortly after the meeting between Cook and Wheatstone in March 1837, Davey felt the competition and began to think more seriously about building a practical system. He noticed that the deflection force of the galvanic needle decreased markedly as the length of the wire increased. As he wrote many years later:
Then I thought that even the slightest movement of the needle through the thickness of a hair would be enough to bring two metal surfaces into contact, completing a new circuit, dependent on a local battery; and so it can be repeated forever.
Davey called this idea of ββturning a weak electrical signal into a strong one "electrical refresher". But he failed to realize this or any other telegraph idea. He received a patent for the telegraph in 1838, independently of Cook and Wheatstone. But in 1839 he sailed to Australia to escape an unhappy marriage and left the field to competitors. Their telegraph company bought this patent a few years later.
relay in the world
In the history of technology, we pay a lot of attention to systems, but often ignore their components. We lead the history of the telegraph, telephone, electric light, bathe their creators in the warm rays of our approval. But these systems could only come into being through the combination, recombination, and alteration of existing elements quietly growing in the shadows.
The relay is one of those elements. It quickly evolved and became diversified when the telegraph networks began to grow rapidly in the 1840s and 1850s. Over the next century, it appeared in electrical systems of various kinds. The earliest modification was the use of a rigid metal anchor, as on a telegraph signal, to complete the circuit. After turning off the electromagnet, the armature was disconnected from the circuit using a spring. Such a mechanism was more reliable and durable than pieces of wire or needles. Models that were closed by default were also developed, in addition to the original design that was open by default.
A typical relay from the late XNUMXth century. Spring T keeps armature B from touching contact C. When electromagnet M is activated, it overcomes the spring and closes the circuit between wire W and contact C.
In the early years of the telegraph, relays were rarely used as amplifiers or "renewers" because one circuit could be stretched for 150 km. But they were very useful for connecting low-current long lines with local high-voltage lines that could be used to power other machines, such as a Morse recorder.
Dozens of US patents from the second part of the 4th century describe new types of relays and their new applications. The differential relay, which divided the coil so that the electromagnetic effect was canceled in one direction and amplified in the other, allowed the use of duplex telegraph communication: two signals traveling in opposite directions on one wire. Thomas Edison used a polarized (or polarized) relay to create a quadruplex capable of sending XNUMX signals simultaneously on a single wire: two in each direction. In a polarized relay, the armature itself was a permanent magnet that responded to the direction of the current rather than the strength. Thanks to permanent magnets, it was possible to make relays with switching contacts, which, after switching, remained open or closed.
Polarized Relay
Relays, in addition to the telegraph, began to be used in railway signaling systems. With the advent of power transmission networks, relays began to be used in these systems, especially as protection devices.
But even these extended and complex networks did not require more from the relays than they were able to provide. The telegraph and the railway entered every city, but not every building. They had tens of thousands of endpoints, but not millions. The transmission systems didn't care where they endedβthey simply provided current to the local circuit, and each home and business could take as much of it as they wanted.
Telephony was a completely different matter. Telephones needed to communicate point-to-point, from any home or office to any other, and so they needed control loops on an unprecedented scale. The human voice oscillating through the wires was a signal rich but weak. Therefore, long-distance telephone communications needed better quality amplifiers. It turned out that the switches can work with such amplifiers. Now, more than any other system, telephone networks have driven the evolution of switches.
What to read
β’ James B. Calvert, "The Electromagnetic Telegraph"
β’ Franklin Leonard Pope, Modern Practice of the Electric Telegraph (1891)
Source: habr.com