But finally, the relay

In the last part of the story, we learned how an American scientist and teacher, Joseph Henry, traveled through Europe for the first time. Visiting London, he specifically visited the man deeply respected by him, the mathematician Charles Babbage . Together with Henry was his friend, Alexander Bach, and his new acquaintance, also an experimenter in the field of telegraph, Charles Wheatstone . Babbage told the guests that he was soon going to demonstrate his counting machine to a Member of Parliament, but with even greater pleasure he shared with them the idea of ​​his new car, “which will significantly surpass the first one.” Henry recorded general information about this plan in his diary:



This machine is divided into two parts, one of which Mr. B. calls the repository, and the second - the mill. The store is filled with wheels with numbers drawn on them. Periodically, the levers pull them out and move them in the mill, where the necessary manipulations take place. At the end, this machine can tabulate any algebraic formula in the table.



The historian cannot help but feel the chill running down his back from such random intersections in human lives. Here two threads of the history of computers have crossed, one of which was nearing completion, and the other was just beginning.



Indeed, although the Babbage machine is often presented as the beginning of the history of modern universal computers, the connection between them is rather weak. His car (which he never built) was the culmination of the dream of mechanical computing. This dream, first voiced by Leibniz, was inspired by ever more complex watch mechanisms created by craftsmen from the end of the Middle Ages. But no general-purpose computer was built on pure mechanics — this task is too complex.



But the electromagnetic relay, conceived by Henry and others, can be quite easily incorporated into computational circuits, the complexity of which without it seems unimaginable. However, until that moment there were still decades left, and such a development could not have been foreseen by Henry and his contemporaries. It became the progenitor of countless transistors that made today's digital world so intertwined with our modern life possible. The relays were filled with the insides of early programmable computers, which ruled for a short time until their purely electronic relatives replaced them.



Relays were invented several times independently of each other in the 1830s. Its goals were diverse (five of its inventors came up with at least three applications) - as well as examples of use. But it is convenient to think about it as a dual-purpose device. It can be used as a switch that controls another electrical device (including, importantly, another relay), or as an amplifier that turns a weak signal into a strong one.

Switch

Joseph Henry combined in one person a deep knowledge of natural philosophy, mechanics and interest in the problem of mechanical telegraph. In the 1830s, perhaps, only Wheatstone had such a set of qualities. By 1831, he had built a 2.5 km long contour capable of activating a bell using the most powerful magnet that existed. Perhaps, if he continued to work so actively on the telegraph, and showed the same perseverance, as Morse showed, then it was his name that would be written in textbooks.



But Henry, a teacher from the Academy at Albany, and then at the College of New Jersey (now Princeton University) built and improved electrical devices for research, teaching, and scientific demonstrations. He was not interested in turning the pedagogical tool into a messaging system.



Around 1835, he invented a particularly ingenious demonstration using two contours. Recall that Henry discovered two measurements of electricity - 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. The closure of the intensive circuit caused the magnet to lift the wire, and this opened the switch and the quantitative circuit. The quantitative electromagnet then suddenly dropped 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 conversion of electrical energy into mechanical energy, as well as how a small force can control a large one. The easy immersion of the wire in acid to close the circuit led to a small movement of the small switch, which resulted in a catastrophe in the form of a metal falling in an amount sufficient to crush someone who was stupid enough to stand under it. For Henry, Relay was a tool for demonstrating scientific principles. It was an electric lever.







Henry was probably the first to unite in this way two circuits — in order to control the other using electromagnetism of one circuit. The second place, as far as we know, belongs to William Cook and Charles Wheatstone, although they were set by completely different goals.



In March 1836, shortly after attending the telegraph demonstration in Heidelberg, which used a galvanic needle to transmit signals, Cook was inspired by a music box. Cook believed that the use of letters in letters on a real telegraph would require several needles, and they would require several circuits. Cook, on the other hand, wanted the electromagnet to activate a mechanism that can be arbitrarily difficult to demonstrate the desired letter.



He conceived a car, resembling a music box, with a keg surrounded by many pins. On one side of the barrel should be located a dial with letters. At each end of the telegraph line should be such a box. The cocked spring should make the barrel rotate, but most of the time it will be fixed by a stopper. When the telegraph key is pressed, the circuit closes, which activates the electromagnets that open both the locks and both machines rotate. When the desired letter is shown on the scale, the key is released, the locks snap into place and stop the movement of the kegs. Cook, not knowing himself, recreated the chronometric model of Ronald's telegraph, invented two decades ago, and the early experiments of the Chapp brothers with the telegraph (only those used to synchronize the scales sound, not electricity).



Cook realized that the same mechanism could help solve a long-standing telegraph problem - 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. The loop closure would drag the stop and the bell would ring.



In March 1837, Cook began working together with Wheatstone on the telegraph, and around this time they began to think about the need for a second circuit. Instead of setting up an independent circuit for the alert signal (and pulling extra kilometers of wires), wouldn't it have been easier to use the main loop to control the signal?







By that time, Cook 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 by an electromagnet, its tail would close the second circuit. This circuit would trigger a signal. After a certain interval, during which the recipient of the message could have time to wake up, turn off the signal and prepare a pencil and paper, the needle could already be used to transmit the message as usual.



For two years, on two continents, twice, with two different goals, people realized that the electromagnet could be used as a switch controlling another circuit. But it was possible to imagine a completely different way of interaction between the two contours.

Amplifier

By the fall of 1837, Samuel Morse was convinced that his idea of ​​an electric telegraph could be made to work. Using Henry's intense battery and magnet, he sent messages a distance of half a kilometer. But in order to prove to Congress the possibility of transmitting messages over his telegraph across the whole continent, he needed much more. It was clear that regardless of the power of the batteries, at some point the circuit would become too long to transmit a legible signal to the other end. But Morse realized that, despite a strong drop in power with distance, the 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 as many times as necessary and cover distances of any length. Therefore, these intermediate magnets were called “relay” - as postal stations for changing horses. They received an electrical message from a weakening partner and carried it on with a new force.



It is impossible to establish whether this idea was inspired by the works of Henry, but Morse was definitely the first to use the 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, Edward Davey , a London-based pharmacist, came up with a similar idea. He probably became interested in the telegraph around 1835. By the beginning of 1837, he regularly conducted experiments with a 1.5-kilometer contour in Regent's Park in north-west London.



Soon after the meeting of 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 with increasing wire length. As he wrote many years later:



Then I thought that even the slightest movement of the needle on the thickness of the hair would be enough to bring into contact two metal surfaces, closing the new circuit, depending on the local battery; and so you can repeat forever.



Davey called this idea of ​​turning a weak electrical signal into a strong "electrical renewer." But he failed to realize this or any other idea about the telegraph. He received a patent for a telegraph in 1838, regardless of Cook and Wheatstone. But in 1839 he sailed to Australia, fleeing from an unhappy marriage, and left the field to competitors. Their telegraph company bought this patent several 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 keep the history of telegraph, telephone, electric light, bathe their creators in the warm rays of their approval. But these systems could only appear due to the combination, recombination and modification of existing elements that quietly grew in the shade.



Relay - one of these elements. It quickly evolved and gained diversity as telegraph networks began to grow rapidly in the 1840s and 1850s. Over the next century, it appeared in various electrical systems. The earliest modification was the use of a rigid metal anchor, as on a telegraph signal, to close the loop. After the electromagnet was turned off, the anchor was disconnected from the circuit by means of a spring. Such a mechanism was more reliable and durable than pieces of wires or needles. The default models, in addition to the default open design, were also developed.





Typical relay of the late XIX century. The spring T holds the armature B from contact with the contact C. When the electromagnet M is activated, it overcomes the spring and closes the circuit between the wire W and the contact C.



In the early years of the telegraph, relays were rarely used as amplifiers or “renewers”, since one circuit could be stretched 150 km. But they were very useful for combining low-voltage long lines with local high-voltage lines that could be used to power other machines, for example, Morse recorder.



Dozens of patents in the USA of the second part of the 19th century describe new types of relays and their new uses. The differential relay, which divided the coil so that the electromagnetic effect was compensated in one direction and amplified in the other, allowed the use of duplex telegraph communication: two signals going in opposite directions along a single wire. Thomas Edison used a polarized (or polar) relay to create a quadruplex capable of sending 4 signals simultaneously over one wire: two in each direction. In a polarized relay, the anchor itself was a permanent magnet that reacted to the direction of the current, and not to the force. Thanks to permanent magnets, it was possible to make relays with switching contacts, which, after switching, remained open or closed.





Polarized relay



The relay, in addition to the telegraph, began to be used in the signal systems of railways. With the advent of power transmission networks, relays were used in these systems, especially as protection devices.



But even these long and complex networks did not require more relays than they were able to provide. The telegraph and the railway went into any city, but not in any building. They had tens of thousands of end points, but not millions. The power transmission systems did not care where they ended - they simply gave a current to the local circuit, and each house and enterprise could take it as much as needed.



Telephony was a completely different matter. Telephones had to create a connection from point to point, from any of the houses and offices to any other, and therefore they needed control circuits of an unprecedented scale. The human voice, coming in the form of oscillations through wires, was a signal rich but weak. Therefore, long-distance telephony needed better quality amplifiers. It turned out that the switches can work with such amplifiers. Now the telephone network more than any other systems have controlled the evolution of switches.

What to read

• James B. Calvert, “The Electromagnetic Telegraph“

• Franklin Leonard Pope, "Modern Practice of the Electric Telegraph" (1891)