Relay history: electronic era

The last time we saw the first generation of digital computers were built based on the first generation of automatic electrical switches - electromagnetic relays. But by the time these computers were created, behind the scenes another digital switch was waiting for its release. The relay was an electromagnetic device (using electricity to control a mechanical switch), and a new class of digital switches was electronic — based on new knowledge about an electron that appeared in the early 20th century. This science indicated that the carrier of the electric force was not current, not wave, not field, but a solid particle.

The device that spawned the era of electronics, based on this new physics, became known as the "electron tube" [in the United States - vacuum tube, or "vacuum tube"]. Two people are involved in the history of its creation: the Englishman Ambrose Fleming and the American Lee de Forest . In fact, the origin of electronics is more complex, it is twisted from a multitude of threads crossing Europe and the Atlantic, and stretching back into the past, right up to the early experiments with Leiden banks in the middle of the 18th century.

But within the framework of our presentation it will be convenient to cover (pun intended) this story, starting with Thomas Edison. In the 1880s, Edison made an interesting discovery while working on electric lighting - this discovery sets the stage for our story. Hence the further development of electronic lamps, which was required for two technological systems: a new form of wireless messaging and ever-expanding telephone networks.

Prologue: Edison

Edison is usually considered the inventor of the light bulb. This at the same time does him too much and too little honor. Too much, since not one Edison came up with a glowing lamp. In addition to the crowd of inventors who preceded him, whose creations did not reach commercial applications, we can mention Joseph Swan and Charles Stern from Britain and American William Sawyer, who brought light bulbs to the market simultaneously with Edison. [The honor of the invention also belongs to the Russian inventor Lodygin Alexander Nikolaevich . Lodygin was the first who guessed to deflate air from a glass tube bulb, and then suggested to make a filament not of coal or charred fibers, but of refractory tungsten / approx. trans.]. All lamps consisted of a sealed glass bulb, inside which resistive filament was located. When the lamp was turned on in the circuit, the heat generated by the resistance of the thread to the current made it glow. Air was pumped out of the flask so that the thread would not catch fire. Electric light was already known in large cities as arc lamps used to illuminate large public places. All these inventors were looking for a way to reduce the amount of light, taking a bright piece of a burning arc, small enough to use it in homes to replace gas lamps, and make the light source safer, cleaner and brighter.

And what Edison really did — or rather, created his industrial laboratory — is not just the creation of a light source. They built a whole electrical system for lighting homes — generators, current wires, transformers, and so on. Of all this, the bulb was only the most obvious and visible component. The presence of Edison’s name in his electricity generating companies was not a simple kneeling in front of a great inventor, as is the case with Bell Telephone. Edison showed himself not only as an inventor, but also as a system architect. His laboratory continued to work on improving the various components of electric lighting, even after their early success.


Copy of early edison lamps

During the research somewhere in 1883, Edison (and possibly one of his employees) decided to enclose a metal plate inside the luminous lamp along with the thread. The reasons for this act are unclear. Perhaps it was an attempt to eliminate the darkening of the lamp - the inside of the glass bulb eventually accumulated a mysterious dark substance. The engineer apparently hoped that these black particles would be attracted to the live plate. To his surprise, he discovered that when the plate was included in the circuit along with the positive end of the thread, the value of the current flowing through the thread was directly proportional to the intensity of the glow of the thread. When the plate was connected to the negative end of the filament, nothing like this was observed.

Edison decided that this effect, later called the Edison effect orthermal emission can be used to measure or even control “electromotive force”, or voltage, in an electrical system. By virtue of habit, he filed a patent application for this "electric indicator", and then returned to more important tasks.

Without wires

Fast forward to 20 years in the future, in 1904 minutes. At this time in England, John Ambrose Fleming worked on the instructions of the Marconi Company to improve the radio wave receiver.

It is important to understand what was and what was not the radio at that time, both in terms of the instrument and in terms of practice. Radio was not even called “radio” at that time, it was called “wireless”, wireless. The term "radio" began to prevail only in the 1910s. Specifically meant wireless telegraph - a system of transmitting signals in the form of points and a dash from the sender to the receiver. Its main use was the connection between ships and port services, and in this sense they were interested in the marine departments of the whole world.

Some inventors of the time, notably Reginald Fessenden, experimented with the idea of ​​a radiotelephone - transmitting voice messages over the air in the form of a continuous wave. But the broadcast in the modern sense arose only 15 years later: the transfer of news, stories, music and other programs to receive a wide audience. Prior to that, the omnidirectional nature of radio signals was considered as a problem that needs to be solved, and not as a feature that can be used.

The radio equipment that existed at that time was well adapted for working with Morse code and bad for everything else. Transmitters created Hertz's waves, sending a spark through a gap in the circuit. Therefore, the signal was accompanied by a crash of statics.

The receivers recognized this signal through a coherer: metal filings in a glass tube, knocked under the influence of radio waves into a continuous mass, and thus closed the circuit. Then it was necessary to knock on the glass so that the sawdust would disintegrate and the receiver was ready for the next signal — at first it was done manually, but soon automatic devices appeared for that.

In 1905, crystal detectors , also known as catlike whiskers , began to appear . It turned out that just touching the wire of a certain crystal, for example, silicon, iron pyrites or galena, it was possible to snatch a radio signal from the air. The resulting receivers were cheap, compact and accessible to everyone. They stimulated the development of amateur radio, especially among young people. The sudden surge in the employment of airwaves, which arose as a result of this, led to problems due to the fact that the radio broadcast was shared among all users. Innocent lovers' conversations could accidentally intersect with the talks of the morflot, and some hooligans even managed to give false orders and send signals for help. The state inevitably had to intervene. As Ambrose Fleming himself wrote, the appearance of crystal detectors

Immediately led to a surge of irresponsible radio telegraphy due to the tricks of an innumerable number of amateur electricians and students, which required tough intervention by national and international authorities to keep what was happening in a reasonable and safe manner.

Of the unusual electrical properties of these crystals, the third generation of digital switches will appear in its time, followed by relays and lamps — the switches that dominate our world. But everything has its time. We have described the scene, now we will return all attention to the actor who has just appeared in the spotlight of the spotlights: Ambrose Fleming, England, 1904.

Valve

In 1904, Fleming was a professor of electrical engineering at University College London, and a consultant for the Marconi Company. Initially, the company hired him to obtain an expert assessment for the construction of a power plant, but then he took up the task of improving the receiver.


Fleming in 1890.

Everyone knew that the coherer was a poor receiver in terms of sensitivity, and the magnetic detector developed at Macroni was not particularly better. To find a replacement for him, Fleming first decided to build a sensitive circuit to detect Hertz waves. Such a device, even without becoming a detector in itself, will be useful in future studies.

To do this, he had to think of a way to constantly measure the strength of the current generated by the incoming waves, instead of using a discrete coherer (he showed only the states on - where the sawdust stuck together, or off). But the known devices for measuring current strength - galvanometers - required a constant, that is, unidirectional current to operate. The alternating current, excited by radio waves, changed direction so quickly that no measurement would have been possible.

Fleming remembered that several interesting things were gathering dust in his closet — Edison indicator lights. In the 1880s, he was a consultant for the Edison Electric Lighting Company in London, and worked on the problem of blackening lamps. At that time, he received several copies of the indicator, perhaps from William Price, the chief electrical engineer of the British Postal Service, who had just returned from an electric exhibition in Philadelphia. At that time, outside the United States, postal control over telegraph and telephone was common, so they were centers of electrical expertise.

Later, in the 1890s, Fleming himself studied the Edison effect using lamps obtained from Pris. He showed that the effect was that the current flowed in one direction: a negative electric potential could flow from a hot filament to a cold electrode, but not vice versa. But only in 1904, when he faced the task of detecting radio waves, he realized that this fact can be used in practice. The Edison indicator will allow only one-way AC pulses to bridge the gap between the thread and the plate, which will give a constant and unidirectional flow.

Fleming took one lamp, connected it in series with the galvanometer, and switched on the spark transmitter. Voila - the mirror turned, and the beam of light shifted on the scale. It worked. He could accurately measure the incoming radio signal.


Prototypes of the Fleming valve. The anode is located in the middle of the thread loop (hot cathode).

Fleming called his invention "valve", because it passed electricity only in one direction. Speaking in more general electrical language, it was a rectifier - a method of converting alternating current into direct current. Then it was called a diode, because it had two electrodes — a hot cathode (filament) that emitted electricity, and a cold anode (plate) that received it. Fleming introduced several improvements to the design, but in essence the device did not differ from the indicator lamp made by Edison. Its transition to a new quality occurred as a result of a change in the way of thinking - we have already seen this phenomenon many times. The change occurred in the world of ideas in Fleming's head, and not in the world of things outside of it.

The valve of Fleming itself was useful. It was the best field device for measuring radio signals, and a good detector in itself. But he did not shake the world. The explosive growth of electronics began only after Lee de Forest added a third electrode and turned the valve into a relay.

Listening

Lee de Forest had an unusual upbringing for a Yale student. His father, Rev. Henry de Forest, was a civil war veteran from New York, pastor of the congregational church , and firmly believed that, as a preacher, he should spread the divine light of knowledge and justice. In obedience to the call of duty, he accepted the invitation to become president of Talladeg College in Alabama. The college was founded after the Civil War by the American Missionary Association, based in New York. It was intended to train and instruct local black people. There, Lee felt himself between a hammer and an anvil - local negros humiliated him for being naive and cowardly, and local whites for being a Yankee .

And yet, the young men de Forest developed a strong self-confidence. He discovered in himself a penchant for mechanics and inventions - his scale model of a locomotive became a local miracle. Even as a teenager, while studying at Talladeg, he decided to devote his life to inventions. Then, being a young man and living in the city of New Haven, the pastor’s son threw off his last religious beliefs. They gradually left due to acquaintance with Darwinism, and then they blew away like the wind after the untimely death of his father. But the feeling of having a destination did not leave de Forrest - he considered himself a genius and sought to become the second Nikola Tesla, a rich, famous and mysterious wizard of the electricity era. His classmates from Yale University considered him a complacent windbag. His maybe


de Forest, ca.1900

After graduating from Yale University in 1899, de Forest chose the development of the art of transmitting wireless signals as a path to wealth and fame. In the following decades, he stormed this path with great determination and confidence, and without any hesitation. It all started with the joint work of de Forest and his partner Ed Smythe in Chicago. Smythe kept their enterprise afloat with regular payments, and together they developed their own radio wave detector, consisting of two metal plates connected by glue, which de Forest called “paste” [goo]. But de Forest could not wait long awards for his genius. He got rid of Smythe and cooperated with a questionable financier from New York named Abraham White [ironically changing his name from the one given to him at birth, Schwartz, to hide his dark deeds. White / White - (eng.) White, Schwartz / Schwarz - (German) black / approx. trans. ], opening the De Forest Wireless Telegraph Company.

The very activity of the company was secondary for both our heroes. White used the ignorance of people to fill their pockets. He lured millions from investors, struggling to keep up with the expected radio boom. And de Forest, thanks to the abundant flow of funds of these “suckers”, has concentrated on proving his genius through the development of a new American system of wireless transmission of information (in contrast to the European one, developed by Marconi and others).

Unfortunately for the American system, the detector de Forrest did not work particularly well. For a while, he solved this problem by borrowing Reginald Fessenden's patented design for a detector called “liquid barette” - two platinum wires immersed in a bath of sulfuric acid. Fessenden filed a lawsuit because of patent infringement - and he would obviously have won this lawsuit. De Forest could not calm down until he would come up with a new detector that belonged only to him. In the autumn of 1906, he announced the creation of such a detector. At two different meetings at the American Institute of Electrical Engineering, de Forest described his new wireless detector, which he called “Audion”. But its real origin is in doubt.

For a while, De Forest’s attempts to build a new detector revolved around the passage of current through a flame.Bunsen burner , which, in his opinion, could be an asymmetric conductor. The idea, apparently, was not crowned with success. At some point in 1905, he learned about the valve of Fleming. De Forest hammered that this valve and its device based on the burner were basically no different - if you replace the hot thread with a flame and cover it with a glass bulb to limit the gas, you will get the same valve. He developed a series of patents that repeated the history of inventions that preceded the Fleming valve with gas flame-based detectors. He obviously wanted to give himself priority in the invention, bypassing the Fleming patent, since the work with the Bunsen burner preceded the work of Fleming (they had been going since 1900).

It is impossible to say whether it was self-deception or fraud, but the result was De Forest's patent of August 1906 for "a devastated glass vessel containing two separate electrodes, between which there is a gaseous medium, which, with sufficient heating, becomes a conductor and forms a sensitive element." The equipment and the operation of the device belong to Fleming, and the explanation of his work is to de Forest. De Forest, as a result, lost the patent dispute, although it took ten years.

The impatient reader may already begin to wonder why we spend so much time on this person, whose self-proclaimed genius was to give out other people's ideas for his own? The reason lies in the transformations that Audion underwent in the last few months of 1906.

By then, de Forest had no work. White and his partners escaped responsibility for the Fessenden lawsuit by creating a new company, United Wireless, and lending American De Forest assets for $ 1. De Foresta was expelled with $ 1,000 compensation and several useless patents on his hands, including the Audion patent. Accustomed to a wasteful way of life, he faced serious financial difficulties and desperately tried to turn the Audion into a great success.

In order to understand what happened next, it is important to know that de Forest believed that he invented the relay - in contrast with the Fleming rectifier. He made his audio by connecting the battery to the cold plate of the valve, and believed that the signal in the antenna circuit (connected to the hot thread) modulated a more powerful current in the battery circuit. He was wrong: it was not two schemes, the battery simply shifted the signal from the antenna, and not amplified it.

But this error became critical, since it led de Forest to experiment with the third electrode in the flask, which should have further separated the two circuits of this “relay”. At first he added the second cold electrode next to the first one, but then, perhaps under the influence of the control mechanisms used by physicists to redirect the rays in the electron-beam devices, he moved the electrode to the space between the filament and the primary plate. He decided that such a situation could interrupt the flow of electricity, and changed the shape of the third electrode from the plate to a wavy wire, resembling a gridiron, and called it a “grid”.


Triode Audion 1908. The thread (torn) on the left is the cathode, the wavy wire is the grid, the rounded metal plate is the anode. He still has a thread like an ordinary light bulb.

And it really was a relay. A weak current (such as that obtained from a radio antenna) applied to the grid could control a much stronger current between the filament and the plate, pushing away the charged particles trying to cross between them. This detector worked much better than the valve, since it not only straightened out, but also amplified the radio signal. And, like the valve (and in contrast to the coherer), it could issue a constant signal, which made it possible to create not only radio, but also a radiotelephone (and later - voice and music transmission).

In practice, he did not work very well. The de Foresta audions were fastidious, they quickly burned, there was a lack of consistent quality in their production, and they were ineffective as amplifiers. In order for a particular Audion to work properly, it was necessary to adjust the electrical parameters of the circuit for it.

However, de Forest believed in his invention. For his advertising, he organized a new company, De Forest Radio Telephone Company, but sales were scanty. The biggest success was the sale of equipment to the fleet for intra-fleet telephony during the circumnavigation of the Great White Fleet.“However, the fleet commander, having no time to force de Forest’s transmitters and receivers to work and train the team to use them, ordered them to be packed and left in storage. Moreover, the new De Forest company, led by a follower of Abraham White, was no more respectable than the previous one. in addition to its failure, he soon fell under the accusation of fraud.

in five years, Audion has achieved nothing. And the phone again play a key role in the development of digital relays, this time to save a promising but untested th technology is on the brink of oblivion.

And again the phone

The long-distance communications network was AT & T’s central nervous system. She tied together many local companies and provided a key competitive advantage after the expiration of Bell's patents. By joining the AT & T network, a new customer could, in theory, get through to all the other subscribers that were thousands of kilometers away from him — although in reality long distance calls were rarely made. Also, the network was the material basis for the comprehensive ideology of the One Policy, One System, Universal Service company.

But with the beginning of the second decade of the twentieth century, this network reached the physical maximum. The further the telephone wires went, the weaker and noisier the signal passing through them became, and as a result the speech became almost indistinguishable. Because of this, in the US there were actually two AT & T networks separated by a continental ridge.

For the eastern network, the peg was New York, and mechanical repeaters and Pupin coils- a leash that determined how far a human voice could go. But these technologies were not omnipotent. The coils changed the electrical properties of the telephone circuit, reducing the attenuation of voice frequencies - but they could only reduce it, not eliminate it. Mechanical repeaters (just a telephone speaker connected to a booster microphone) added noise with each repetition. The 1911 line from New York to Denver brought this leash to its maximum length. There was no talk of extending the network to the whole continent. However, in 1909, John Carty, the chief engineer of AT & T, publicly promised to do just that. He promised to do it in five years - by the time the Panama-Pacific International Exhibition began in San Francisco in 1915.

The first who managed to make such an enterprise possible with the help of a new telephone booster was not an American, but a heir to a rich Viennese family interested in science. Being young, Robert von Lieben, with the help of his parents, bought a telephone production company and set out to make an amplifier for telephone conversations. By 1906, he had made relays based on cathode-ray tubes, which by that time were widely used in physical experiments (and later became the basis for the dominant screen technology in the 20th century). The weak incoming signal was controlled by an electromagnet, a bending beam that modulated a stronger current in the main circuit.

By 1910, von Lieben and his colleagues, Eugene Reise and Sigmund Strauss, learned about Audion de Forest and replaced the magnet in the tube with a grid that controlled the cathode rays - this design was the most effective and surpassed all the developments made at that time in the USA. The German telephone network soon adopted the von Lieben amplifier. In 1914, thanks to her, a nervous phone call from the commander of the East Prussian Army to the German headquarters, located 1000 kilometers away, was held in Koblenz. This forced the chief of staff to send generals Hindenberg and Ludendorff to the east, to eternal glory and with grave consequences. Similar amplifiers later connected the German headquarters with field armies in the south and east, up to Macedonia and Romania.


A copy of the enhanced von Lieben cathode ray relay. The cathode is at the bottom, the anode is the coil at the top, and the grid is a round metal foil in the middle.

However, language and geographical barriers, as well as war, led to the fact that such a design did not reach the United States, and soon it was already ahead of other events.

Meanwhile, de Forest left the curving Radio Telephone Company in 1911 and fled to California. There he got a job at the Federal Telegraph Company in Palo Alto, founded by Stanford graduate Cyril Elvel. Nominally, de Forest was supposed to work on an amplifier that raises the volume of the output signal of the federal radio. In fact, he, Herbert van Ettan (an experienced telephone engineer) and Charles Logwood (developer of the receiver) set about creating a telephone amplifier in order to get a prize from AT & T, which was rumored to be $ 1 million.

For this de Forest got Audion from the mezzanine, and by 1912 they and their colleagues already had a device ready for demonstration in a telephone company. It consisted of several connected audions, which created amplification in several stages, and several other auxiliary components. The device, in principle, worked - it could amplify the signal enough for you to hear how a handkerchief falls or how a pocket watch ticks. But only at currents and voltages too small to be useful in telephony. With increasing current, the audions began to emit a blue glow, and the signal turned into noise. But the phone people were interested enough to give the device to their engineers and see what they could do with it. It so happened that one of them, a young physicist Harold Arnold, knew for sure

It's time to discuss how the valve and the Audion worked. The key insights needed to explain their work appeared in the Cavendish Laboratory in Cambridge, the intellectual center of new electronic physics. In 1899 JJ Thomson showed there in experiments with cathode ray tubes that a particle of mass, which later became known as an electron, transfers current from the cathode to the anode. Over the next few years, Owen Richardson, a colleague of Thomson, developed this assumption into the mathematical theory of thermionic emission.

Ambrose Fleming, an engineer who worked a short train ride from Cambridge, was familiar with this work. It was clear to him that his valve was working due to the thermionic emission of electrons from a heated filament that crossed the vacuum gap to the cold anode. But the vacuum in the indicator lamp was not deep - it was not necessary for an ordinary light bulb. It was enough to pump out so much oxygen so that the thread would not catch fire. Fleming realized that in order for the valve to work best it should be emptied as carefully as possible so that the remaining gas does not interfere with the flow of electrons.

De Forest did not understand this. Since he came to the valve and Audion through experiments with the Bunsen burner, his belief was the opposite - that the hot ionized gas was the working body of the device, and that its complete removal would lead to the termination of work. That is why Audion worked so unstable and unsatisfactory as a radio receiver, and therefore it radiated blue light.

Arnold at AT & T found himself in an ideal situation to correct de Forest's mistake. He was a physicist who studied with Robert Milliken at the University of Chicago and was hired specifically to apply his knowledge of new electronic physics to the task of building a telephone network from coast to coast. He knew that the Audiion's lamp would work best in an almost perfect vacuum, he knew that the newest pumps could achieve such a vacuum, he knew that a new type of filament coated with oxide, together with an increased plate and grid, would also be able to increase the flow of electrons. In short, he turned the Audion into an electronic lamp, the wonder-worker of the electronic era.

AT & T has a powerful amplifier necessary for the construction of a transcontinental line - there was only the right to use it. Representatives of the company behaved incredulously in negotiations with de Forest, but started a separate conversation through an outside lawyer who managed to acquire the rights to use Audi as a telephone amplifier for $ 50,000 (about $ 1.25 million in 2017 dollars). The New York-San Francisco line opened just in time, but more as a triumph of technical virtuosity and corporate advertising than as a means of communication. The cost of conversations was so cosmic that almost no one could use it.

Electronic era

This electronic lamp has become the root of a completely new tree of electronic components. Like the relay, the electronic lamp was constantly expanding the possibilities of its application when engineers found new ways to adjust its device to solve specific problems. The growth of the tribe "-odes" did not end with diodes and triodes. It continued with the tetrode , which added an additional grid that supported the gain with the growth of elements in the circuit. Follow the pentode , pentagrid converter , and even OCTODE. Thiratrons filled with mercury vapor appeared, glowing with an ominous blue light. Miniature lamps the size of a pinky foot or even an acorn. Lamps with indirectly heated cathodes, in which the buzzing of an alternating current source did not disturb the signal. The Saga of the Vacuum Tube, which describes the growth of the lamp industry until 1930, lists more than 1,000 different models by their index — although many of them were illegal copies of unreliable brands: Altron, Perfecttron, Supertron, Voltron , etc.



More important than the variety of forms was the variety of applications of the electronic lamp. Regenerative circuits turned the triode into a transmitter — creating smooth and constant sine waves, without noisy sparks, capable of perfectly transmitting sound. With coherers and sparks in 1901, Marconi could barely transmit a small excerpt of Morse code through a narrow part of the Atlantic. In 1915, using an electronic lamp as a transmitter and receiver, AT & T could transmit a human voice from Arlington, Virginia to Honolulu — twice the distance. By the 1920s, they combined telephony over long distances with high-quality audio broadcasting and created the first radio networks. Thus, soon the whole nation could listen to the same voice on the radio, be it Roosevelt or Hitler.

Moreover, the ability to create transmitters tuned to an accurate and stable frequency allowed telecommunications engineers to realize the long-held dream of a frequency multiplex that attracted Alexander Bell, Edison and the rest forty years ago. By 1923, AT & T had a ten-channel voice line from New York to Pittsburgh. The ability to transfer multiple votes over a single copper wire radically reduced the cost of long-distance calls, which, because of the high cost, was always available only to the richest people and businesses. After seeing what electronic lamps are capable of, AT & T sent their lawyers to buy additional rights from de Forest in order to secure the right to use Audiion in all available fields of application. In sum, they paid him $ 390,000, which in today's money is equivalent to about $ 7.5 million.

Why then, with such versatility, electronic lamps did not dominate the first generation of computers as they dominated the radio and other telecommunications equipment? Obviously, the triode could be a digital switch just like a relay. It is so obvious that de Forest even thought that he created the relay before he actually created it. And the triode was much more responsive than the traditional electromechanical relay, since it did not need to physically move the anchor. A typical switching relay required a few milliseconds, and the change in flow from the cathode to the anode due to a change in electrical potential on the grid was almost instantaneous.

But the lamps had a clear disadvantage in front of the relay: their tendency, by analogy with their predecessors, light bulbs for lighting, burn out. The lifetime of the original Audion de Forest was so short - about 100 hours - that he had in his lamp a spare thread that had to be connected after the first burned out. It was very bad, but even after that, it was impossible to expect more than a few thousand hours from the best quality lamps. For computers with thousands of bulbs, and computing that lasted for hours, this was a serious problem.

And the relay, on the contrary, according to George Stibits, was “fantastically reliable.” So much so that he claimed that

If the set of U-shaped relays would start its work in the first year of our era and switch the contact once a second, it would still work until now. The first failure in contact could have been expected not earlier than in a thousand years, somewhere in the 3000th year.

Moreover, there was no experience of using large electronic circuits comparable to the electromechanical circuits of telephone engineers. Radio receivers and other equipment could contain 5-10 lamps, but not hundreds of thousands. No one knew whether it would be possible to make a computer work with 5,000 lamps. By choosing relays instead of lamps, computer developers made a safe and conservative choice.

In the next part we will see how and why these doubts were overcome.