The Vacuum Tube’s Forgotten Rival
Share
FEATURE
HISTORY OF TECHNOLOGY
THE VACUUM TUBE’S FORGOTTEN RIVAL
Magnetic amplifiers, the alt-tech of the Third Reich, lasted into the Internet era
KEN SHIRRIFF
27 MAR 202212 MIN READ
A photo of an older woman in front of an early generation computer.
Magnetic amplifiers were used in the Univac Solid State, shown here being operated in 1961 by pioneering computer scientist Grace Hopper. COMPUTER HISTORY MUSEUM
DURING THE SECOND World War, the German military developed what were at the time some very sophisticated technologies, including the V-2 rockets it used to rain destruction on London. Yet the V-2, along with much other German military hardware, depended on an obscure and seemingly antiquated component you’ve probably never heard of, something called the magnetic amplifier or mag amp.

In the United States, mag amps had long been considered obsolete—“too slow, cumbersome, and inefficient to be taken seriously,” according to one source. So U.S. military-electronics experts of that era were baffled by the extensive German use of this device, which they first learned about from interrogating German prisoners of war. What did the Third Reich’s engineers know that had eluded the Americans?

After the war, U.S. intelligence officers scoured Germany for useful scientific and technical information. Four hundred experts sifted through billions of pages of documents and shipped 3.5 million microfilmed pages back to the United States, along with almost 200 tonnes of German industrial equipment. Among this mass of information and equipment was the secret of Germany’s magnetic amplifiers: metal alloys that made these devices compact, efficient, and reliable.

U.S. engineers were soon able to reproduce those alloys. As a result, the 1950s and ’60s saw a renaissance for magnetic amplifiers, during which they were used extensively in the military, aerospace, and other industries. They even appeared in some early solid-state digital computers before giving way entirely to transistors. Nowadays, that history is all but forgotten. So here I’ll offer the little-known story of the mag amp.

An amplifier, by definition, is a device that allows a small signal to control a larger one. An old-fashioned triode vacuum tube does that using a voltage applied to its grid electrode. A modern field-effect transistor does it using a voltage applied to its gate. The mag amp exercises control electromagnetically.

A photo of a rocket on a launcher with trees in the background. A photo of a man sitting at an early computer next to a typewriter.A photo of two men sitting at a terminal in front of an early computer.Magnetic amplifiers were used for a variety of applications, including in the infamous V-2 rockets [top] that the Germany military employed during the Second World War and in the Magstec computer [middle], completed in 1956. The British Elliot 803 computer of 1961 [bottom] used related core-transistor logic. FROM TOP: FOX PHOTOS/GETTY IMAGES; REMINGTON RAND UNIVAC; SMITH ARCHIVE/ALAMY
To understand how it works, first consider a simple inductor, say, a wire coiled around an iron rod. Such an inductor will tend to block the flow of alternating current through the wire. That’s because when current flows, the coil creates an alternating magnetic field, concentrated in the iron rod. And that varying magnetic field induces voltages in the wire that act to oppose the alternating current that created the field in the first place.

If such an inductor carries a lot of current, the rod can reach a state called saturation, whereby the iron cannot become any more magnetized than it already is. When that happens, current passes through the coil virtually unimpeded. Saturation is usually undesirable, but the mag amp exploits this effect.

Physically, a magnetic amplifier is built around a metallic core of material that can easily be saturated, typically a ring or square loop with a wire wrapped around it. A second wire also wrapped around the core forms a control winding. The control winding includes many turns of wire, so by passing a relatively small direct current through it, the iron core can be forced into or out of saturation.

The mag amp thus behaves like a switch: When saturated, it lets the AC current in its main winding pass unimpeded; when unsaturated, it blocks that current. Amplification occurs because a relatively small DC control current can modify a much larger AC load current.

The history of magnetic amplifiers starts in the United States with some patents filed in 1901. By 1916, large magnetic amplifiers were being used for transatlantic radio telephony, carried out with an invention called an Alexanderson alternator, which produced a high-power, high-frequency alternating current for the radio transmitter. A magnetic amplifier modulated the output of the transmitter according to the strength of the voice signal to be transmitted.

One Navy training manual of 1951 explained magnetic amplifiers in detail—although with a defensive attitude about their history.

In the 1920s, improvements in vacuum tubes made this combination of Alexanderson alternator and magnetic amplifier obsolete. This left the magnetic amplifier to play only minor roles, such as for light dimmers in theaters.

Germany’s later successes with magnetic amplifiers hinged largely on the development of advanced magnetic alloys. A magnetic amplifier built from these materials switched sharply between the on and off states, providing greater control and efficiency. These materials were, however, exquisitely sensitive to impurities, variations in crystal size and orientation, and even mechanical stress. So they required an exacting manufacturing process.

The best-performing German material, developed in 1943, was called Permenorm 5000-Z. It was an extremely pure fifty/fifty nickel-iron alloy, melted under a partial vacuum. The metal was then cold-rolled as thin as paper and wound around a nonmagnetic form. The result resembled a roll of tape, with thin Permenorm metal making up the tape. After winding, the module was annealed in hydrogen at 1,100 °C for 2 hours and then rapidly cooled. This process oriented the metal crystals so that they behaved like one large crystal with uniform properties. Only after this was done were wires wrapped around the core.

By 1948, scientists at the U.S. Naval Ordnance Laboratory, in Maryland, had figured out how to manufacture this alloy, which was soon marketed by an outfit called Arnold Engineering Co. under the name Deltamax. The arrival of this magnetic material in the United States led to renewed enthusiasm for magnetic amplifiers, which tolerated extreme conditions and didn’t burn out like vacuum tubes. Mag amps thus found many applications in demanding environments, especially military, space, and industrial control.

During the 1950s, the U.S. military was using magnetic amplifiers in automatic pilots, fire-control apparatus, servo systems, radar and sonar equipment, the RIM-2 Terrier surface-to-air missile, and many other roles. One Navy training manual of 1951 explained magnetic amplifiers in detail—although with a defensive attitude about their history: “Many engineers are under the impression that the Germans invented the magnetic amplifier; actually it is an American invention. The Germans simply took our comparatively crude device, improved the efficiency and response time, reduced weight and bulk, broadened its field of application, and handed it back to us.”

The U.S. space program also made extensive use of magnetic amplifiers because of their reliability. For example, the Redstone rocket, which launched Alan Shepard into space in 1961, used magnetic amplifiers. In the Apollo missions to the moon during the 1960s and ’70s, magnetic amplifiers controlled power supplies and fan blowers. Satellites of that era used magnetic amplifiers for signal conditioning, for current sensing and limiting, and for telemetry. Even the space shuttle used magnetic amplifiers to dim its fluorescent lights.

The image shows a Redstone rocket at the launch pad, with three space-suit-wearing astronauts in the foreground.Magnetic amplifiers were also used in Redstone rockets, like the one shown here behind astronauts John Glenn, Virgil Grissom, and Alan Shepard.UNIVERSAL IMAGES GROUP/GETTY IMAGES
Magnetic amplifiers also found heavy use in industrial control and automation, with many products containing them being marketed under such brand names as General Electric’s Amplistat, CGS Laboratories’ Increductor, Westinghouse’s Cypak (cybernetic package), and Librascope’s Unidec (universal decision element).

The magnetic materials developed in Germany during the Second World War had their largest postwar impact of all, though, on the computer industry. In the late 1940s, researchers immediately recognized the ability of the new magnetic materials to store data. A circular magnetic core could be magnetized counterclockwise or clockwise, storing a 0 or a 1. Having what’s known as a rectangular hysteresis loop ensured that the material would stay solidly magnetized in one of these states after power was removed.

Researchers soon constructed what was called core memory from dense grids of magnetic cores. And these technologists soon switched from using wound-metal cores to cores made from ferrite, a ceramic material containing iron oxide. By the mid-1960s, ferrite cores were stamped out by the billions as manufacturing costs dropped to a fraction of a cent per core.

But core memory is not the only place where magnetic materials had an influence on early digital computers. The first generation of those machines, starting in the 1940s, computed using vacuum tubes. These were replaced in the late 1950s with a second generation based on transistors, followed by third-generation computers built from integrated circuits.

Transistors weren’t an obvious winner for early computers, and many other alternatives were developed, including magnetic amplifiers.

But technological progress in computing wasn’t, in fact, this linear. Early transistors weren’t an obvious winner, and many other alternatives were developed. Magnetic amplifiers were one of several largely forgotten computing technologies that fell between the generations.

That’s because researchers in the early 1950s realized that magnetic cores could not only hold data but also perform logic functions. By putting multiple windings around a core, inputs could be combined. A winding in the opposite direction could inhibit other inputs, for example. Complex logic circuits could be implemented by connecting such cores together in various arrangements. //