What is a cryotron?

   The cryotron is a tiny, low-temperature device that is used most commonly as a switch. A simple cryotron consists of a straight piece of wire one-quarter to one inch in length with a second wire coiled around the first. In a typical cryotron the central wire, called the gate, is made of tantalum and is .009 inch in diameter. The outer wire, the control coil, is made of niobium .003 inch in diameter.
   Uses of the Cryotron. In its electrical characteristics the cryotron is similar to a vacuum tube. However, since it is so much smaller than a vacuum tube, the cryotron is useful in electronic computers where great numbers of switches must fit into a small space.
   How the Cryotron Works. The cryotron operates at temperatures near absolute zero (—459.67° F.). It is chilled to such temperatures by being encased in a capsule of liquid helium. At the temperature of liquid helium, the wires of the cryotron become superconductors and lose all of their electrical resistance. Their resistance can be instantly restored, however, by placing them in a magnetic field of sufficient strength.
   In operation, the current to be controlled is allowed to flow through the gate. When the gate current is to be switched off, a current is passed through the control coil. The control coil produces an electromagnetic field and stops current flow in the gate.
   History of the Cryotron. The first cryotron was made in 1935 by W. J. de Haas and J. M. Casimir-Jonken of the Kamerlingh Onnes Laboratory in Leiden, Holland. In 1956, Dudley Buck, an engineer, pointed out the cryotron's usefulness as a computer component. It was Buck who named the cryotron and refined it to its present design. Modern cryotrons are made with extremely thin films instead of wires, because this makes them switch faster.