SQUID

squid, carnivorous marine cephalopod mollusk. The squid is one of the most highly developed invertebrates, well adapted to its active, predatory life. The characteristic molluscan shell is reduced to a horny plate shaped like a quill pen and buried under the mantle.

The mantle, the chief swimming organ of the animal, is modified into lengthwise fins along the posterior end of the body and projects forward like a collar around the head. As the mantle relaxes and contracts, the squid swims forward, upward, and downward. Water is expelled in jets from the muscular funnel located just below the head, propelling the squid backward in abrupt jetlike motions. Two of the ten sucker-bearing arms (used to steer in swimming) are tentacles that can seize prey, which is then cut into pieces by the animal's strong beaklike jaws.

The squid breathes through gills, and may emit a cloud of inky material from its ink sac when in danger. The circulatory and nervous systems are highly developed. The eye of the squid is remarkably similar to that of humans—an example of convergent evolution, as there is no common ancestor. Squids are also distinguished by internal cartilaginous supports. Some deep-sea forms have luminescent organs.

The common squid is found from Maine to the Carolinas, often moving in shoals. In the United States tons of squid are used for fish bait, particularly by the cod fisheries in New England. Squid is a favorite food in East Asia and in the Mediterranean area. Species range in size from about 2 in. (5 cm) to the proportions of Mesonychoteuthis hamiltoni, the colossal squid, which is the largest of all invertebrates and may attain a mantle length of 13 ft (4 m) and total length of 33–46 ft (10–14 m), and the giant squid, Architeuthis dux, which has a mantle length of 7.4 ft (2.25 m) and is known to reach 43 ft (13 m) in total length.

Squids are classified in the phylum Mollusca, class Cephalopoda, order Teuthoidea.

---

squid

Squid (Illex coindeti) swimming forward

(credit: Douglas P. Wilson)

Any of nearly 400 species of 10-armed cephalopods, found in both coastal and oceanic waters, that prey on fishes and crustaceans. They range from less than 0.75 in. (1.5 cm) to more than 65 ft (20 m) long (in the case of the giant squid). Two of the 10 arms are long, slender tentacles; each has an expanded end and four rows of suckers with toothed, hard-edged rings. An internal shell supports the slender tubular body of most species. Squid eyes, almost as complex as human eyes, are usually set into the sides of the head. Squids may be swift swimmers (propelling themselves by contracting and relaxing their mantle or by undulating their two fins) or mere drifters; water expelled from a funnel below the head can propel the squid backward. Like the octopus, the squid may emit an inky cloud from its ink sac when in danger from sperm whales, fishes, or humans, among other predators.

---

(Superconducting QUantum Interference Device) An electronic detection system that uses Josephson junctions circuits. It is capable of detecting extremely weak signals.

---

SQUID

An acronym for superconducting quantum interference device, which actually refers to two different types of device, the dc SQUID and the rf SQUID.

The dc SQUID consists of two Josephson tunnel junctions connected in parallel on a superconducting loop (see illustration). A small applied current flows through the junctions as a supercurrent, without developing a voltage, by means of Cooper pairs of electrons tunneling through the barriers. However, when the applied current exceeds a certain critical value, a voltage is generated. When a magnetic field is applied so that a magnetic flux threads the loop, the critical value oscillates as the magnetic flux is changed, with a period of one flux quantum, weber, where h is Planck's constant and e is the electron charge. The oscillations arise from the interference of the two waves describing the Cooper pairs at the two junctions, in a way that is closely analogous to the interference between two coherent electromagnetic waves. See Interference of waves, Josephson effect, Superconductivity

Direct-current (dc) SQUID with enclosed magnetic flux Φ

The rf SQUID consists of a single junction interrupting a superconducting loop. In operation, it is coupled to the inductor of an LC-tank circuit excited at its resonant frequency by a radio-frequency (rf) current. The rf voltage across the tank circuit oscillates as a function of the magnetic flux in the loop, again with a period of one flux quantum. Although SQUIDs were for many years operated while immersed in liquid helium, ceramic superconductors with high transition temperatures make possible devices operating in liquid nitrogen at 77 K.

SQUIDs have important device applications. Usually with the addition of a superconducting input circuit known as a flux transformer, both dc and rf SQUIDs are used as magnetometers to detect tiny changes in magnetic field. The output of the SQUID is amplified by electronic circuitry at room temperature and fed back to the SQUID so as to cancel any applied flux. This makes it possible to detect changes in flux as small as 10^-6 of one flux quantum with SQUIDs based on low-transition-temperature superconductors, corresponding to magnetic field changes of the order of 1 femtotesla in a 1-hertz bandwidth. Suitable modifications to the input circuit enable the SQUID to measure other physical quantities, including voltages, displacement, or magnetic susceptibility. SQUIDs are also used for logic and switching elements in experimental digital circuits and high-speed analog-to-digital converters. See Superconducting devices

---

SQUID

An acronym for superconducting quantum interference device, which actually refers to two different types of device, the dc SQUID and the rf SQUID.

The dc SQUID consists of two Josephson tunnel junctions connected in parallel on a superconducting loop (see illustration). A small applied current flows through the junctions as a supercurrent, without developing a voltage, by means of Cooper pairs of electrons tunneling through the barriers. However, when the applied current exceeds a certain critical value, a voltage is generated. When a magnetic field is applied so that a magnetic flux threads the loop, the critical value oscillates as the magnetic flux is changed, with a period of one flux quantum, weber, where h is Planck's constant and e is the electron charge. The oscillations arise from the interference of the two waves describing the Cooper pairs at the two junctions, in a way that is closely analogous to the interference between two coherent electromagnetic waves.

The rf SQUID consists of a single junction interrupting a superconducting loop. In operation, it is coupled to the inductor of an LC-tank circuit excited at its resonant frequency by a radio-frequency (rf) current. The rf voltage across the tank circuit oscillates as a function of the magnetic flux in the loop, again with a period of one flux quantum. Although SQUIDs were for many years operated while immersed in liquid helium, ceramic superconductors with high transition temperatures make possible devices operating in liquid nitrogen at 77 K.

SQUIDs have important device applications. Usually with the addition of a superconducting input circuit known as a flux transformer, both dc and rf SQUIDs are used as magnetometers to detect tiny changes in magnetic field. The output of the SQUID is amplified by electronic circuitry at room temperature and fed back to the SQUID so as to cancel any applied flux. This makes it possible to detect changes in flux as small as 10^-6 of one flux quantum with SQUIDs based on low-transition-temperature superconductors, corresponding to magnetic field changes of the order of 1 femtotesla in a 1-hertz bandwidth. Suitable modifications to the input circuit enable the SQUID to measure other physical quantities, including voltages, displacement, or magnetic susceptibility. SQUIDs are also used for logic and switching elements in experimental digital circuits and high-speed analog-to-digital converters. See Analog-to-digital converter, Integrated circuits, Superconducting devices