Clock

A clock is an instrument to indicate, keep, and co-ordinate time. The word clock is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a striking mechanism has traditionally been known as a timepiece. In general usage today a "clock" refers to any device for measuring and displaying the time. Watches and other timepieces that can be carried on one's person are often distinguished from clocks.

The clock is one of the oldest human inventions, meeting the need to consistently measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia. A sundial shows the time by displaying the position of a shadow on a flat surface. There are a range of duration timers, a well-known example being the hourglass. Water clocks, along with the sundials, are possibly the oldest time-measuring instruments. A major advance occurred in Europe around 1300 with the invention of the escapement, which allowed construction of the first mechanical clocks, which used oscillating timekeepers like balance wheels. Spring-driven clocks appeared during the 15th century. During the 15th and 16th centuries, clockmaking flourished. The next development in accuracy occurred after 1656 with the invention of the pendulum clock. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The electric clock was patented in 1840. The development of electronics in the 20th century led to clocks with no clockwork parts at all.

Clock (cryptography)

In cryptography, the clock was a method devised by Polish mathematician-cryptologist Jerzy Różycki, at the Polish General Staff's Cipher Bureau, to facilitate decrypting German Enigma ciphers.

Method

This method sometimes made it possible to determine which of the Enigma machine's rotors was at the far right, that is, in the position where the rotor always revolved at every depression of a key.

Różycki's "clock" method was later elaborated by the British cryptologist Alan Turing at Bletchley Park in the development of a cryptological technique called "Banburismus."

Machine settings

The Enigma cipher machine relied on the users having some shared secrets. Here are the secret daily settings from a 1930 Enigma manual:

The daily settings told the code clerks how to configure the machine so message could be exchanged. Initially, the machine had three rotors that could be arranged in any order (the wheel order or rotor order). Each rotor had a ring with numbers or letters on it, and that ring could be in any of 26 positions. A plugboard interchanged additional characters.

Clock (restaurant)

Clock was a Swedish hamburger restaurant chain originally founded by an offshoot from the US-based Carrols. The chain suffered from mismanagement and declared bankruptcy; as the government had acquired the forfeited company, it was incorporated into the state-owned restaurant chain SARA.

As Carrols, Clock used the McDonald's concept with names for hamburgers such as 'Big Clock' ('Big Mac'). Using a huge clock as its logo, the chain grew to be very successful and widespread during the 1970s and 1980s, even branching out to China, but got into economic problems in the 1990s and started closing or selling restaurants. In 1996 Clock actually sold six restaurants (four in Stockholm and two in Gothenburg) to McDonald's. The same year, as part of what turned out to be a new business strategy, the company bought the hotel and restaurant company Provobis, which had the same main owner, Rolf Lundström, who thereby consolidated his holdings. It also attempted to reduce its own ownership in restaurants and increase the number of franchise restaurants, but by 1998 only 14 Clock restaurants remained, of which six were sold the same year and the remaining eight at the beginning of 1999. The company took the name Provobis, and was in 2000 bought by the large Scandic Hotels corporation.