The IBM Selectric typewriter (occasionally known as the IBM Golfball typewriter ) was an influential electric typewriter design. It was introduced in 1961.
Instead of a "basket" of pivoting typebars the Selectric had a pivoting type element (frequently called a "typeball") that could be changed so as to display different fonts in the same document, resurrecting a capacity that had been pioneered by the moderately successful Blickensderfer typewriter sixty years before. The Selectric also replaced the traditional typewriter's moving carriage with a paper roller ("platen") that stayed stationary while the typeball and ribbon mechanism moved from side to side.
Selectrics and their descendants eventually captured 75 percent of the United States market for electric typewriters used in business.
Features and uses
The ability to change fonts, combined with the neat regular appearance of the typed page, was revolutionary, and marked the beginning of desktop publishing. Later models with dual pitch (10/12) and built-in correcting tape carried the trend even further. Any typist could produce a polished manuscript. By 1966, a full typesetting version with justification and proportional spacing was released.
The possibility to intersperse text in Latin letters with Greek letters and mathematical symbols made the machine especially useful for scientists writing manuscripts that included mathematical formulas. The typical look of Selectric typed documents is hence still familiar to any scientist who reads conference proceedings, monographies, theses and the like from these times. (Proper mathematical typesetting was very laborious before the advent of TeX and done only for much-sold textbooks and very prestigious journals.)
The machine had a feature called "Stroke Storage" that prevented two keys from being depressed simultaneously. When a key was depressed, an interposer, beneath the keylever, was pushed down into a slotted tube full of small metal balls (called the "compensator tube") and spring latched. These balls were adjusted to have enough horizontal space for only one interposer to enter at a time. If a typist pressed two keys simultaneously both interposers were blocked from entering the tube. Pressing two keys several milliseconds apart allows the first interposer to enter the tube, tripping a clutch which rotated a fluted shaft driving the interposer horizontally and out of the tube, making way for the second interposer to enter the tube some milliseconds later. While a full print cycle was 65 milliseconds this filtering and storage feature allowed the typist to depress keys in a more random fashion and still print the characters in the sequence entered. The powered horizontal motion of the interposer selected the appropriate rotate and tilt of the printhead for character selection.
The spacebar, dash/underscore, index, backspace and line feed repeated when continually held down. This feature was referred to as "Typamatic."
Design
The Selectric typewriter was introduced on 23 July 1961. Its industrial design is credited to influential American designer Eliot Noyes. Noyes had worked on a number of design projects for IBM; prior to his work on the Selectric, he had been commissioned in 1956 by Thomas J. Watson, Jr to create IBM's first house style — these influential efforts, in which Noyes collaborated with Paul Rand, Marcel Breuer, and Charles Eames, have been referred to as the first "house style" program in American business.
Both Selectric and the later Selectric II were available in standard, medium, and wide-carriage models and in various colors, including red and blue as well as traditional neutral colors.
Mechanically, the Selectric borrowed some design elements from a toy typewriter produced earlier by Marx Toys. IBM bought the rights to the design. The typeball and carriage mechanism was similar to the design of the Teletype Model 26 and later, which used a rotating cylinder that moved along a fixed platen.
The mechanism that positions the typing element ("ball") is partly binary, and includes two mechanical digital-to-analog converters, which are basically "whiffletree" linkages of the type used for adding and subtracting in linkage-type mechanical analog computers. Every character has its own binary codes, one for tilt and one for rotate.
When the typist presses a key, it unlatches a metal bar for that key. The bar is parallel to the side of the mechanism. This bar has several short projections ("fingers"). Only some of the fingers are present on any given code bar, those present corresponding to the binary code for the desired character.
When the key's bar moves, its projections push against a second set of bars that extend all the way across the keyboard mechanism; each bar corresponds to one bit. All bars for the keys contact some of these crosswise bars. Those bars that move, of course, define the binary code.
The bars that have been moved cause cams on the driveshaft (which is rotating) to move the ends of the links in the whiffletree linkage, which sums (adds together) the amounts ("weights") of movement corresponding to the selected bits. The sum of the weighted inputs is the required movement of the typing element. There are two sets of similar mechanisms, one for tilt, one for rotate. The reason for this is the type element has four rows of 22 characters. By tilting and rotating the element to the location of a character, the element could be thrust against the platen, leaving an imprint of the chosen character.
The motor at the back of the machine drove a belt connected to a two-part shaft located roughly halfway through the machine. The Cycle Shaft on the left side provided the energy that was used to tilt and rotate the type element. The Operational Shaft on the right side provided functions such as spacing, back spacing and case shifting. Additionally, the Op Shaft was used as a governor; limiting the left-to-right speed with which the carrier moved. A series of spring clutches were used to power the cams which provided the motion needed to perform functions such as backspacing. The Cycle Shaft was rotated when a spring clutch was released, driving a set of cams whose rotational motion was then converted into left-and-right motion by the whiffle tree. The system was highly dependent upon lubrication and adjustment and much of IBM's revenue stream came from the sale of Service Contracts on the machines. Repair was fairly expensive, so maintenance contracts were an easy sell.
The locations of the characters on the element were not random. Punctuation marks and the underscore were deliberately placed so the maximum amount of energy was used to position the element, thus reducing the impact made by them and lessening the chance that the underscore would cut through the paper. Later on, a deliberate mechanism was added that reduced the force of the impact made by punctuation.
Tilt and rotate movements are transferred to the ball carrier, which moves across the page, by two taut metal tapes, one for tilt and one for rotate. The tilt and rotate tapes are both anchored to the right side of the carrier (the metal contraption upon which the type element is located). They both wrap around separate pulleys at the right side of the frame. They then wrap behind the carrier the are wrapped around two separate pulleys at the left side of the frame. The tilt tape is then anchored to a small, quarter-circle pulley which, through a gear, tips the tilt ring to one of four possible locations (The tilt ring is the device to which the type element is connected). The rotate tape is wrapped around a spring-loaded pulley located in the middle of the carrier. The rotate pulley under the tilt ring is connected through a universal joint (called a "dog bone"; it looked like a small bone) to the center part of the tilt ring. The type element is sprint-latched onto that central post. The type element rotates counter-clockwise when the rotate tape is tightened. The clock spring underneath the rotate pulley rotates the element in the clockwise direction. As the carrier moves across the page (such as when it returns), the tapes travel over their pulleys, but the spring-loaded pulleys on the ball carrier do not pivot or rotate.
To position the ball, both of the pulleys on the left side of the frame are moved by the whiffletree linkage. When the rotate pulley is moved to the right or left, the rotate tape spins the type element to the appropriate location. When the tilt pulley is moved, it tips the tilt ring to the appropriate location. When it moves, the tape rotates the spring-loaded pulley on the ball carrier independent of the carrier's location on the page.
Case was shifted between caps and lower case by rotating element by exactly half a turn. This was accomplished by moving the right-hand rotate pulley using a cam mounted on the end of the operation shaft.
There was a proportional-spacing Selectric called a Composer that would backspace proportionally for perhaps 40 characters. The spacing code for the last characters typed was stored by small sliding plates in a carrier wheel.
Selectric II
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