Two different classes of mechanisms had become established by this time, reciprocating and rotary. The former type of mechanism was operated typically by a limited-travel hand crank; some internal detailed operations took place on the pull, and others on the release part of a complete cycle. The illustrated 1914 machine is this type; the crank is vertical, on its right side. Later on, some of these mechanisms were operated by electric motors and reduction gearing that operated a crank and connecting rod to convert rotary motion to reciprocating.
The latter, type, rotary, had at least one main shaft that made one [or more] continuous revolution[s], one addition or subtraction per turn. Numerous designs, notably European calculators, had handcranks, and locks to ensure that the cranks were returned to exact positions once a turn was complete.
The first half of the 20th century saw the gradual development of the mechanical calculator mechanism. The Dalton adding-listing machine introduced in 1902 was the first of its type to use only ten keys, and became the first of many different models of "10-key add-listers" manufactured by many companies.
In 1948 the miniature Curta calculator, which was held in one hand for operation, was introduced after being developed by Curt Herzstark in 1938. This was an extreme development of the stepped-gear calculating mechanism. It subtracted by adding complements; between the teeth for addition were teeth for subtraction.
From the early 1900s through the 1960s, mechanical calculators dominated the desktop computing market. Major suppliers in the USA included Friden, Monroe, and SCM/Marchant. These devices were motor-driven, and had movable carriages where results of calculations were displayed by dials. Nearly all keyboards were full — each digit that could be entered had its own column of nine keys, 1..9, plus a column-clear key, permitting entry of several digits at once. One could call this parallel entry, by way of contrast with ten-key serial entry that was commonplace in mechanical adding machines, and is now universal in electronic calculators. Full keyboards generally had ten columns, although some lower-cost machines had eight. Most machines made by the three companies mentioned did not print their results, although other companies, such as Olivetti, did make printing calculators.
In these machines, addition and subtraction were performed in a single operation, as on a conventional adding machine, but multiplication and division were accomplished by repeated mechanical additions and subtractions. Multiplications is repeated addition and division is repeated subtraction. Friden made a calculator that also provided square roots, basically by doing division, but with added mechanism that automatically incremented the number in the keyboard in a systematic fashion. The last of the mechanical calculators were likely to have short-cut multiplication, and some ten-key, serial-entry types had decimal-point keys. However, decimal-point keys required significant internal added complexity, and were offered only in the last designs to be made. Handheld mechanical calculators such as the 1948 Curta continued to be used until they were displaced by electronic calculators in the 1970s.
The original ten key was an adding machine. The three by three arrangement of keys was first used in 1911 on a machine invented by David Sundstrand of the Sundstrand Adding Machine Company. Similar machines are still used by accountants and other professionals who do a lot of mathematics. Instead of using an "=" key to complete an equation, they usually have a key marked "T" for total, which serves the same purpose. These machines have become somewhat less common as computers have become more popular, but are still useful in some professions. Calculators often feature a similar number pad layout, but typically offer many additional keys to handle more complicated mathematics.
A ten-key layout is commonly found on personal computer keyboards, usually to the right of the alphabet keys. Laptops and other small computers may only offer number keys above the main keyboard or include a number pad that shares keys with letters on the right side, as a separate keypad takes up too much space. Left-side keypads are available, although they are uncommon; in addition, separate USB ten keys can be plugged into a laptop or other machine. Most computers, even those without a separate number pad, usually have a ten-key calculator accessory that displays on the screen.
The telephone key board is upside down from the adding machine and computer. Why that happened? Many theories have been trying to explain, but it remains uncertain. However, the first theory deals with the telephone's circuitry and tone-recognition hardware. When the touch-tone telephone was being designed in the late 1950s, the calculator and adding-machine designers had already established a layout that had 7, 8 and 9 across the top row. Data-entry professionals, and others who used calculators regularly, were quite adept at navigating these keypads. They could hit the numbers extremely quickly, which was great for data entry, but not so great for dialing a touch-tone phone. The tone-recognition technology could not operate effectively at the speeds at which these specialists could dial the numbers. The telephone designers figured that if they reversed the layout, the dialing speeds would decrease and the tone-recognition would be able to do its job more reliably. This theory has little proof to substantiate it, but it does make sense.
"About 1950 in the family wholesale grocery, we bought a new electric Ten Key adding machine to add the invoices for delivery every morning. Included was exercises to learn the ten key by touch. Our operator said she didn't need the exercises and she wouldn't look at the adding machine anyway. Some of us would stand behind her for a while and just observe. Of course her head would swing back and forth and yours does too. In the course of time, after practicing the exercises few minutes per day, she could operate the adding machine without looking at it. It increased her speed typing the numbers and most important, it increased her self-confidence.If you will practice these exercises each day for a few days, you may be as quick and accurate as I am (This was written years ago)."
TIP: When proofing a column of figures, always subtract the total. you will have a zero balance. It prevents you reading the balance as what you know it should be rather than what it is.
"Never look at your adding machine again. I have often added a list of numbers, looked for the total and found that I didn’t have the adding machine turned on."- Gelard Dix
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