Slide Rules

Our slide rule display has been put together and is maintained by UKSRC, the UK Slide Rule Circle.

Slide Rules were the pre-eminent calculating tool from their invention in around 1620, right through to their demise in the late 1980s, some 350 years later, but their decline really started in the 1960s with the advent of electronic calculators and computers, .

Those of you who used a slide rule, where it was not supplied as part of your job, will remember the very careful consideration that you gave to its choice, and the occasion when you actually went to buy it. Many people can still remember both where and when it was bought, and also exactly how much it cost.

With so many different varieties around, most people only had one or two slide rules, either a ‘pocket model’ 5” or a ‘desk-model’ 10” slide rule.

If the first slide rule needed updating, often as a result of more complex calculations being needed, or to get an improved quality one, this would have been the only reason to purchase another one. This is very different to the situation today with many electronic pocket calculators lying around the house.

Our slide rule display includes a wide variety of makes, types and formats from all over the world, but you would be hard-pressed to tell where a slide rule was made by just looking at it, as many used a similar construction method.

There are slide rules of all shapes and sizes for performing very different calculations, with many industries having purpose made devices. Some examples include:

• Gauging - the calculation of quantities of liquids and the strength of alcohol for tax purposes.

• Timber volume calculation - how much timber will a particular tree produce

• Standard mathematical calculation - multiplying, dividing, squares and cubes

• Business and purchasing calculations

• Specialist calculations, such as spring designs, milk strength, engineering and structural calculations

• There is even one on display that can calculate the amount of meat on a cow!

We have slide rules in a variety of shapes and sizes, with rectilinear scales, circular, spiral and helical scales, including:

• Rectilinear in different sizes - this being the most common shape, where multiple different sized scales are in a straight line, often on both top and bottom

• Circular slide rules, favoured by many users as the slide did not need to be swapped during any calculation

• Circular slide rules with spiral scales - this gave a much longer scale length with a smaller physical size

• Helical scales

• Grid-iron scale where rectilinear scale has been split into a number of separate scales either on a cylinder or on a flat surface

And if you ever wanted to try your hand at using one, or see if you can remember how, we have a large working model of a PICKETT American slide rule on the wall opposite the slide rule display cabinets

Slide Rule Displays

Calculators

History

The Chinese Abacus is often thought of as the first calculating device, but it is believed much simpler devices were in use as early as 2500 BC when shells, pebbles or stones were used within makeshift hollowed out containers, often of sand but also of stone or wood.

The Chinese improved upon the concept by putting a string of beads on wire columns in a frame to make it easier and often quicker to use and making a much more portable device for merchants and tradesman. It still had its limitations in only being able to do addition and subtraction.

It took another 4000 years before a much improved calculating device was invented by John Napier, a mathematician from Scotland, in 1617. His mathematical device, which came to be known as Napier’s Bones, was a series of rods on which the multiplication table was written and calculations performed by moving the rods.

While the abacus and Napier’s Bones may not be considered as true calculators, the first true calculator, and possibly the first mechanical calculator is credited to Blaise Pascal, a French mathematician, and inventor. in the mid 17th Century. Pascal’s device did calculations using geared wheels. Each wheel having numbers 0 to 9 on it and as each wheel turned back to 0, it would 'carry’ or move the next wheel on one position. Although it performed well, it turned out to be bulky and unreliable in operation.

The first reliable mechanical calculator was believed to be Thomas de Colmar’s Arithmometer, invented in 1820. It’s accuracy and build quality was far superior to previous devices and was a popular calculating device for 19th-century businesses and retailers. It used rotating drums to perform the calculations and was so robust that it was exclusively used until the early 20th century. It also started the mechanical calculator industry which continued until the early 1970s.

The Arithmometer was still a large device so in 1938, Curt Herzstark began working on ways to reduce the size of the mechanical parts to try and come up with a hand-held version. He finally achieved his goal when, in 1945, he released his Curta calculators. What was unusual about these Curta calculators was it’s shape; It looked like a large pepper grinder. But it was still capable of doing the calculations that business people needed and was small enough to carry around easily.

At the same time as the Curta mechanical calculators were starting to appear, the 1940s also saw the emergence of electrical calculating machines, but with one drawback… the size of the vacuum tubes needed meant the machines were big. But as the tubes got smaller and inventions such as the Nixie tube appeared allowing compact display of numbers, electronic calculators really took off.

That is until the early 1970s when the Japanese produced the first digital calculators. The invention of the microprocessor and design of dedicated calculator chips, together with better and smaller displays like LEDs allowed these digital calculators to be truly pocket-sized, and with mass-production, very quickly inexpensive compared to the electrical calculators of the 60s. The technology improved so fast that by the early 1980s, it was possible to get a credit-card sized calculator for a few pounds. It was soon possible to have not just a packet sized calculator, but a pocket sized computer able to be programmed to perform very complex calculations.

And we have today, where it is rare to have a dedicated pocket calculator. Instead you now have smartphones that contain, amongst other things, a complex calculator.

On Display

In one of the corridors, we have 3 display cabinets showing just a small collection of calculators from all ages. There are also examples of the more recent pocket calculators from the likes of Sinclair, HP, Casio in the corridor on the way to the Classroom.

Below are descriptions of what we have on display, plus more details of a select few artefacts:

Cabinet 1

U - Sumlock Anita Mark VIII c1962

The world’s first completely non-mechanical calculator was this model, designed and build in the UK. It was invented by a veteran of the Pilot ACE computer project, Norman Kitz. The design evolved from the first prototype in 1958 until a production machine was release by Sumlock Comptometer in 1961.

The Anita used a novel set of decimal counting circuits rather than the logic circuits common in later electronic calculators. It calculated much like a mechanical calculator, but at electronic speeds. It’s not transistorised, nor is it based on thermionic valves (tubes).  Instead, it uses cold cathode tubes (usually neon based). It even has a Dekatron hiding under the covers. When operated with the case open, it’s a real festival of lights.

You operate the Anita much like any other desktop calculator of its day, such as the Monroe two machines to the left. First you set the machine’s operating mode (add, subtract, multiply or divide), using the four buttons at the bottom left. If you are adding or subtracting, you used that big field of keys in the middle.

Machines of this period typically had what they called ‘full keyboards’. Instead of a single digit keypad, with number 0-9, it has columns of 9 keys (you don’t push a key for zero). Each column aligns with the corresponding digit of the number you are entering. Once you’ve entered all the columns you want, you push the ‘enter’ key which is the tall one at the bottom toward the right. However, if you wanted to multiply or divide, you entered the number, one digit at a time using that last vertical column of keys. Each time you pushed the key in that column, it would do an operation and automatically shift the input up a digit (1s, 10s, 100s, etc.)

Cabinet 2

A - Fuller Calculators

These are slide rules wrapped round and round a cylinder. They were made from approximately 1880-1974.

A slide rule is a way of doing complicated division or multiplication quickly by comparing positions and markings on a pair of rulers. The rulers aren’t like a normal ruler in that they are marked logarithmically. Each division is half the size of the last. That’s would be like having a foot long ruler where the first segment was 1-2 inches, the second is 2-4, the third is 4-8 and so on.  You match up the rulers to get outputs. Until the 1970s, most routine technical calculations relied on slide rules.

While this is a quick way to do big calculations, it has limited accuracy. A typical slide rule is 10-12 inches long. Its precision is based on the fineness of the markings and the accuracy with which you can read them. Most have three significant digits of precision.

The Fuller Calculators are special in that they are much more accurate. That’s because they are effectively much longer. Because the rulers are wrapped round and round cylinder, these instruments are like slide rules 41 feet long. The longer the rule, the more divisions you can see, and the more precise you can get.

Accuracy is always relative though. A Fuller Calculator could produce a little more than one additional significant digit. For instance, 3.141 vs 3.14. But for some calculations, that makes all the difference.

M - Felt and Tarrant Comptometer Model J (1926-1938)

A Comptometer is rapid-entry adding machine. All it does is add. But these machines were incredibly popular and successful from their introduction in 1887 until the many later clones were driven out in the early 1970s. This is because a good operator could whiz columns of accounting numbers at something approach the speed in which letters could be typed in a line on a typewriter. 

The secret is that you can press all the digits in your number at once – like playing a musical chord on a keyboard. The act of pressing the numbers causes the addition. There’s no separate enter, plus or equals key to push. The number you press in is immediately added to the last number. The running total is shown as digits in little windows across the front of the machine. 

Unlike most modern numeric entry keyboards, there’s a whole column of keys for each digit. You don’t have to enter one digit at a time, in sequence. Trained operators of these machines were prized employees and paid well. Most operators were women. Training as a Comptometer operator was one of a very few ways for women to gain financial independence in the late nineteenth the first half of the twentieth century.

Cabinet 3

L - The Millionaire

The Millionaire was was produced from 1893-1935. Most mechanical calculators designed before the Millionaire were just adding machines. A few could subtract. The ability to multiply and divide made the Millionaire special in its day. The concept we may have today of a pocket calculator or calculator app with four functions (add, subtract, multiply and divide) didn’t exist before the Millionaire.

Other machines had to do multiplication and division by repeating the same operation over and over again. For instance, 3x3 is the same as 3+3+3. So you had three operations to key. Imagine doing that with large numbers!

The Millionaire was different. It has a built in multiplication table made of a spindle with varying length teeth. It does one digit multiplication at a time. Any number that fits in its mechanical memory can be multiplied or divided by 0-9. You set that single digit using the big sweeping lever at the top left. You set the operation type with the sliding lever second from right. To complete the operation, you crank the handle at the top right. It applies the operation to what ever number is already in the little circular digit windows across the bottom silver bar.

Of course it also adds and subtracts — but in much the same way as other 19th century calculating machines. To add or subtract, you put the full number in by using the sliding handles across the top middle.

When this machine was made, it was so novel it found a ready market among statisticians. Most adding machines were sold to bookkeepers and financial clerks. Those machines were very much faster to use for routine accounting. However, this Millionaire made quick work of calculating percentages, and other statistical functions. Those are important to actuarial work like insurance

Calculator Displays