### John Wolff's Web Museum

As the name suggests, these simple machines were used primarily for addition. Subtraction was possible by addition of complements, generally using secondary or "duplex" scales that were provided for the purpose. Multiplication and division were theoretically possible by repeated addition or subtraction, but the process was tedious and prone to error.

Slide adders were made in many different versions - decimal, Sterling currency, Imperial weights and measures, and fractions. There was even a hexadecimal model for computer programmers!

"Addiator" was originally the brand name of the major German manufacturer, but is often used as a generic term for devices of similar construction.

Operation

The slide adder consists of a number of metal slides located behind slots in a metal face plate. The illustration shows a close-up of a typical arrangement. An internal view is provided below.

To add (eg) 5, the operator inserts a metal stylus in the hole to the right of the number and pulls the slide down to the bottom. The result appears in the circular window above.

Numbers appearing in the clear portion of the slide can be added directly by the same method. Numbers appearing in the red section can not be added directly, as the slide only has 10 positions and will not move far enough down. The addition has to be performed by subtracting the complement of the number and adding ten in the next column. Although this sounds complicated, it is mechanised by a simple process of inserting the stylus opposite the required number (eg 7), moving the slide up to the top of the slot (thus subtracting 3), "around the bend" to engage with the slide in the next column, and back down the short slot to the end (thus adding 1 in the "tens" column). The result (5 - 3 + 10 = 12) appears in the windows. Note that the openings in the red section are slots rather than holes, to allow the stylus to move sideways.

The situation becomes more complicated when successive carries are required, such as when adding to a number containing a series of nines. The whole operation has to be inverted when subtraction is performed by addition of complements, or (in the "red" section) by direct addition and subtraction of tens.

The machines illustrated below show a variety of construction methods and operating rules that were devised to reduce the potential for confusion, and to assist the operator in obtaining the correct results.

Internal Construction

This view shows the component parts of a typical slide adder.

At the left is the top cover, made of brass, steel, or aluminium.

Next is the internal plate, which has been stamped to form vertical guides for the sliding numeral strips. The strips are curved so that they only touch the plate at the ends. They are held against the circular windows in the cover by small fingers formed near the centre of the guide plate. The clearing mechanism is a loop of stiff square-section brass wire, which collects the sliders and returns them to zero as the handle is raised.

The painted bottom cover has a clip on the right-hand side to hold the operating stylus.

"Baby Calculator", 8 columns, decimal currency
Metal and plastic body, plastic slip case, metal stylus
Dimensions: 3-1/4"W x 5-3/4"D x 1/4"H
Weight: 4oz (125g)
Manufactured: U.S.A.

The "Baby" calculator is an 8-column slide adder for decimal currency, supplied with an instruction leaflet, metal stylus, and a red plastic slip case. The slots are marked with complements for use in subtraction, and have bends at both ends. The upper bends are used for addition, and the lower for subtraction. The slides on this machine are coloured white at the top, and red below (opposite to the example above).

Addition proceeds as described, with the slides moving down for numbers in the red section, or "up and around the bend" in the white. Subtraction involves adding the complement shown in the small numbers to the left of the main scales. (Note that the complement scales relate to the slides on their right, not to the adjacent slide on the left). The direction of movement is still the same (down for red, up for white), but the rule is different - down and around the bend for red, up but not around the bend for white.

Although the "Baby" calculator has the simplest possible mechanical construction (single sided, one set of slots, bends at both ends), the location of the complement scales and the presence of two sets of bends leaves plenty of opportunity for confusion - as evidenced by this warning notice in the instructions.

Exactus "Mini-Add", 8 columns, Sterling currency
Dimensions: 2-3/4"W x 4-1/4"D x 1/4"H
Weight: 2oz (60g)
Manufactured: England

The Exactus is also a single-sided machine with only a single set of operating slots. It uses an ingenious "false front" panel to reduce the potential for confusion by switching positively between adding and subtracting modes.

The panel is hinged across the centreline of the machine, and normally covers the lower half of the operating slots. The "+" tab at its lower right shows the machine is in "Add" mode, with the numbers in "normal" order and a single set of bends at the tops of the slots. Operation is the same as previously described, with the slides moving down for clear numbers, or up and around the bend for red.

To perform a subtraction, the front panel is flipped over to cover the upper half of the slots. The tab now shows "-" next to the result windows, and the numerals have been magically reversed to show their complements. The direction of movement is still the same (down for clear numbers, up for red), but "around the bend" is now only possible at the lower ends of the slots.

10 columns, Sterling currency
Dimensions: 6-1/2"W x 8-1/2"D x 5"H
Weight: 3-1/2 pounds (1.6kg)
Manufactured: England

This is a larger version of the Exactus above, although it still goes by the name "Mini-Add". The device itself is about 50% larger than the pocket version (4-3/4" x 6-1/2") and has a knob as well as side tabs to flip the cover plate.

The die-cast desk stand is finished in green hammertone and has a pocket (top right) for a combined operating stylus and propelling pencil. The space under the stand is empty, apart from a 2-pound chunk of scrap iron fastened inside the base for ballast. It is not known whether this was "original equipment" or was added later by an exasperated owner.

6 columns, clip-on metal stylus
Dimensions: 1-5/8"W x 6-3/8"D x 3/16"H
Weight: 2oz (60g)
Manufactured: Germany

A "pocket caclulator" for Sterling currency, of brass construction, with metal stylus and an embossed leather slip case.

The upper scales are used for addition, with the result appearing in the windows in the centre of the machine.

A separate set of "inverted" slots and scales are provided for subtraction. The double-ended construction clearly separates addition and subtraction, and provides a simple operating rule for both - numbers in the clear sections move towards the centre of the machine, while those in the red sections move outwards and around the bend.

6 columns, clip-on metal stylus
Dimensions: 1-5/8"W x 6-3/8"D x 3/16"H
Weight: 1-1/4oz (40g)
Manufactured: Germany

A pocket Addiator for decimal currency. This unit is finished in anodised aluminium, and feels much lighter and less substantial than the brass unit above.

6 columns, clip-on metal stylus
Dimensions: 1-5/8"W x 6-3/8"D x 3/16"H
Weight: 1-1/4oz (40g)
Manufactured: Hong Kong, mid 1960s.

An exact copy of the decimal pocket Addiator, except for the currency conversion table printed on the top face. These units would have been built specifically for the Australian market, to assist in the conversion from Sterling to decimal currency in 1966.

Duplex Addiator, 8 columns, Sterling currency
Brass finish, fancy metal stylus
Dimensions: 3-1/2"W x 5"D x 1/4"H
Weight: 5oz (140g)
Manufactured: Germany, late 1950s.

An 8-column pocket Addiator for Sterling currency, with instruction leaflet and an embossed leather slip case. The original owner purchased this unit at the International Fair at the Melbourne Exhibition Buildings in March 1959.

The term "duplex" refers to the double-sided construction, with a second set of (linked) slides and result windows on the reverse side. Addition is performed on the front face, and the whole machine is flipped over (top to bottom) to do subtraction.

The inversion of the whole machine rather than just the scales allows the simplest and most consistent operating rule, which applies for both addition and subtraction - numbers in the clear section move downwards, while those in the red move up and around the bend.

The Duplex construction is a little more complicated, but the opportunity for error is greatly reduced.

9 columns for Feet, Inches and Sixteenths
Dimensions: 5"W x 7-1/4"D x 3/4"H
Weight: 12oz (340g)

This "Fractomator" is a larger 9-column duplex Addiator calibrated in Feet (decimal), Inches (base 12), and Sixteenths. The colour coding suggests that the decimal columns would also have been used for currency calculations.

The machine is mounted in a lined and embossed leather wallet, which also holds a notepad and a combined operating stylus and propelling pencil.

Operation is the same as for the Duplex Addiator above.

The Fractomator in its case is about the size and weight of a largish pocketbook.

The mechanism slides into a hinged bracket, so that it can easily be flipped forward to access the subtraction side. The front face of this unit shows signs of use, but the subtraction side is in perfect condition.

Duplex Slide Adder, 9 columns, Sterling currency
Dimensions: 4-1/2"W x 7-1/2"D x 1/2"H
Weight: 17oz (500g)

A 9-column duplex slide adder for Sterling currency. The mechanism is mounted in a metal case which unfolds to make a convenient desk stand. The mechanism tilts forward to access the Subtraction side.

Resources for further information

The history and products of the Addiator company are described in detail on Friedrich Diestelkamp's web site The site is mostly in German, but is well illustrated.