In the pantheon of computing pioneers, names like Charles Babbage, Alan Turing, Grace Hopper, and John von Neumann are often hailed as the forefathers and foremothers of the digital age. But the story of modern computing has a much longer and richer history, filled with fascinating characters whose groundbreaking inventions paved the way for today‘s electronic wonders. Among these unsung heroes is Alonzo Johnson, a brilliant machinist from Springfield, Massachusetts, who in 1868 designed and patented two of the most advanced and innovative mechanical calculating machines of the 19th century.
The Remarkable Calculating Machines of Alonzo Johnson
Johnson‘s two calculators, patented just 11 months apart in January and December of 1868, employed a clever carry mechanism to perform addition operations on numbers up to 9,999. This was a major computational feat for the time, when most calculations were still performed mentally, on paper, or with simple tools like the abacus.
The key innovation in Johnson‘s machines was the use of two concentric discs – one stationary disc engraved with the numbers 0-99 around its outer rim, and a rotatable inner disc with 100 evenly spaced holes around its edge. Two metal arms pivoted from the center of the inner disc. The longer arm had a knob for rotating it and a pin on the underside that fit into the holes, allowing the user to input numbers by turning the arm to the desired value. The shorter arm served as the carry mechanism. When the sum on the inner disc passed 99, the short arm would automatically advance forward one space to register the hundred value.
This carry mechanism may seem relatively simple today, but in Johnson‘s time it was a marvel of mechanical engineering. To achieve reliable and precise operation, Johnson had to carefully calculate the gear ratios and design the mechanism with very tight tolerances. A single misaligned gear or slipped tooth could render the entire machine inoperable.
In his second patented design, Johnson further refined the carry mechanism so that the hundreds value was displayed in its own small window above the main discs. This made it easier for the user to read the result and reduced the chance of errors.
Based on the dimensions and descriptions in Johnson‘s patents, I estimate that his machines could perform somewhere between 10-30 addition operations per minute, depending on the dexterity of the operator. This may not sound impressive compared to the billions of calculations per second of modern computers, but it would have been a major labor-saving improvement over mentally adding long lists of numbers.
Putting Johnson‘s Calculators in Historical Context
To fully appreciate the significance of Johnson‘s inventions, it is important to understand the state of calculating technology in the mid-19th century. Mechanical calculating aids had existed for centuries, but most were simple single-purpose devices like the Abacus, Napier‘s Bones, or slide rules. There had been a few attempts to create more complex calculating machines, but none had achieved widespread success.
The most notable early mechanical calculator was the Pascaline, invented by a teenage Blaise Pascal in 1642. It used a series of geared wheels to perform addition and subtraction, with carry transfer between wheels. But it was an expensive device prone to breaking under heavy use. In 1820, Charles Xavier Thomas de Colmar invented the Arithmometer, the first commercially produced mechanical calculator. It used a stepped drum design and could perform all four basic arithmetic operations. But it was still expensive, temperamental, and hand-produced in small numbers.
This was the landscape in which Alonzo Johnson designed his calculators. By comparison, his carry mechanism was more reliable and easier to manufacture than earlier designs. And while his machines were more limited single purpose devices that could only perform addition, they did so more quickly and accurately. Here is a brief timeline showing how Johnson‘s work fits into the larger history of mechanical computation:
| Year | Inventor | Invention |
|---|---|---|
| 1623 | Wilhelm Schickard | Calciphor clock (first mechanical calculator) |
| 1642 | Blaise Pascal | Pascaline |
| 1672 | Gottfried Leibniz | Leibniz wheel |
| 1820 | Charles X. Thomas | Arithmometer |
| 1853 | Per Georg Scheutz | Difference Engine |
| 1868 | Alonzo Johnson | Johnson Calculator |
| 1872 | Frank S. Baldwin | Baldwin Calculator |
| 1887 | Willgodt Odhner | Odhner Arithmometer |
| 1889 | Leon Bollée | Millionnaire |
The Engineering Marvel of the Carry Mechanism
At the heart of what made Johnson‘s calculators so innovative was his carry mechanism. In a modern electronic calculator or computer, the carry operation is performed by a special electronic circuit called a full adder. This element takes in three 1-bit binary numbers – A, B, and an input carry bit – and outputs their sum as well as a carry out bit. Multiple full adders can be chained together to add together larger multi-bit numbers.
Johnson‘s mechanical carry mechanism performed a similar function. As the operator turned the input arm, it rotated the inner disc which was engraved with the numbers 0-99. If the inner disc passed 99, a small lever attached to the outer stationary disc would engage a gear on the inner disc, causing the short arm to advance forward by one position, representing the 1 carried over into the hundreds place.
To better visualize how this works, imagine the inner disc is divided into 100 discrete "cells", each corresponding to one of the numbers from 0-99. As the inner disc rotates, it is almost like the cells are passing underneath the carry lever. Most of the time they pass by without engaging it. But the cell corresponding to the number 99 has a small tab that snags the lever just as the 99 is about to roll over to 00. This momentary catch is just enough to trip the carry arm forward by one position before the tab releases the lever.
Conceptual diagram of Johnson‘s carry mechanism engaging at the transition from 99 to 00 to increment the hundreds register
From an engineering perspective, this design is incredibly clever and was surely a major challenge for Johnson to perfect. The precise angles of the tab and lever, the spring tension holding the lever in place, the allowable tolerances between the discs – all of these variables had to be just right for reliable operation. It‘s the mechanical equivalent of a transistor rapidly switching states.
The Precision Craftsmanship of a Master Machinist
As impressive as the carry mechanism design was, actually building a working model required immense skill and precision. In the 1860s, there was no such thing as computer-aided design, CNC machining, or even widespread interchangeable parts manufacturing. Each component had to be painstakingly crafted by hand using manually operated machine tools like lathes, mills, and presses.
From the patent drawings and descriptions, we know that Johnson used brass for the discs and plates, and steel for the arms, gears, and pivot shafts. He would have had to turn the discs on a lathe, engrave the markings, drill the pivot holes, cut the gear teeth, shape the arms, and much more.
For reliable operation and smooth rotation, the spacing of the 100 holes around the rim of the inner disc had to be extremely precise – likely within a tolerance of a few thousandths of an inch. Any eccentricities would throw off the timing of the carry. The meshing of the carry gears between discs was another crucial interface that had to be tight enough to engage reliably but not so tight as to bind.
Sadly Johnson‘s original production notes and logs have been lost to time, so we can only speculate as to how many prototypes he had to build to perfect the design. But as a master machinist, he would have had an intuitive feel for how mechanisms moved and interacted and could visualize paths of motion in his mind‘s eye. This spatial reasoning, honed over years working with metal, was as essential to his inventing process as the creativity to devise novel mechanisms like his carry system.
After Johnson: The Dawn of Complex Calculators
In the decades following Johnson‘s pioneering work, other inventors took up the baton to design mechanical calculators of increasing complexity and power. Frank S. Baldwin filed a patent just four years later in 1872 for a calculator that extended Johnson‘s basic concept to perform a kind of multiplication through repeated addition. Willgodt Odhner‘s Arithmometer, patented in 1887, introduced a clever pinwheel design that would be widely copied in 20th century calculator designs. But no inventor was more ambitious than Charles Babbage.
While Johnson was tinkering away on his calculating machines in Springfield, across the Atlantic in London, Charles Babbage was dreaming up plans for a massive machine he called the Analytical Engine. Far more complex than any prior design, the Analytical Engine had all the essential components of a modern computer, including input, memory, processing, and output. It even had a concept similar to programming with punched cards.
Babbage worked on the design for decades but sadly it was never fully built in his lifetime due to limitations in manufacturing technology and lack of funding. It wasn‘t until over a century later in 1991 that a working model was finally completed, proving that his brilliant design would have really worked!
Babbage‘s work represented the pinnacle of mechanical computation. By the turn of the 20th century, mechanical calculators had reached their practical limits. Making them larger and more complex ran into issues of reliability, maintainability, and speed. The path forward would have to be electric rather than mechanical.
A portion of Charles Babbage‘s Analytical Engine on display at the Science Museum in London, England. Photo by Geni, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
The Legacy of a Tinkerer Ahead of His Time
So what are we to make of Alonzo Johnson and his marvelous brass contraptions today? In many ways, he was the consummate Yankee tinkerer – a mechanical savant who loved nothing more than dreaming up new devices and then diving into his workshop to build them.
Over his life, Johnson patented at least 10 different inventions, ranging from his calculating machines to nut locks, train car brakes, and a gummed paper making machine. He had an incredibly wide-ranging and playful mechanical mind that was always searching for novel solutions to tricky problems.
But I believe Johnson‘s greatest legacy is as a shining example of the spirit of ingenuity and inventiveness that characterized the best of 19th century America. This was a time when science and technology were rapidly advancing, and the pace of change was dizzying. The western frontier was opening up, the Industrial Revolution was in full swing, and it seemed like anything was possible if you could dream it up and tinker it into existence.
Johnson and his fellow inventors didn‘t have advanced degrees or research labs or venture capital funding. What they had was insatiable curiosity, mechanical aptitude, and a determination to bend metal to their will to create something new under the sun. They were the original "makers" and "hackers" long before those terms took on their modern meanings.
Of course, Johnson could never have imagined how his clever little calculating machines would sow the seeds for the computer revolution to come a century later. Even Babbage‘s far-sighted Analytical Engine was more of a mathematical curiosity than a practical tool in its day. It would take the harnessing of electricity and the invention of the vacuum tube and transistor to kick start the next phase of the computing story.
But that‘s the funny thing about the future – it‘s built on a myriad of small discoveries and innovations that seem quaint or curious in their time but end up enabling wonders beyond imagining. So maybe the 21st century equivalent of Alonzo Johnson is tinkering away right now on some garage project that they think is just a nifty gadget but will one day be seen as the seed of a whole new technological revolution.
Only time will tell, but in the meantime, let us celebrate the spirit of the 19th century maker, so embodied in Johnson, who peered into the realm of mechanical motion and saw a symphony of calculation just waiting to be played. His machines may be museum curiosities now, but his legacy as a computing pioneer and exemplar of the inventive spirit lives on.
[You can see Alonzo Johnson‘s original patent models of his two calculating machines on display at the National Museum of American History in Washington D.C.]
