The Wood Computer

by Jo Brodie, Queen Mary University of London

A wooden woodland bridge with trees in the background.
Image by Peter H from Pixabay

Punch cards inspired Babbage as he invented the first Victorian computer, and were a way the first computers stored data a hundred years later. Variations, called edge-notched cards, had their uses before the first working computers, though. They provided an efficient way to look up information. One use was to help identify timber: Oxford’s human-operated ‘wood computer’ was used in forests world-wide.

Interested in nature and enjoying a nice walk, you come across an unfamiliar tree, and want to identify it. How do you do it? You might work through a set of questions, first looking at the leaves: what shape are they, what colour, do they have stalks, do they sit opposite each other on a twig or are they diagonally placed, and so on. You then move to questions about the bark… Gradually, you narrow it down to one tree. What, though, if your job is to check that your company is buying the right timber and the tree is cut up into logs (no leaves or bark)? The task is the same, going through a checklist of questions, just harder unless you are an experienced botanist. Now you consider things like the pattern of the grain, the hardness, the colour and any scent from the tree’s oils.

Historically, one way of working out which piece of timber was in front of you was to use a wood identification kit or ‘wood computer’. This was prepared (programmed!) from a pack of index cards with 60 or more features of timber printed on them. However, they weren’t just cards to read but cards to compute with.

Holes and notches

The cards were special because they had regularly placed holes round all four sides. Each card had notches cut into different holes. Each feature of the timber was linked to one or more holes. The features were grouped together around related properties: so, for example, all the possible colours of timber might be grouped together on one section of the card. Properties about how fine-grained the timber is would be grouped in another section.

Each card represented one type of wood and the ‘programmer’ of the cards would notch the holes next to the features that defined it. If a particular type of timber was fine-grained you would add the notch to the hole next to “fine-grained”, if it wasn’t that hole would be left un-notched. Notches were added for all relevant timber properties making each card unique, with a slightly different pattern of notches, uniquely describing the features of the tree it represents. (See an example of an edge-notched card below.)

How it works

To use a wood computer, take the pile of cards, pick a feature of the timber in front of you and insert a thin knitting needle into the hole linked to the feature. Then lifting the pile up shake out any cards with notches in that hole. All of the cards for timber that don’t have that feature will have an un-notched hole and will hang from your knitting needle. All cards representing timber that does have that feature are now sitting on the table. Yours is somewhere amongst them. If your timber is NOT fine-grained then instead, when you put the knitting needle in the fine-grained hole, keep those left on the knitting needle.

You repeat the process several times to whittle (sorry!) your cards down, each time choosing another feature of the timber in front of you. Eventually you have only one card left and have identified your timber.

Just the cards for the job

The cards are incredibly low tech, requiring no electricity or phone signal and are very easy to use even without specialist botanical knowledge. All the knowledge is programmed into them. You also find the answer very quickly. Margaret Chattaway, a botanist at the Imperial Forestry Institute, Oxford, in the 1930s realised that was exactly what was needed for their team inspecting timber and so the original wood computer was created.

So next time you are out for a walk, make sure you have your knitting needle and a suitable pile of cards with you. Then identifying trees, birds, fungi or even animal poo will be so much quicker and simpler.

An edge-notched card for Bob Marley and the Wailers notched as a Reggae Group with initials B-M-W: those three letters are notched, the other letters are un-notched.

Why not create an edge notched card system for something you are interested in, for identifying birds perhaps or quickly finding details of films, or music or of something you collect?

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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.

Babbage’s barrels

by Adrian Johnstone, Royal Holloway, University of London and Paul Curzon, Queen Mary University of London

Workings of a music box with bumps on a barrel flicking metal prongs
Image by Monika Schröder from Pixabay

Despite his hatred of Barrel organs, Babbage used barrels with relocatable pins in his machines. They worked in a similar way to a music box, where the pins flip the teeth of a metal comb to sound a note and by moving the pins you get a different tune. In Babbage’s version the barrel’s pins push levers that send information round the machine, determining what it does.

By programming the positions of the pins, different overall operations are created from the combinations of lever pushes. This is a similar idea to what later became called microcoding, in modern computers, where very simple low level instructions are used to program the operations available in a computer’s instruction set.


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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.

Ada and the music machine

by Paul Curzon, Queen Mary University of London

A man playing a barrel organ with a soft toy monkey.
Image by Holger Schué from Pixabay

Charles Babbage found barrel organs so incredibly irritating that he waged a campaign to clear them from the streets, even trying to organise an act of parliament to have them banned. Presumably, it wasn’t the machine Babbage hated but the irritating noise preventing him from concentrating: the buskers in the streets outside his house constantly playing music was the equivalent to listening to next door’s music through the walls. His hatred, however, may have led to Ada Lovelace’s greatest idea.

It seems rather ironic his ire was directed at the barrel organ as they share a crucial component with his idea for a general purpose computer – a program. Anyone (even monkeys) can be organ grinders, and so play the instrument, because they are just the power source, turning the crank to wind the barrel. Babbage’s first calculating machine, the Difference Engine was similarly powered by cranking a handle.

The barrel itself is like a program. Pins sticking out from the barrel encode the series of notes to be played. These push levers up and down, which in turn switch valves on and off, allowing air from bellows into the different pipes that make the sounds. As such it is a binary system of switches with pins and no pins round the barrel giving instructions meaning on or off for the notes. Swap the barrel with one with pins in different positions and you play different music, just as changing the program in a computer changes what it does.

Babbage’s hate of these music machines potentially puts a different twist on Ada Lovelace’s most visionary idea. Babbage saw his machines as ways to do important calculations with great accuracy, such as for working out the navigation tables ships needed to travel the world. Lovelace, by contrast, suggested that they could do much more and specifically that one day they would be able to compose music. The idea is perhaps her most significant, and certainly a prediction that came true.

We can never know, but perhaps the idea arose from her teasing Babbage. She was essentially saying that his great invention would become the greatest ever music machine…the thing he detested more than anything. And it did.


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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.

Ant Art

by Paul Curzon, Queen Mary University of London

(from the archive)

The close up head of an ant staring at you
Image by Virvoreanu Laurentiu from Pixabay 

There are many ways Artificial Intelligences might create art. Breeding a colony of virtual ants is one of the most creative.

Photogrowth from the University of Coimbra does exactly that. The basic idea is to take an image and paint an abstract version of it. Normally you would paint with brush strokes. In ant paintings you paint with the trails of hundreds of ants as they crawl over the picture, depositing ink rather than the normal chemical trails ants use to guide other ants to food. The colours in the original image act as food for the ants, which absorb energy from its bright parts. They then use up energy as they move around. They die if they don’t find enough food, but reproduce if they have lots. The results are highly novel swirl-filled pictures.

The program uses vector graphics rather than pixel-based approaches. In pixel graphics, an image is divided into a grid of squares and each allocated a colour. That means when you zoom in to an area, you just see larger squares, not more detail. With vector graphics, the exact position of the line followed is recorded. That line is just mapped on to the particular grid of the display when you view it. The more pixels in the display, the more detailed the trail is drawn. That means you can zoom in to the pictures and just see ever more detail of the ant trails that make them up.

You become a breeder of a species of ant

that produce trails, and so images,

you will find pleasing

Because the virtual ants wander around at random, each time you run the program you will get a different image. However, there are lots of ways to control how ants can move around their world. Exploring the possibilities by hand would only ever uncover a small fraction of the possibilities. Photogrowth therefore uses a genetic algorithm. Rather than set all the options of ant behaviour for each image, you help design a fitness function for the algorithm. You do this by adjusting the importance of different aspects like the thickness of trail left and the extent the ants will try and cover the whole canvas. In effect you become a breeder of a species of ant that produce trails, and so images, you will find pleasing. Once you’ve chosen the fitness function, the program evolves a colony of ants based on it, and they then paint you a picture with their trails.

The result is a painting painted by ants bred purely to create images that please you.


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EPSRC supports this blog through research grant EP/W033615/1, and through EP/K040251/2 held by Professor Ursula Martin. 

Diamond Dogs: Bowie’s algorithmic creativity

by Paul Curzon, Queen Mary University of London

(Updated from the archive)

Bowie black and white portrait
Image by Cristian Ferronato from Pixabay

Rock star David Bowie co-wrote a program that generated lyric ideas. It gave him inspiration for some of his most famous songs. It generated sentences at random based on something called the ‘cut-up’ technique: an algorithm for writing lyrics that he was already doing by hand. You take sentences from completely different places, cut them into bits and combine them in new ways. The randomness in the algorithm creates strange combinations of ideas and he would use ones that caught his attention, sometimes building whole songs around the ideas they expressed.

Tools for creativity

Rather than being an algorithm that is creative in itself, it is perhaps more a tool to help people (or perhaps other algorithms) be more creative. Both kinds of algorithm are of course useful. It does help highlight an issue with any “creative algorithm”, whether creating new art, music or writing. If the algorithm produces lots of output and a human then chooses the ones to keep (and show others), then where is the creativity? In the algorithm or in the person? That selection process of knowing what to keep and what to discard (or keep working on) seems to be a key part of creativity. Any truly creative program should therefore include a module to do such vetting of its work!

All that, aside, an algorithm is certainly part of the reason Bowie’s song lyrics were often so surreal and intriguing!


Write a cut-up technique program

Why not try and write your own cut-up technique program to produce lyrics. You will likely need to use String processing libraries of whatever language you choose. You could feed it things like the text of webpages or news reports. If you don’t program yet, do it by hand cutting up magazines, shuffling the part sentences before gluing them back together.


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This blog is funded by UKRI, through grant EP/W033615/1.

The algorithm that could not speak its name

by Paul Curzon, Queen Mary University of London

(Updated from the archive)

Image by PIRO4D from Pixabay 

The first program that was actually creative was probably written by Christopher Strachey, in 1952. It wrote love letters…possibly gay ones.

The letters themselves weren’t particularly special. They wouldn’t make your heart skip a beat if they were written to you, though they are kind of quaint. They actually have the feel of someone learning English doing their best but struggling with the right words! It’s the way the algorithm works that was special. It would be simple to write a program that ‘wrote’ love letters thought up and then pre-programmed by the programmer. Strachey’s program could do much more than that though – it could write letters he never envisaged. It did this using a few simple rules that despite their simplicity gave it the power to write a vast number of different letters. It was based on lists of different kinds of words chosen to be suitable for love letters. There was a list of nouns (like ‘affection’, ‘ardour’, …), a list of adjectives (like ‘tender’, ‘loving’, …), and so on.

It then just chose words from the appropriate list at random and plugged them into place in template sentences, a bit like slotting the last pieces into a jigsaw. It only used a few kinds of sentences as its basic rules such as: “You are my < adjective > < noun >”. That rule could generate, for example, “You are my tender affection.” or “You are my loving heart”, substituting in different combinations of its adjectives and nouns. It then combined several similar rules about different kinds of sentences to give a different love letter every time.

Strachey knew Alan Turing, who was a key figure in the creation of the first computers, and they may have worked on the ideas behind the program together. As both were gay it is entirely possible that the program was actually written to generate gay love letters. Oddly, the one word the program never uses is the word ‘love’ – a sentiment that at the time gay people just could not openly express. It was a love letter algorithm that just could not speak its name!

You can try out Strachey’s program [EXTERNAL] and the Twitter Bot loveletter_txt is based on it [EXTERNAL] Better still why not write your own version. It’s not too hard.

Here is one of the offerings from my attempt to write a love letter writing program:

Beloved Little Cabbage,

I cling to your anxious fervour. I want to hold you forever. You are my fondest craving. You are my fondest enthusiasm. My affection lovingly yearns for your loveable passion.

Yours, keenly Q

The template I used was:

salutation1 + ” ” + salutation2 + “,”

“I ” + verb + ” your ” + adjective + ” ” + noun + “.”

“You are my ” + noun + “.”

“I want ” + verb + ” you forever.”

“I ” + verb + ” your ” + adjective + ” ” + noun + “.”

“My ” + noun1 + ” ” + adverb + ” ” + verb  + ” your ” + adjective + ” ” + noun2 + “.”

“Yours, ” + adverb + ” Q”

Here characters in double quotes stay the same, whereas those that are not in quotes are variables: place holders for a word from the word associated word list.

Experiment with different templates and different word lists and create your own unique version. If you can’t program yet, you can do it on paper by writing out the template and putting the different word lists on different coloured small post-it notes. Number them and use dice to choose one at random.

Of course, you don’t have to write love poems. Perhaps, you could use the same idea for a post card writing program this summer holiday…


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This blog is funded by UKRI, through grant EP/W033615/1.

I Ching binary

by Paul Curzon, Queen Mary University of London

Bright green bamboo stalks on a brihgt green background
Image by Sushuti from Pixabay

I Ching the ancient Chinese divination text, several thousand years old, is based on a binary pattern…

I Ching is one of the oldest Chinese texts. The earliest copies date from around 3000 years ago. It uses 64 hexagrams: symbols consisting of six rows of lines (see right). Each row is either a solid horizontal line or two shorter lines with a gap in the middle. The 64 hexagrams are all the possible symbols that can be made from six rows of lines in this way. In I Ching, they each represent possible predictions about the future (a bit like horoscopes). To use I Ching, a series of hexagrams were chosen. This was done in some unknown but random way, using stalks of the Yarrow plant, standing in for dice. The chosen hexagrams then told the person something about their future.

In the earliest versions of I Ching, the order of the hexagrams suggests that they were not thought of as numbers as such. However, in a later version, from around 1000 AD the order in which they appear is different. Thought to be written by a Chinese scholar, Shao Y\0x014Dng, it is this version that Leibniz was given and that aroused his interest because the order of the hexagrams follows the pattern we know of as binary (see Predicting the future). Shao Y\0x014Dng had apparently picked the sequence deliberately because of the binary pattern, so understood it as a counting sequence, if not necessarily how to do maths with it.

How do the hexagrams correspond to binary? It is not in the lines themselves but the pattern of line breaks down the middle that matters. Think of a break in the lines as a 0 (yin) and no break as a 1 (yang). The order, as Leibniz realised, is a counting system, equivalent to our decimals but where you only have two digits (0 and 1) rather than our ten digits (0…9).

I Ching Hexagrams for numbers 0 to 7
I Ching Hexagrams for numbers 0 to 7

Whereas in decimal each column of a number like 123 represents a power of 10 (ones, tens, hundreds, …) in binary each column represents a power of 2 (ones, twos, fours, eights, …). To work out the value that the number represents you multiply each digit by its column value and add the results. So in decimal, 123 represents one hundred plus two tens plus three ones (one hundred and twenty three). 1011 in decimal represents one thousand plus no hundreds plus one ten plus a one (one thousand and eleven). In binary, however, 1011 represents instead one eight plus no fours plus one two plus a one (8+0+2+1) so eleven. It is just a different way of writing down numbers.

Investigating the I Ching pattern helped Leibniz to work out the mathematics of binary arithmetic and on to thinking about machines to do calculations using it.


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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.

Predicting the future: marble runs, binary and the I Ching

by Paul Curzon, Queen Mary University of London

Image by Sergio Camacho Camacho S. A. from Pixabay

Binary is at the core of the digital world, underpinning everything computers do. The mathematics behind binary numbers was worked out by the great German mathematician Gottfried Wilhelm Leibniz in the 17th century. He even imagined a computing machine a century before Babbage, and two centuries before the first actual computers. He was driven in part by an ancient Chinese text used for divination: predicting the future.

I Ching

Leibniz was interested in the ancient Chinese text, the I Ching because he noticed it contained an intriguing mathematical pattern. The pattern was in the set of 64 symbols it used for predicting the future. The pattern corresponded to the counting sequence we now know of as binary numbers (see I Ching binary). Leibniz was obviously intrigued by the patterns as a sequence of numbers. Already an admirer of the great Chinese philosopher, Confucius, he thought that the I Ching showed that Chinese philosophers had long ago thought through the same ideas he was working on. Building on the work of others who had explored non-decimal mathematics, he worked out the maths of how to do calculations with binary: addition, subtraction, multiplication, and division as well as logical operations such as ‘and’, ‘or’ and ‘not’ that underpin modern computers.

Algorithms embedded in machines

Having worked out the mathematics and algorithms for doing arithmetic using binary, he then went further. He imagined how machines might use binary to do calculations. He also created very successful gadgets for doing arithmetic in decimal, but saw the potential of the simplicity of the binary system for machines. He realised that binary numbers could easily be represented physically as patterns of marbles and the absence of marbles. These marbles could flow along channels under the power of gravity round a machine that manipulated them following the maths he had worked out.

His computer would have been a giant marble run!

A container would hold marbles at the top of a machine. Then, by opening holes in its base above different channels, a binary number could be input into the machine. An open hole corresponded to a 1 (so a marble released) and a closed hole corresponded to a 0 (no marble). The marbles rolled down the channels with each channel corresponding to a column in a binary number. The marbles travelled to different parts of the machine where they could be manipulated to do arithmetic. They would only move from one column to another as a result of calculations, such as carry operations.

Addition of digits in binary is fairly simple: 0+0 = 0 (if no marbles arrive then none leave); 0+1 = 1+0 = 1 (if only one marble arrives then one marble leaves; (1+1 = 2 = 10) if two marbles arrive in a channel then none leave that channel, but one is passed to the next channel (a carry operation). The first rules are trivial, open the holes and either a marble will or will not continue. Adding two ones is a little more difficult but Leibniz envisioned a gadget that did this, so that whenever two marbles arrived in a channel together, one was discarded, but the other passed to the adjacent channel. By inputting two binary numbers into the same set of channels connected to a mechanical gadget doing this addition on each channel, the number emerging is a new binary number corresponding to the sum.

Multiplication by two can be done by shifting the tray holding the number along a place to the left. In decimal, to multiply a number by ten, we just put a 0 on the end. This is equivalent to shifting the number to the left: 123 becomes 1230. In binary the same thing happens when we multiply by 2: put a 0 on the end so shift to the left and the binary number is twice as big (11 meaning 2+1 = 3 becomes 110 meaning 4+2+0 = 6). Multiplication of two numbers could therefore be done by a combination of repeated shifts of the marbles of one number, releasing copies of it or not based on the positions of the 1s in the other number. The series of shifted numbers were then just added together. This is just a simplified version of how we do long multiplication.

To multiply 110 by 101, you multiply 110 by each digit of 101 in turn. This just involves shifts, and then discarding or keeping numbers. First multiply 110 by 1 (the ones digit of 101) giving 110. Write it down. Now shift 110 one place to give 1100 and multiply by 0 (the twos digit of 101). This just gives 0000. Write it below the previous number. Shift 110 by another place to give 11000. Multiply it by 1 (the fours digit of 101). That gives 11000. Write it down below the others. Add all three numbers to get the answer (see box).

The basics of a computer

Leibniz had not only worked out binary arithmetic, the basis of a computer’s arithmetic-logical unit (ALU), he had also seen how binary numbers could flow around a machine doing calculations. Our computers use pulses of electricity instead of marbles, but the basic principles he imagined are pretty close to how modern computers work: binary numbers being manipulated as they flow from one computational unit to the next. Leibniz didn’t make his machine, it was more a thought experiment. However, helped by I Ching, a book for divining the future, he did essentially predict how future computers would work.


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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.

Mary Coombs, teashops and Leo the computer

by Jo Brodie, Queen Mary University of London

Image by Pexels from Pixabay

Tea shops played a surprisingly big role in the history of computing. It was all down to J. Lyons & Co., a forward thinking company that bought one of the first computers to use for things like payroll. Except they had a problem. Computers need programs, but no such programs existed, and neither did the job of programmer. How then to find people to program their new-fangled computer? One person they quickly found, Mary Coombs, suited the job to a T, becoming the first female commercial programmer.

J Lyons and Company, a catering company with a chain of over 200 tea shops in London, wanted to increase its sales and efficiency. With amazing foresight, they realised computers, then being used only for scientific research in a few universities, would help. They bought the technology from Cambridge University, built their own and called it LEO (the Lyons Electronic Office). They hoped it would do calculations much more quickly than the 1950s clerks could, using calculating machines. But it could only happen if Lyons could find people to program it. At the time there were only a handful of people in the world who were ‘good at computers’ (programmer didn’t exist as a job yet) so instead they had to find people who could be good with computers and train them for the job. Lyons created a Computer Appreciation Course which involved a series of lectures and some homework, all designed to find staff within the company who could think logically and would learn how to write programs for LEO.

One of those was Mary Coombs. Born in 1929, she studied at Queen Mary University of London in the late 1940s. You might think, given that this is about a computer, that she studied computer science, but she actually studied French and History. She couldn’t study computer science: what we’d call a computer science course didn’t exist. There wasn’t one anywhere until 1953, when the University of Cambridge opened a Diploma in Computer Science.

By then, Mary was already working at Lyons. She’d had a holiday job there in 1951, as a clerk (in the Ice Cream Sales department) as she finished her degree. A year later she returned to the company where her career changed direction. In addition to her language skills, she was good at maths so transferred to Lyon’s Statistical Office. That’s where she heard about LEO and the need for programmers to learn about it and help test it as it was being built and refined. Intrigued, she signed up for the company’s first computer appreciation course, did well, and was one of only two people on the course then offered a job on the project. As a result she became the first woman to work on the LEO team as a programmer and the first female commercial programmer in the world.

LEO was an enormous computer, built from several thousand valves, and took up an entire room, though it could only store a few kilobytes of memory. It was also a little temperamental. It needed to be very reliable if it was going to be of any use, so it underwent months of testing and improvement, with Mary’s help, before it was put to work on solving real problems, again with Mary and others on the team writing the programs for everything it did.

One of its first tasks was to make sure everyone got paid! LEO was able to calculate forty people’s payslips in one minute (one every 1.5 seconds) where previously it would have taken one clerk six minutes to do one: a huge improvement in efficiency for Lyons.

LEO was both pioneering and a big success, but the real pioneers were the programmers like Mary. They turned computers, intended to help scientists win Nobel prizes, into ones that helped businesses run efficiently, ensuring people got paid. Obvious now, but remarkable in the 1950s.


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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.

A custard computer

by Paul Curzon, Queen Mary University of London

This simplistic custard contraption is inspired by a more sophisticated custard computer invented by Adrian Johnstone, Royal Holloway, University of London.

Custard dripping downwards
Image by Bruno / Germany from Pixabay

Imagine a room-sized vat of custard suspended from the ceiling. Below are pipes, valves and reservoirs of custard. At the bottom is a vast lake where the custard collects as it splurges out of the pipes. A pump sucks custard back up to the vat on the ceiling once more. Custard flows, sits, splurges…all the while doing computation.

Babbage worked out how to make a computer using wheels. How might you make a general purpose digital computer out of custard? (It sounds more fun!) Adrian Johnstone at Royal Holloway has designed one and if built it would look something like our above description, like something from Willie Wonka’s chocolate factory.

Here we give a slightly simplistic version. The first step is to have something to represent 0 and 1. That’s easy with custard: no custard in a storage tank is a 0 and custard is a 1. Out of that you can represent numbers using collections of such tanks: lots of tanks containing custard or no custard, with a code (binary) giving them meaning as numbers.

Once you have a way to represent numbers, the next step to making a computer is to make the equivalent of transistors. Transistors are just switches, but ones that revolutionised electronics to the point that they have been hailed as one of the greatest inventions ever. Starting with humble transistors, computers (and lots more) can be built.

Transistors have three inputs. One acts as the data input, or source, connected ultimately to the source of the current (in our case the vat of custard). Another, the drain, connects ultimately to the place the current drains to (in our case the lake of custard). A third input is the gate. It switches the transistor on and off, either allowing current (custard) to flow towards the drain or not. The gate thus acts as the switch to allow custard to flow.

One way to make a custard transistor would be to use a contraption based on your toilet but full of custard (don’t think about that too much). Look in your toilet cistern and see how it switches water on and off when you flush the toilet to get the idea. For a (custard) transistor, have a small tank of custard with a ball floating on the surface. It acts as the gate. Fastened to the end of the ball is a lever The lever’s other end can push up against the end of the pipe that runs from source to drain, blocking the flow. When the tank is full of custard it pushes the other end of the lever down, letting custard flow. If the tank empties then the ball drops, so the other end of the lever rises and blocks the flow.

Transistors have been hailed as one of the greatest inventions ever.

There are two kinds of transistors. They differ in that the gate just operates in opposite fashions. With one kind, custard can flow from source to drain only when there is current (custard) at the gate (as above). In the other, custard flows only when there is no custard at the gate.

Transistor circuit symbol

Once you have (custard) transistors, you can make (custard) logic gates (NOT, AND, OR,…). A (custard) NOT, for example, would need to let custard into its out pipe only if there were no custard on its input pipe (and vice versa). We can do this using a transistor with the NOT circuit’s input connected to the gate, and where custard flows only when the gate has no custard. The drain of the transistor becomes the output of the NOT circuit. The source of the transistor connects to the vat of custard to provide the custard that will flow when the transistor switches on. When custard arrives at the gate which is acting as a switch, it stops custard flowing to the drain, and vice versa, as required.

AND logic needs to let custard out only when there is custard at both its input pipes. OR logic allows custard through when there is custard at either of its input pipes. This can be done with appropriate plumbing together of the transistors as follows.

Diagram of a custard transistor
A custard transistor: when there is
custard at the gate, custard flows
from source to drain. When no
custard is at the gate, the floating ball
drops and closes the link.

A (custard) AND uses two transistors It allows custard to flow when there is custard at both gates which are the input pipes of the AND circuit. Connect one input of the AND circuit to the gate of the first transistor with the source connected to the vat of custard. Connect its output to the source of the second transistor. The gate of this second transistor is linked to the second input of the AND. Custard will flow from the vat down towards the drain only when there is custard at both gates. If either gate has no custard, then the custard will not flow, just as required for custard AND logic. We will leave you to work out how to make (custard) OR logic.

Once you can create gadgets that do (custard) NOT, AND and OR, you can then start to build more interesting circuits by combining them: building up the components of a computer like (custard) adders and (custard) multipliers, circuits that compare numbers and ones that trigger custard to be moved about… put it together in the right way and you can build a computer with control unit, arithmetic logic unit, memory unit and so on… (as long as you have enough custard).

Out of the glooping vat of custard, computation emerges….Would it really work? You would have to build it to find out!


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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.