ON A COLD winter morning a weak battery is unable to start the car engine at once. Continued use of the starter weakens the battery even further.
The autumn leaves blowing along a street pile up behind some small obstacle and so make it larger. More leaves pile up behind the larger obstacle and make it larger still. The same thing happens in winter with driven snow.
Rich people get richer. Once they have started to get rich then they have more money to play around with, their credit is better and their reputation encourages other people to lend them more capital to work with.
Big newspapers get bigger. The bigger the circulation the more advertising they attract. The more advertising they carry the more pages and better features they can afford. The more interesting the paper the bigger the circulation gets.
When people buy stocks and shares in times of inflation the stock-exchange prices rise; so more people want to buy stocks and shares in order to benefit from the price rise; so the price continues to rise.
These are all examples of one type of circular system. This is the explosive type of system which is also known as the positive feedback type. One effect leads through a series of other effects back to itself. So any change is fed back to increase that change. This type of circular system is the ‘change’ type. The examples used so far are all of change in an upward direction, but exactly the same system can increase change in the downward direction.
Outside the welfare state a sick person will get sicker. Being sick he earns less money. Having less money he cannot afford proper food or medical attention, so he gets sicker and that lowers his earning capacity even further.
A small newspaper tends to get smaller. As its circulation falls, so the advertising falls off. So the pages get fewer and they buy less costly features. So the circulation falls further still. When the prices fall on the stock market people tend to sell, and this drives the prices lower so more people sell. There are safeguards to prevent it happening again, but this sort of process was responsible for the great Wall Street crash. Confidence grows by feeding on itself in a circular fashion, but it also diminishes in the same way.
As soon as a few strands start to break in a rope that is supporting weight, then the strain on the remaining strands increases and some of these give way. This puts even more strain on the other strands. And so it goes on until the rope gives way completely.
Several examples of this type of system are to be found in the models already described in this book. In the case of the tilting board with the toppling columns there is an obvious positive feedback: a taller column is more likely to topple over; the more it topples over the taller it is made. In the polythene and pins model, when the weight of water depresses an area of polythene then more water runs into that area, so the area gets even more depressed. Taken as a whole, what happens on the polythene memory-surface is that the patterns shape contours, which then organize incoming patterns, which themselves affect the contours.
If the first type of circular system is the ‘change’ system then the other type is the ‘no change’ system. This is the stabilizing type of system as opposed to the explosive nature of the first type. Here an effect leads through a series of other effects back to itself in the same circular fashion as before. But this time the effect has been reversed by the time it comes back. So a tendency to increase comes back as a tendency to decrease; this counteracts the change and keeps the system stable. This type of system is also known as the negative feedback type.
Bicycles do not ride themselves. They require someone to ride them and convert a tendency to fall over into an action that will counteract this tendency – like a movement of the handlebars. Aeroplanes do not fly themselves. They also require someone at the controls to counteract any tendency to veer off an even keel. The pilot acts as that link in the circular system that reverses the direction of something that is happening and feeds it back to prevent it happening.
If awful Johnny is being naughty in a neighbour’s garden and is picking the heads off the tulips, his mother shouts at him. Johnny stops, one hopes, and the tulip system is stabilized. The decapitated tulips have acted via Johnny’s mother to prevent further decapitation.
Water rising in the lavatory cistern lifts the ballcock and turns the water off. Thus the rise in water level is used to prevent a further rise in water level.
These are the two basic types of circular system, and their operation is straightforward. It is when these basic systems are combined that interesting things start to happen. Most of the behaviour of the special memory-surface described in this book depends on circular system effects.
In dealing with systems of this sort it is useful to have some sort of visual notation so that one can follow what happens without having to imagine the circular effects in one’s head. A very simple notation is shown here.
Any one thing which tends to make a second thing go in the same direction as itself is connected to the second thing by a line, which is interrupted by a solid circle. The direction of effect is shown by an arrow. If the effect can also act in the reverse direction, then there has to be a separate line and a separate arrow. If the first thing tends to get bigger, then the second thing will also tend to get bigger. If the first thing tends to get smaller, then the second thing will also tend to get smaller.
When the first thing has an opposite effect on the second thing then the connecting line is interrupted by an open circle. Thus when the first thing gets bigger the second thing gets smaller.
When the first thing gets smaller the second thing gets bigger.
In other words the filled circle indicates an exciting increase effect, and the open circle an inhibiting decrease effect.
Suppose there was an area in which there were plenty of good jobs unfilled. At first people from another area might be reluctant to move into the opportunity area, as they would feel lonely without their friends and neighbours. A few people would move anyway. Once a few people had moved then it would be easier for others to move as well. Once others had moved it would become even easier for still others to move in. This increasing influx of workers would gradually saturate the labour market, and the work prospects which had been so good would become less and less good. Finally the work opportunities would decline to such an extent that people would actually start to move out of the area. The number of those moving in would also decrease.
In this situation there are two circular systems working alongside each other. In the first system, the more people who move to the opportunity area, the more attractive it becomes for their friends. In the second system, the more people who move to the area, the worse the work opportunities become and the less attractive the area becomes. One system is of the positive feedback type and the other is of the negative feedback type. Since work is the dominant factor the stabilizing system will prevail, and the situation will settle down with a limited number of people in the area. That is the basic behaviour of the system (see (a) here). The most fascinating thing about this system is that it is a selective one. As the work opportunities get less the people who are already there but are not desperate for jobs and can find these anywhere will tend to leave. Meanwhile only the most desperate will continue to move into the area. The number of people in the area will remain stable as some move out and some move in. But the character of the people will change, for the easygoing ones move out and the desperate ones move in. Thus the area will come to be filled with those who are desperate for work because they cannot find it anywhere else. The system, as it were, selects out these people. One could also describe the system in terms of skilled and unskilled workers, with the unskilled ones being forced out as the skilled ones moved in.
Another example of the same type of system can be seen in a party given by undergraduates in Cambridge on the bank of the river towards the end of the summer term. Towards the end of the party a group of people stand watching on the bank as some undergraduates get into a punt. Those in the punt call to their friends on the bank to join them. At first the friends are quite happy to do so. But as more and more people get into the punt it gets lower and lower in the water until it seems very likely to sink. At this point some of the more sober and timid people start to leave the punt while their more drunk and carefree fellows are still clambering aboard. In the end the punt is filled only by the jollier undergraduates (see (b) here).
Again the system is a selecting one. On one hand there is the positive inducement of getting onto the punt to join one’s friends, but on the other hand there is the negative inducement that the punt is about to sink. The relative strengths of the positive and negative inducements vary with the state of sobriety of each undergraduate. This means that the more inhibited undergraduates get pushed out and the more excitable ones are left. This is the type of system that provides the special memory-surface with selective capacity.
Fig. 16b
The behaviour of the systems described above can be imitated with a simple mechanical model. This model is shown here. It consists of a flap which is attached to the wall by a hinge in such a way that the flap can fold downwards. The flap is, however, supported by a light spring which keeps it level. Small lead blocks are now put onto the platform one by one. As more blocks are added, the platform tilts more and more until some of the blocks start to slide off. Eventually only the blocks with the stickiest bottoms will be left on the platform. In this system, the more blocks there are on the platform, the more difficult it becomes for any block to stay on the platform. The presence of blocks on the platform inhibits the presence of blocks on the platform. But there is an artificial rule which states that the presence of blocks on the platform encourages the presence of other blocks on the platform: that is why they are placed there.
The system is again a self-selecting one. It would not be the same to place the blocks on a sloping surface in order to see which slid down and which did not. This would be selection by an outside agency which adjusted the slope of the surface. The lead blocks which do remain on the platform are the ones with the stickiest bottoms, just as the group in the punt was made up of the drunker undergraduates, and the group in the work area was made up of the most work-hungry people.
All three examples of the system show two effects:
1. A limited number of units in the designated area.
2. Selection of these units according to some quality which they possess to a greater extent than the others.
Fig. 18
The discriminating process can be quite fine. If a block that is added to the platform is only slightly more sticky than one of those already there, the platform will tilt just enough for the other one to slide off no matter how small the difference may be.
