3.2 Analogue things

In I claimed that we lived in an analogue world, but put off defining what the term ‘analogue’ means. Here is a starting point: analogue quantities are ones that change continuously.

It is easy to misread this definition. What it does not mean is that analogue quantities are ones that change continually, or all the time. So what does it mean? We need an example.

Figure 3 shows part of a simple thermometer, which measures temperature, an analogue quantity. Most thermometers still use the length of a column of mercury or coloured alcohol to indicate the temperature which we read off a scale beside the column.

Figure 3 A simple analogue thermometer

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Image showing the scale on a simple analogue thermometer.

Figure 3 A simple analogue thermometer

Now focus on a small part of the temperature scale in Figure 3, the gap between 14°C and 15°C, say. Are there possible temperatures between these two values? Of course there are: 14.5°C lies between the two, along with many other values. What about the gap between 14.5°C and 15°C? Once again, there are countless possible temperatures between these two quantities. In fact, however small the gap between any two temperatures there will always be more possible temperatures inside that gap.

Now imagine that the sun has risen and is starting to warm the room in which we've put our thermometer. The temperature starts to rise. If you stare fixedly at the thermometer while the room is getting warmer, you would never see the temperature suddenly leap from 14°C to 15°C, or from 14°C to 14.5°C. Instead the temperature rises smoothly and continuously, with no gaps or jumps. In moving from 14°C to 15°C it passes steadily through all of the infinite number of possible temperatures between the two. This is what we mean when we say the temperature is an analogue quantity.

Just to press the point, here is another example. Most of you will own a radio or stereo with a mechanism for controlling the volume. Usually this is just a simple knob, as in Figure 4, which is rotated clockwise to increase the volume. Now, if you put on your favourite piece of music and slowly turn the control from low to deafening, what happens? The intensity of the sound does not increase in a series of jumps but, as with the temperature, goes up smoothly and progressively. Volume, too, is an analogue quantity.

Figure 4 An analogue volume control

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Image of a simple circle volume control with a ‘lo’ and ‘hi’ position.

Figure 4 An analogue volume control

If you ponder for a minute about the world we experience, you can see the justification for calling it an analogue world. We live in a world of light and sound, touches, smells and tastes. I will look more closely at the nature of some of these later in the course. However, at this stage it is easy to see that they are mostly analogue things.