Video: Inspecting the Reading of a Thermometer

The diagram shows three different angles at which the temperature shown on a thermometer could be read. From which angle is it best to read the thermometer to get an accurate reading?

03:27

Video Transcript

The diagram shows three different angles at which the temperature shown on a thermometer could be read. From which angle is it best to read the thermometer to get an accurate reading?

All right, so we see in this diagram these three potential angles A, B, and C for positioning one’s eye to read off the temperature on this analogue thermometer. From angle A, our eye is looking down on the level of the liquid in the thermometer. From angle B, our eye is on level with that liquid. And from angle C, our eye is below that liquid level. So we’re looking up at it. Our question asks, from which of these three angles is it best to read the thermometer in order to get an accurate reading?

To see which angle of observation will give us the most accurate temperature reading, let’s take a very close up, zoomed-in view on the level of the liquid in our thermometer. That view looks something like this, where the level of liquid in our thermometer is on line with a smaller hash mark. And that’s one hash mark below 20. And if we look carefully at the top of our thermometer, we see that this thermometer measures temperature in degrees Celsius. Therefore, this 20 that we see here indicates a temperature of 20 degrees Celsius. Now, we don’t know the particular liquid that this thermometer is filled with. It might be mercury; that’s one common type. Or it might be ethanol, a type of alcohol. Or it could be something else.

But the important thing to notice is that whatever the liquid is, it doesn’t run up to the edge of our thermometer. That is, there’s this gap that we’ve just drawn in between where the liquid actually rises and falls in a column through the center of the thermometer and the place on the outside of that thermometer where the temperatures are marked in. This distance, this gap, is very important because it can affect the accuracy of our reading.

For example, consider what would happen if our eye was at the position marked out as A. In that case, our line of sight drawn in on our zoomed-in diagram might look very much like this. That is, our eye is over here looking down on the liquid in our thermometer. And this would mean that our line of sight, when it looks at the level of the liquid in the thermometer, passes through the marking identified as 20 degrees Celsius. So if our eye was up here, if it was around position A, then we might say the thermometer is reading a value of 20 degrees Celsius. But from our up-close view, we can see that that’s not accurate.

Now, let’s consider putting our eye at the lower position at angle C. In this case, again using our zoomed-in view, our line of sight might look very much like this. That is, our eye is positioned here, looking up at the level of the liquid. And from this position, we see the liquid level crossing here on our thermometer. Now, if we look at the big scale view of our thermometer, we see that each one of these smaller tick marks indicates a change in temperature of one degree Celsius. Which means that this tick mark here represents 18 degrees Celsius. It’s two tick marks below 20.

So if our eye was in position A, we might record the temperature as 20 degrees Celsius. And if it was in position C, we might record 18 degrees Celsius. But neither of these we can see is accurate. The better reading would be on level with the liquid in our thermometer. That is, it’s best for our eye to be here on that level. So that the distance gap between the liquid in the center of our thermometer and the thermometer markings on the outside doesn’t lead to a distorted or inaccurate reading. So we’ll say it’s from angle B that this thermometer should be read in order to get an accurate reading.

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