Question Video: Measuring a Length Using a Micrometer Physics

What is the reading on the micrometer shown in the diagram?


Video Transcript

What is the reading on the micrometer shown in the diagram?

We can find this reading by looking at the two scales on a micrometer, both of which are shown in this diagram. The vertical markings on the left are the main scale, while the horizontal markings on the right are the rotating scale. The main scale measures in millimeters, while the rotating scale, also called the thimble scale, measures in fractions of a millimeter.

Now, in order to get a reading on a micrometer, we have to start by reading the main scale first. These long labeled lines on the main scale represent a full millimeter, with the lines in between those representing half millimeters. So, for example, this line here represents 1.5 millimeters. To get a reading on the main scale, we look at which lines are closest to the long vertical line that separates the main scale from the rotational scale. For this reading, this line happens to be the closest line, which represents 2.5 millimeters. So, the reading that we have for the main scale is 2.5 millimeters.

Now that we have this, let’s move on to the rotating scale. Each line on the rotating scale represents a whole number, with labeled lines every five numbers. The numbers on a rotating scale go from one to 50, with each one on the rotating scale being equal to one one hundredth of a millimeter. The reason the scale goes from one to 50 and not one to 100 even though each one represents one one hundredth of a millimeter is because what the rotating scale is actually measuring is the space in between the half-millimeter markings. So, the rotating scale only has to measure up to one-half of a millimeter, not the distance between the full millimeters here. It doesn’t need the full one-millimeter distance, and so it doesn’t needed 100.

To see what we mean, let’s look at our actual reading on our rotating scale. To determine which of these lines to read, we have to figure out which one is closest to the main scale’s bottom horizontal line. For this reading, we can see that the 25 line is just barely closer than the 26 line. Therefore, the reading on a rotational scale is 25. 25 on the rotating scale is equal to twenty-five one hundredths of a millimeter, which in decimal form is equal to 0.25 millimeters, which we can store up here.

Now that we have readings for both the main scale and the rotating scale, we just add them together. 2.5 millimeters plus 0.25 millimeters equals 2.75 millimeters. We can see from this answer that if the numbers went past 50 on the rotating scale and our reading gave us some number like, say, 65, which would indicate 0.65 millimeters on the rotational scale, then it would’ve looked like some whole new line right about here, which doesn’t make very much sense. And using the rotating scale as intended along with the main scale for this question, we find that the reading on the micrometer is 2.75 millimeters.

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