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Metric Clock

An inexpensive wall clock gets a new paper face with ten numbers and the words Think Metric. It is a conversation piece for lessons on the metric system and powers of ten, and it sets up a good puzzle about what a decimal second would really be.

Written from Bruce's original workshop handout. Bruce's final review of this plan is in progress.

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Metric Clock
DifficultyBeginner
Build time1 hour
CostAbout $10
SafetyEveryday care
Good forTeachers, Families

What it shows

Hang a clock with only ten numbers on it and watch every student in the room start doing math.

Key ideas: metric system, powers of ten, unit conversion, time

Construction

Build it

  1. Open the clock

    Remove the screws on the back and take off the case front and clear cover. Bruce's clock came apart with four screws.

  2. Remove the hands

    Gently pull the hands straight off the shaft, starting with the second hand. Note their order.

  3. Make the new face

    Trace the old face onto card stock and cut it out, including the center hole. Print the numbers 1 to 10 and glue them evenly around the circle, 36 degrees apart.

  4. Add the title

    Print the words Think Metric and glue them to the face.

  5. Install the face

    Lay the new face over the old one, line up the center hole, and press the hands back on in the same order, all pointing straight up when set to the top position.

  6. Reassemble and hang

    Replace the cover and screws, add a battery, and hang the clock where everyone will notice it.

The physics

How it works

Our time system mixes bases: 60 seconds per minute, 60 minutes per hour, 24 hours per day, for 86,400 seconds in a day.

A decimal time system would divide the day by powers of ten: for example 10 hours per day, 100 minutes per hour and 100 seconds per minute, which gives 100,000 decimal seconds per day.

Because 100,000 decimal seconds must fit into the same day as 86,400 ordinary seconds, each decimal second would be shorter: about 0.864 of an ordinary second.

The clock's motor still runs at ordinary speed, so the new face is a thinking prop, not a working metric clock. Figuring out why is part of the fun.

Using it

  • How many ordinary seconds are in a day? (86,400.)
  • How many metric seconds should a metric day have, and would a metric second be longer or shorter than an ordinary second?
  • Should a metric clock have AM and PM? How does your answer change the number of seconds?
  • Does this clock actually keep metric time? What would you have to change inside it?
  • Research the decimal time France tried in the 1790s and why it was dropped.
Go further

Variations

  • Make a matching face for a 10-hour day with the hour hand going around once per day.
If it doesn't work

Troubleshooting

If this happensThe hands rub on the new face.Try thisUse thinner paper, or press the hour hand on slightly less far so it clears the face.
If this happensThe hands will not come off.Try thisRock them gently while pulling straight out; do not twist, which can strip the shaft.
Free PDFDownload this planPrintable build plan with photos

More build plans

Measurement

Metric Measuring Sticks

Ten wooden sticks, four metric units per stick, and one deliberate mistake for students to catch.

Beginner · 2-3 hours+ · About $10-15

© Bruce Yeany · Homemade Science with Bruce Yeany · youtube.com/@YeanyScience
Free for personal and classroom use. Please don't resell or republish. Build and use at your own risk; follow the safety notes and supervise children.