Lockdown Challenge: The Paperclip Clock

Lockdown Challenge #40: The Paperclip Clock

The quartz clock in your phone or computer is parts-per-million accurate. That’s like measuring from London to Berlin to a metre. But the clock you have online by radio or internet or GPS is 1 part in 10¹³, and atomic clocks exist that can do 1 in 10¹⁶. That’s like London to Berlin to the nearest atom!

The Paperclip Clock won’t do atomic accuracy. But pendulum clocks have fascinated mankind ever since Galileo discovered the constancy of a pendulum swinging. And the Paperclip Clock will keep good enough time. Plus, it’s the simplest clock in the world you can make yourself.

The heart of the Paperclip Clock is the paperclip on the pendulum string. The paperclip is the ‘escapement’, the device which stops a rotating power source (like an electric or clockwork motor) from running at any old high speed and makes it rotate at the speed that the pendulum swings. The paperclip also provides little pushes to keep the pendulum swinging.

Pretty much any small electric motor will work. The motor needs minimal power, only just enough to make it go. Try a small toy motor that will work with one battery, and then you add a series resistor of 10-100 ohms to further limit the current. One neat way to achieve this is to use croc clips on a length of resistance heating wire, such as you might find in a broken hair drier. Changing the length of wire between the clips adjusts the resistor value. Test some old tungsten bulbs and you may find suitable resistance. You can even use a small mains motor from a kitchen gadget. These are ‘universal’ AC/DC motors and need only 10 to 20 volts for a Paperclip Clock.

The motor needs to be fitted with a wheel with a nail as a crank pin. The pin engages with the wide slot in the paperclip. The string must dangle down, with the paperclip just in front of the motor, spanning the wheel. The pendulum needs to be compact: something like two AA or C cells, more weight for bigger motors.

Now set it all up, put current on to the motor, and swing the pendulum 10 or 20 degrees to the vertical. You should find that the pin on the wheel rotates to its top, then stops and waits for the pendulum to swing, then rotates to the bottom, and stops again to wait for the pendulum. If the motor rotates too fast, reduce power. If the pendulum swings too little, or the motor sticks, apply more power (and lubricate motor?). If you need more power but that causes fast rotation, swing the pendulum wider and/or increase its weight.

Once your Paperclip is running well you can do more with it. What about making the pendulum longer? The swing time goes up as √(length), so making it 4x longer gives twice the swing time. And what about adding a counter/display? Adding a pair of contacts isn’t difficult: put a croc clip on the string above the paperclip as the moving contact, with the fixed contact made with a croc clip and stiff wire as the other. If you get ‘contact bounce’, use a piece of conductive rubber (calculator key?). Alternatively, try a reed-switch and magnet.

The contacts will actuate the stepper motor and dial of a quartz clock, a commercial counter, or, rather more ingeniously, a calculator. Most calculators count up if you first key in ‘1+1’ and then the ‘=’ key repeatedly. You could push the ‘=’ key with a crank-drive, but it’s probably easier to install tiny wires to bring the ‘=’ key connections out of the case. And finally, what about a Microbit computer displaying the time scrolling on LEDs, perhaps sensing the pendulum with its magnetic sensor?

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org

There is also the back catalogue of Lockdown Challenges from 2020 to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.