Build a tabletop ram

Parts & tools

• Two check valves — ½″ brass spring checks are ideal; poppet or swing both work. One becomes the waste valve (mount it so its flap can fall open), one the delivery check.
• A ½″ tee joining drive line, waste valve and delivery riser.
• ~1 m of ½″ rigid pipe for the drive line. Rigid matters even here — a short length of copper or steel pipe behaves; clear hose will barely work at all (see why rigidity decides the hammer).
• Drilled wine bottle or funnel + a small weight for the waste valve, and a 1–1.5 L bottle for the air chamber — a small balloon or a piece of bicycle inner tube inside makes it recharge-free.
• Delivery tubing (any small-bore plastic) and a jug to lift into.
• Silicone tape or rubber washers for the threads, and a tray to catch the waste stream.

Assembly

1. Build the heart

   Screw the two check valves onto either side of the tee so both would pass flow in the same direction: supply → tee → checks. The waste valve hangs vertically below the tee with its flap held open by a light weight; the delivery check points upward into a short riser.

2. Attach the drive pipe

   Run the rigid pipe from the tee up to where the header bottle will sit — on a book stack, 30–50 cm above the tee. Keep it straight and sloping steadily; no sags.

3. Add the chamber

   Fit the small bottle (with its balloon or inner-tube scrap) on the riser above the delivery check. A pressure vessel at this scale is tiny; the balloon does the work of a barrel-sized tank.

4. Set the fall

   Place the bottle so its water surface sits 30–50 cm above the waste valve. This is the whole drive: at this scale the Joukowsky spike is a few hundred millibars — enough to lift through the check a few centimetres, which is all a demo needs.

5. Tune the waste valve

   Adjust the weight until the flap falls open easily but closes promptly in the flow. Start light. The art is identical to the full-size machine's (tuning the beat), just in grams instead of kilograms.

Running it

Pour water into the bottle and let it run through. Nudge the waste valve once to start the cycle. Expect a beat every second or two — a brisk clack-clack — and a slow pulse of water rising through the delivery tube with each beat. For the clearest demonstration, dip the delivery tube into a measuring jug and watch the level climb against the volume disappearing down the waste.

• Nothing happens? Air in the drive pipe (purge it), or the waste valve is too heavy at this scale.
• Beats then stalls? A sag in the drive pipe is holding an air pocket — straighten the run.
• Barely delivers? Correct — the lift ratio at bench scale is poor by design. Raising the jug (more lift) slows delivery exactly as the efficiency equation predicts.

What it teaches

• Perpetual motion, disproved with a jug. Track the volumes: for every millilitre delivered, dozens go down the waste. The ram trades volume for height; it never creates either. Put the jug and the waste bucket side by side and the first law of thermodynamics is visible in two columns of water.
• Rigidity is physics, not preference. Swap the drive pipe for clear hose and the beat dies. The wave speed argument from The Physics survives at 1/100th scale.
• Tuning is one dial. The single weight controls everything — faster beat versus stronger hammer — exactly as on the full-size machine.
• The cycle is visible. At this scale you can watch the waste valve lift, pause, and slam — the four phases of How It Works with nothing hidden inside a housing.

A natural classroom companion to Myths & misconceptions, and a stepping stone to the full-size 2-inch build.