Galileo Thermoscope

If you are forming your own bulb, hot glass looks identical to cold glass. Let everything cool on a heatproof surface and never test by touch.

Use a round-bottomed flask with a long stem, or blow a bulb about 40 mm across on the end of glass tubing 4-6 mm in bore.

A narrow tube gives a big movement for a small change in air volume. Wide bore, insensitive instrument — this single choice sets the resolution.

Any grease breaks the water column into beads that stick and jump. Wash with detergent, rinse well and dry completely.

Add a drop of food colouring so the column is visible against the glass. The vessel must be deep enough that the tube mouth stays submerged throughout.

Cup the bulb until the air inside has clearly expanded. You should see bubbles escape if the tube is already in the water.

With the bulb still warm, lower the open end into the coloured water and clamp it upright, bulb uppermost.

As the trapped air cools and contracts, water rises up the tube. Wait until it settles — that is your starting level.

Adjust the warming so the water settles roughly halfway. You need headroom in both directions to see rises and falls.

Tape an evenly divided strip alongside. The divisions are arbitrary — this instrument has no units, which is precisely what makes it a thermoscope.

Note the level in melting ice and in a warm room. Two fixed points is the idea that later turned thermoscopes into thermometers.

Cup the bulb and watch the column fall within seconds. The instrument is sensitive and fast — far more responsive than most people expect.

Record the reading and a real thermometer's reading daily. Plot both. They will not track each other perfectly, and the discrepancy is the interesting part.

On a day when barometric pressure shifts sharply, the column moves with no temperature change at all. Your thermoscope is also a barometer — and that is its fatal flaw.

What Galileo made, around 1593. An air thermoscope: a bulb and open tube standing in water, with no sealed volume, no units and no fixed points. It showed that something had changed and roughly in which direction, which was revolutionary in itself — warmth became a quantity rather than a sensation. Galileo did not invent the thermometer, and the pretty sealed glass with floating weighted bulbs sold today as a "Galileo thermometer" is a later device based on buoyancy and density, not on his thermoscope at all. Two different instruments share his name.

Why the open design fails. The water column is held up by the difference between atmospheric pressure outside and the trapped air's pressure inside. That means the reading depends on the atmosphere as well as the temperature, so a passing weather system moves the column just as a warm hand does. There is no way to separate the two effects from a single reading. Santorio Santorio added a scale around 1612 and applied it to medicine; the decisive fix was sealing the liquid inside the tube, which removed atmospheric pressure from the equation entirely and produced the true thermometer.

The physics you can verify. The trapped gas obeys what we now write as the ideal gas law. At roughly constant pressure, volume is proportional to absolute temperature — Charles's law, formalised over a century later. Your bulb is a crude gas thermometer, and gas thermometry remained the most accurate primary standard well into the twentieth century precisely because gases behave so predictably.

Fixed points were the real breakthrough. An instrument that shows change is a thermoscope; one that gives a number that another person can reproduce is a thermometer, and that requires agreed reference points. Fahrenheit's mercury scale of 1714 and Celsius's water-based scale of 1742 supplied them. The distinction is not pedantry — it is the difference between noticing and measuring, and this bulb of air sits exactly on the line between the two.