science

Wedgwood scale

Wedgwood scale is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wedgwood scale rather than just read about it. In short: The Wedgwood scale (°W) is an obsolete temperature scale, which was used to measure temperatures above the boiling point of mercury of 356 °C (673 °F). The scale and associated measurement technique were proposed by the English potter Josiah Wedgwood in the 18th century.

Key takeaways

Wedgwood scale belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Wedgwood scale to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wedgwood scale from memory before moving on to harder problems.

Reference excerpt

The Wedgwood scale (°W) is an obsolete temperature scale, which was used to measure temperatures above the boiling point of mercury of 356 °C (673 °F). The scale and associated measurement technique were proposed by the English potter Josiah Wedgwood in the 18th century. The measurement was based on the shrinking of clay when heated above red heat, and the shrinking was evaluated by comparing heated and unheated clay cylinders. It was the first standardised pyrometric device. The scale began with 0 °W being equivalent to 1,077.5 °F (580.8 °C) and had 240 steps of 130 °F (72 °C) each. The origin and the sizing of the steps were later both found to be inaccurate.

History The boiling point of mercury limits the mercury-in-glass thermometer to temperatures below 356 °C, which is too low for many industrial applications such as pottery, glass making and metallurgy. To solve this problem, in 1782, Wedgwood created an accurately scaled pyrometric device, with details published in the Philosophical Transactions of the Royal Society of London in 1782 (Vol. LXXII, part 2). This led him to be elected a fellow of the Royal Society.

Method

A 0.5-inch-diameter cylinder made from pipe clay was dried at the temperature of boiling water. This would prepare it for firing in the kiln in which the temperature was to be measured. During the firing, sintering of fine particles resulted in contraction of the clay. After cooling, the temperature was evaluated from the diameter difference before and after firing assuming that the contraction is linear with temperature. To facilitate the temperature calculation, Wedgwood built a device which would directly read the temperature. Two metal bars with scales on them were fixed one above another on a metal plate and inclined at a small angle. The spacing between the bars was 0.5 inches at one end and 0.3 inches at the lower end. The scale was divided into 240 equidistant parts. The unheated piece of clay would fit the 0.5-inch gap giving the zero temperature reading. After annealing, the clay cylinder would shrink and fit somewhere in between the left and right ends of the bars, and the temperature could be read from the scales on the bars.

Scale The origin on the Wedgwood scale (0 °W) was set at the onset temperature of red heat, 1,077.5 °F (580.8 °C). The scale had 240 steps of 130 °F (72 °C) and extended up to 32,277 °F (17,914 °C). Wedgwood tried to compare his scale with other scales by measuring the expansion of silver as a function of temperature. He also determined the melting points of three metals: copper (27 °W or 4,587.5 °F (2,530.8 °C)), silver (28 °W or 4,717.5 °F (2,603.1 °C)) and gold (32 °W or 5,237.5 °F (2,891.9 °C)). All these values are at least 2,500 °F (1,400 °C) too high.

Corrections Louis-Bernard Guyton de Morveau used his pyrometer to evaluate the temperature scale of Wedgwood and came to the conclusion that the starting point should be significantly lower, at 517 °F (269 °C) instead of 1,077.5 °F (580.8 °C), and that the steps should be nearly halved from 130 °F (72 °C) to no more than 62.5 °F (34.7 °C). However, even after this revision the Wedgwood measurements overestimated the melting points of elements.

See also Outline of metrology and measurement Pyrometric cone

Notes and references

Illustrations

Worked examples

Example 1 — a first encounter with Wedgwood scale

Start with the simplest possible case. Write down what Wedgwood scale claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wedgwood scale before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wedgwood scale ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wedgwood scale

In research: Wedgwood scale appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wedgwood scale in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wedgwood scale is common in secondary-school and first-year university syllabi. It links to neighbouring topics Obsolete units of measurement, Scales of temperature, Science and technology in England, so understanding it makes those chapters shorter.
In everyday life: Look for Wedgwood scale outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Wedgwood scale in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Wedgwood scale means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Wedgwood scale out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wedgwood scale in simple terms?

Why does Wedgwood scale matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wedgwood scale?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wedgwood scale.