Choosing the right instrument

This lesson asks you to select suitable instruments and materials for each inquiry and to identify the methods and procedures. The museum has only examples from the history of instruments in the Islamic world; the instruments of your own laboratory are for the class to bring. ¹

Look at what your question needs: what do you want to measure or separate? Then compare what each instrument offers with what the question requires, and record why you set the others aside.

Notice that a piece of apparatus is built of parts, each with its own job. Write down what each part of your own set-up does in the whole procedure.

Read what one instrument assumes. Al-Khazini's balance of wisdom was not a single reading: it combined weighing in air, weighing in liquid and calibrated pan positions to investigate metals and gems, as Robert E. Hall explains ². Testing a binary alloy assumes that its two components are known ². A significant qualification is that the mixture's volume must approximately equal the sum of its components' volumes; Hall notes that this is approximately true of many alloys, not a universal chemical-identification rule ². His account stresses that the water must reach all parts of the tested object; merely suspending something over a vessel does not accomplish the comparison ². A sensitive tongue cannot compensate for choosing the reference liquid or positioning the object incorrectly ².

The record gives no passage in which al-Khazini says why he built this balance; what it shows is the setting he worked in. Hall places the work at Merv under Sanjar's patronage and says the balance was constructed for the treasury ³. Testing metals and gems puts the instrument within the problem of trusting materials rather than their colour or advertised name ⁴. Hall reads the name "wisdom" in both technical and ethical senses: sound discrimination of genuine and adulterated materials, alongside a well-founded and carefully constructed instrument ⁵. The record adds that the author's preface brings knowledge, action and justice together, placing reliable measurement within a wider conception of fairness ⁶.

A five-pan balance can look like the first of its kind. The treatise itself traces a history beginning with Archimedes, and its author does not present himself as the first person to discover weighing or water's support of an immersed body ⁵. Hall credits the indicator tongue to al-Razi; a third pan was known before al-Khazini, while al-Asfizari supplied the two movable pans and proposed specific-gravity marks on the beam ⁷. Hall tentatively identifies al-Khazini's special refinement chiefly with marks for different substances and more than one reference liquid ⁷.

We know the balance through a book, not an instrument. The record says the 515/1121–1122 date dates that treatise, not an exact manufacturing day, and not the later manuscript pictured here ^{5 8}. The illustrated witness is LJS386 fol80v: a graduated beam, suspension, indicator and pans drawn in red and black, not a photograph of surviving metalwork ⁹. Penn describes an undated copy probably made between the later thirteenth and earlier fourteenth centuries, expressed as 1250?–1350? ⁸. Hall also relates an older story that al-Asfizari's preceding balance was destroyed out of fear that it would expose fraud; this remains a transmitted account, not a modern eyewitness finding ⁵.

Hall says that testing an alloy with this balance assumes the two components are known, that the water must reach all parts of the object, and that a sensitive tongue cannot make up for a wrong reference liquid ². He also says sensitivity depends on suspension, careful construction and the long tongue, not merely the number of pans ². Take a position: when you choose an instrument for your own inquiry, do you put sensitivity first, or clear conditions of use? Answer the strongest objection to your position: why does your choice still hold if the better-built instrument is also the more sensitive one?

• Al-Khazini's balance of wisdom: the five pans, the conditions for testing an alloy, and what the balance was built on.
• Abd al-Rahman al-Khazini: the author of the treatise, and his work at Merv under Sanjar.
• The anbik: the three parts of a distilling apparatus, the cucurbit, the head and the receiver.

Read the records on the anbik and the balance of wisdom. For each instrument, name the question it was made to answer, and propose a question from your own laboratory that needs a similar instrument.