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Dial M for Measure

As the saying goes, you wait ages for a London bus and then two come along together. The same would appear to hold true for Elliott Bros dividers with a big dial on the front, if my latest acquisition is anything to go by.

Bagshaw's patent diagrammeter (left) and electrum dial dividers (right), both by Elliott Bros.

Following hot on the heels of Bagshaw’s Patent Diagrammeter, there was once again no time to figure out what this instrument was for, let alone to consider whether owning more than one set of dial dividers might be considered excessive. It was essentially a done deal the moment I set eyes on the unusual morocco leather case, fitted exactly to the dividers’ profile so that it outwardly resembled nothing less than a miniature banjo.

Elliott Bros electrum dial dividers morocco case with hook closures

Where I was able to discover the use of the Bagshaw through its name and the associated patent, this latest example was not so forthcoming. Aside from the tiny hand-engraved “Elliott Bros London” in letters 1/16th of an inch high on its dial face, the instrument is devoid of any such helpful information.

Elliott Bros electrum dial dividers signature detail

Inside the lid of its case is the less commonly encountered Elliott Brothers “Opticians” logo with the address 449 Strand, occupied by the firm between 1864 and 1886. Based on other instruments I have encountered with this style of logo, it most likely belongs to the earliest part of this date range.

Elliott Bros electrum dial dividers in morocco case marked 449 Strand logo detail

Just as minimal is the instrument’s construction, fabricated from unadorned plates of electrum with plain prismatic steel points fitted by tongue-and-groove joints.

Elliott Bros electrum dial dividers

Even more unexpected is what lies behind the dial – a fully exposed arrangement of gears with a ratio of approximately 6:1 that translates the opening of the legs to the rotational motion of a single pointer on the dial.

Elliott Bros electrum dial dividers gear mechanism detail

Like the Bagshaw, the dial is not continuously divided, but stops short of a full circle. In this case it is marked from 0 to 4, each unit being subdivided into tenths with no vernier or finer divisions. Measurements taken against an imperial scale confirm that the numbers on the dial represent whole inches, decimally divided. This offers no significant boost in precision, the distance travelled by the pointer around the dial’s perimeter being approximately 5.5 inches at the maximum 4-inch leg opening. Even so, the reading is remarkably accurate, as demonstrated by my test with a modern inch scale.

Elliott Bros electrum dial dividers 2 inch test measurement

Its purpose would therefore seem to be primarily a means of enabling reliable measurements to be taken without recourse to a separate linear scale. This could be useful in any number of situations, but technical drawing is not one of them. Equally, in engineering the same job could be done by vernier calipers or similar, so who would need such an instrument?

A clue may be found on the website of the Science Museum, London, which includes an object from Sir Henry Wellcome’s Museum Collection described only as “Caliper, metal, in velvet case”. As can be seen from their photo, it comes in a similar fitted case with the same “opticians” logo inside the lid, only this time with Elliott Bros’ earlier address of 56 Strand (1854-57).

Elliott Bros calipers from the Wellcome Collection, image courtesy of the Science Museum

The instrument itself is rather larger, with a visibly greater gear ratio and an index window cut into the dial to enable readings directly from the gear quadrant attached to one of the legs. Consequently the dial is continuously divided into 40 unmarked sub-units, while the major units (inches again?) are taken from the index. The flat electrum construction extends only to the top of the legs, which then become tubular in section before terminating with spherical tips.

The presence of these spheres suggests a medical use. For example, the earliest form of phrenology calipers featured a spherical end to one of its legs, for the purpose of insertion into the subject’s ear while taking measurments of the skull (see for example Fig. 1 from the frontispiece to George Combe’s 1824 Elements of Phrenology).

Calipers from George Combe's 1824 Elements of Phrenology

A medical attribution is also supported by its presence in the Wellcome Collection; Sir Henry Wellcome (1853-1936) was an American-born pharmaceutical entrepreneur and avid collector of all things medical, who had planned a “Museum of Man” to cover the entire history of human health. The museum remained unrealised on his death, but the core of the collection – around 100,000 objects – was eventually transferred to the Science Museum in 1976. The calipers are categorised under opthalmology, although without any explanation why. It may simply be the result of a misinterpretation of the term “optician” found inside the lid. During the nineteenth century this was used to denote a retailer of scientific, mathematical and optical instruments generally, not necessarily one specifically concerned with the human eye as it does today.

Returning to my dial dividers, a medical or optical use could explain the simplicity of their design, allowing them to be more easily cleaned for use in operations or with specimens. The sharp points are certainly not something that a conscious patient would want to come into contact with!

Elliott Bros electrum dial dividers, front and back views

Ideally, I would like to find an unambiguous match in a contemporary Elliott Bros catalogue, but these are thin on the ground (at least online). Until then, this strange forebear of the diagrammeter will have to remain in the file marked “unsolved cases”.

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