Lack of space
The world's largest slide rule, until the Texas Magnum TM1 beat it.
Collectors of mechanical calculators soon encounter a problem that is virtually unknown to slide rule collectors: lack of space. Apart from a calculating drum or a demonstration slide rule, the size of a slide rule is negligible compared with a mechanical calculator. I am a calculator collector, so I found some comfort in the existence of the instrument shown in figure 1.[1]
This is a calculating wheel with 11 feet scales,[2] so with a diameter of about 1.1 meter. It is known as the Smith-Davis Piecework Balance calculator, and is especially meant for wage calculations. According to Peter Hopp it is the largest slide rule ever made.[3] There is also a smaller version, the Smith-Davis Premium Calculator, with scales of a mere 4½ feet.[3]
Figure 1: Smith-Davis Balance Calculator
A conspicuous part of the instrument is the pedal for disengaging the two wheels, to allow them to rotate independently. Normally, the wheel are pressed together by a strong spring which causes them to turn together.
In 1905 a British patent on this instrument was applied for by John William Smith, a Mechanical Engineer from Newcastle-on-Tyne, and Henry Davis, an Instrument Manufacturer at All Saints Works, Derby. The patent specification clearly shows that this instrument has pre-decimal monetary scales (figure 3). The patent doesn’t give the dimensions of the instrument, but it mentions a small table attached to the wheel support (nr. 4 in figure 4), so the patent is about a BIG calculating wheel. The patent focuses on the monetary scales and suggests the use of a spiral scale. The pedal is not shown in the drawings.
The patent was granted in 1906.
Figure 2
Figure 3: Scales. British Patent 2610/1905
Figure 4: Sketch. British Patent 2610/1905
The giant was made by John Davis & Son, Derby. This instrument manufacturing company was established around 1844 by John Davis in Derby. After his death in 1873 the company continued under the same name, directed by his son Henry Davis. John Davis & Son was specialized in instruments for the mining industry, and began manufacturing slide rules in 1914, as a direct consequence of the start of the First World War and its resulting import restrictions.[5] Before 1914 John Davis & Son sold German and Japanese slide rules, among which a surveyors’ slide rule according to Jakins.[5]
Now one question remains: Who has a Smith-Davis Balance Calculator in his collection?
References:
Alphonse Drevon, “Automatisk Löne- och vinstberäkning”, Vetenskapen och livet, IV (1919) page 369–370.
“The Smith-Davis Premium Calculator”, Page's Engineering Weekly (ca. 1917) page 140.
Peter Hopp, “Slide Rules: Their History, Models, and Makers”, Astragal Press, 1999, page 119.
John William Smith, Henry Davis, “Wages and Premium Calculator”, British Patent 2610/1905.
John Davis & Sons, “Surveying Instruments”, Catalogue ca. 1912.