THE ART OF INSTRUMENT MAKING
by David St. John
The early American instruments makers relied upon their own ingenuity and personal skill in crafting scientific instruments. The period of interest (1800-1900) was void of any commercial standards of measurement or universal interchangeability of components. The technical problems and scarcity of quality materials dictated that this was the time of the American craftsman. Their challenges were legion. Even the best optical components were handmade or fabricated on crude machines which would create error systems of their own. Appropriate rod and tubing werenít available, nor convenient materials of suitable alloy. (Brass was the primary metal, but the bulk of it was imported, as America was still in its infancy of pouring brass). The early instrument makers knew little about compass variation or declination, and circle centering, and graduation errors remained undefined. All in all, only a few early 19th century instrument makers left behind desirable examples of their work. These were the craftsmen who resorted to building their own tooling to achieve the required finishes, precision, and accuracy demanded by scientific users.
Interchangeability is so commonplace in modern times as to have lost its once significant impact on the manufacturing complex. It was not so in the time of the early American instrument maker, even though in limited segments of industry and science promising materials and machines existed for research and high end commercial ventures. Complicating matters, an antagonistic posture was taken between many instrument makers when their products were sent to another instrument maker for repairs and calibration. It did not take long for strides in interchangeability to disappear within the instrument industry. A case in point would be the evolution of the American standard thread form and pitch diameter tolerance. At the outset components which were subject to constant use, i.e. leveling screws, tangent screws and clamp screws were premeditatedly fabricated with different pitch diameters than the developing standard to make repairs difficult, or impossible to accomplish on competitive instruments. Although the development of the four-element erecting eyepiece provided for specific spacings and focal lengths for optimum correction on the exterior focusing terrestrial system, individual instrument makers changed the diameters of components to void the possibility of competitive repairs after point of sale. There were numerous obstacles deliberately placed by individual makers and company philosophies in the name of competition. These efforts characterized an instrument makerís product in terms of dimension and frequently extended to other features such as knurls, chamfers, and the esthetic properties of bezels, objective cells, eyepiece caps, hand finished casting, and embellishments of the compass face.
There were other features which were important from a technical perspective. As the error systems of surveys became better defined, efforts were made to improve designs in the area of stability and repeatability. So once again instrument makers took different paths to improve the accuracy of their instruments. The manufacturing and fitting of tapirs on repeating center group assembles designed to hold a precision spirit vial was a significant starting point, but once achieved there remained problems due to wear. Materials had to be improved which introduced further changes. The attachment of sterling silver to horizontal circles and vertical arcs by soldering, inlaying, and pinning were tried to improve reading errors which eventually were resolved by improving the dividing engine. The different methods of circle attachment to spindles and telescope axis for centering graduations over the rotational axis, led to changes in the design and materials of the telescope, its slide and rack & pinion systems. The numerous changes made to achieve accuracy and durability left fingerprints of the maker, the period, and his craftsmanship.
Metallurgical advances followed a similar path during the same period when more reliable materials became available for scientific and commercial products. As characteristics of metals improved, so too did the metallurgy of cutting tools and machines to meet the demand for more precision, accuracy, and lower manufacturing costs.
The finest product to emerge was the American open engineerís transit, unique to the United States. It was designed to be a self-checking device with the capability to investigate its own errors, at the magnitude of vernier acuity (when used by a knowledgeable technician). The control of accuracy was compromised by a significant number of working surfaces, which as a system is oriented to gravity by an often unreliable spirit level vial. The system required stability in hostile environments, for many years. The early transit was a most remarkable accomplishment of American craftsmen working with crude machines and tooling augmented by fine handwork.
In addition to the instrumentís inherent system of errors, one had to consider the transmission properties of glass, internal reflections, and sun angle. Anti-reflection coatings were just being investigated, while approximately 4% light loss per optical surface significantly limited the usefulness of the terrestrial system. The ambient conditions (i.e. solar radiation, water vapor, scintillation, temperature gradients, and earthís curvature combined with parallax introduced by the observer) further confused the problems related to improving the design of the American open engineerís transit.
But, when an investigator considers the achievement in creating an instrument capable of an accuracy of 1 minute of arc, and considers the number of functioning surfaces of an American open engineerís transit, he or she can be overwhelmed with an appreciation of the makerís craft. Ponder for a moment that 1 minute of arc represents 0.0002909" per inch. This provides only 0.0008727" value at a 3.0" radius of a 6" engineerís transit. The line placement is normally at 30 minute intervals which is divided by a 1 minute double opposite vernier. The instrument represents a system of errors which is read with the optics of the telescope in a line of sight collimator while using a magnifier to read the vernier. The general resolution of the optics on axis is something on the order of 5.5 seconds, with an appropriate collimator target designed for vernier acuity, the precision of readout approaches 0.46 seconds of arc.
While certain instrument makers conducted successful businesses throughout their lifetime (William J. Young is a fine example), others faded into obscurity. Some of the more prominent individuals developed relationships which advanced the art of instrument making in a significant way. William Wurdemann, C.L. Berger, and Hyde worked in concert to produce the first dividing engine for accurate circles used by the firm of Buff & Berger, circa 1885.
The technological impact of the Industrial Revolution in Europe and the United Kingdom left its mark on the works of instrument makers.This was a period of rapid change, where many characteristics of successful instrument designs were being emulated or newly-improved among competing instrument makers. Knowledge of these individual problems of instrument construction, material application, fabrication methods and finishing are useful to an investigator during the process of determining authenticity. Remember, "The instrument maker always leaves his mark."