Indicators were customarily accompanied by a variety of accessories. Advertising literature published in 1917, for example, shows that each Crosby instrument came in a lockable velvet-lined walnut case with a compartmented lid. In addition to the indicator, the case contained one spring and a small scale-ruler, one straight steam cock, fifty diagram cards, a hank of indicator cord, a spring bracket, a cord adjuster, one small oil bottle, a turn-screw, a hollow wrench, and an instruction booklet.

Dobbie McInnes indicators of the same vintage were usually supplied in mahogany boxes with a hinged platform and a compartment in the lid. In addition to an indicator cord, each box also contained a cylinder cock, a spare recording drum spring, a hexagon spanner, a turn-screw, an oil bottle, a set square, a detent-cord adjuster, a small sheet-metal tube containing spare pencil leads, a cord-adjusting plate, a radial dividing board, and a cylinder cleaning rod. Some cases will also be found with a short tubular wrench.


A selection of accessories accompanying a German-made Dreyer, Rosenkranz & Droop external-spring indicator dating from c. 1907. They include two steam cocks, springs, rulers, pencil leads, an oiler and a selection of tools. Museum of Making collection.

Additional springs or scales could be purchased separately, as the supply of indicator springs varied according to individual requirements. Some cases had provision for as many as twelve springs, but the absence of springs from a case does not necessarily mean that they were always there: indicators that were purchased specifically for use with a solitary single-cylinder engine (or for one particular cylinder of a triple-expansion engine) may only have had one spring! Indicators accompanied by a wide range of springs were often used by consulting engineers or by representatives of insurance socie-ties.

The springs

A catalogue published by the Crosby Steam Gage & Valve Company in 1915 recorded the range of indicator springs as 4, 8, 12, 16, 20, 24, 30, 40, 50, 60, 80, 100, 120, 150, 180, 200, 250 and 300lb 'to the inch', which referred to the amount of pressure required to move the indicator pointer by the specified amount. The metric sizes were rated at 2, 2·5, 3, 4, 5, 6, 7, 8, 10, 12, 15, 16, 18, 20, 30, 45 and 60, but these figures referred to the height in millimetres the pointer moved for each additional 1kg/cm² of pressure. The classification of the metric springs was the opposite of the imperial patterns, as the '2mm' version was thirty times stronger than the '60mm' type.

Trial and error showed that hot springs performed differently to those that were tested cold, but most manufacturers were aware of the problem and calibrated accordingly. Though there were undoubtedly errors from spring to spring, and also from indicator to indicator, the system was efficient enough to provide acceptable results.

Instructions pasted into the lid of the box of a 1893-vintage McInnes indicator, showing the range of springs that could be obtained. The trading style 'Dobbie McInnes & Clyde, Ltd.' dates from 1921-37, though other evidence suggests that the label was printed in June 1928.

Reducing gear

The comparatively small size of the recording drum on even the largest indicator faced the analytical engineer with a major problem. The stroke of even the smallest engine was considerably greater than the 4.5-5 inches that represented the limits of diagram length, and the stroke of a beam engine or a large horizontal mill engine could exceed ten feet. The engine-testing handbooks illustrate many ways of reducing the motion of the crosshead to suit the indicator, with the assistance of rods, bars, pulleys, pantographs or lazy tongs.

A typical adjustable pantograph, a means of reducing the long stroke of an engine to the comparatively small rotative movement of the indicator drum. Most pantographs were wooden, though metal examples are known. Courtesy of Bruce Babcock, Amanda, Ohio, USA.


There were a few champions of systems that involved rods and gearing, avoiding the stretching of cord or even braided wire, but these methods failed to prosper. Excepting the pantographs and the lazy tongs, which could be purchased commercially, most of the rod and bar systems were the extemporisations of individual engineers. This was actively championed by many of the textbooks, as it permitted the quirks of individual engines to be accommodated. In addition, many of the solutions were simple and accurate.

Another solution, more popular in Europe and the USA than in Britain, was the reducing wheel. This usually consisted of a large-diameter wheel, connected by cord or wire to the crosshead, which shared a common axis with a much smaller wheel that accepted the indicator-drum cord. The wheels were made of aluminium, to keep their weight (and inertia) as low as possible, and could be adapted to a variety of different strokes simply by substituting pulleys. It was not unusual for reducing wheels to come with two crosshead wheels and eight carefully-graduated pulleys. The only other system that could compete was the adjustable pantograph.


Left: made in surprisingly large numbers in the USA in the 1890s, the 'Straightline' indicator is occasionally found with reducing gear. This particular design relies on pulleys and gearing to scale-down the motion of the piston until it is appropriate for the size of the recording drum. Right: a Trill external-spring indicator with continuously-operable reducing gear. This example may date as late as 1936. By courtesy of Bruce Babcock, Amanda, Ohio, USA.

Diagram-analysing aids

The traces provided a surprising amount of information, among the most useful being the 'atmospheric line' drawn simply by allowing the drum to revolve without allowing steam to reach the indicator piston. Each spring was marked with a 'scale' that, in imperial-measure terms, signified the weight required to compress the spring by one inch. A '24' spring, therefore, required a weight of 24lb to compress it by an inch; each inch of diagram-height, therefore, equalled a steam pressure of 24lb. Once the horizontal lines of pressure had been deduced, the 'mean pressure' of the cylinder could also be obtained in any of several ways. The simplest was to divide the diagram into narrow vertical sec-tions, total the heights of the sections, and then divide the result by the number of sections. This gave a good approximation, and was helped by the manufacturers who provided grids to facilitate an accurate division into sections.

Another way, potentially more accurate and often quicker, was to use a polar planimeter. Invented in the 1850s by a Swiss mathematician, Jacob Amsler, these instruments automatically calculated the area of an enclosed figure with the aid of graduated wheels and verniers. Their popularity as mathematical instruments ensured that huge quantities were made. Many engine-indicator manufacturers, such as Crosby and Dobbie McInnes, offered 'own brand' planimeters; however, these were customarily purchased from specialist manufacturers in Europe or (subsequently) the USA.

The simplest forms of these instruments, set for a single pre-determined value (e.g., 10 square inches) have integrating wheels divided into units and tenths, relying on a vernier scale on the carriage or trace arm to give an accurate reading. A more sophisticated version, sharing the same basic construction, had an additional counter-wheel driven by a worm gear on the integrating wheel shaft. The best polar planimeters had additional features. The integrating wheel, worm, counting-wheel and vernier may be carried on a carriage that can be slid along the trace bar, allowing the instrument to be set to different base units.

A Swiss-made Coradi polar planimeter. Dated '14th October 1910', it was sold in the USA by the Eugene Dietzgen Company (the mark 'E.D. Co.' lies on top of the pole arm). Museum of Making collection.

The manufacture of planimeters has been restricted to specialist instrument-makers such as Amsler and Coradi in Switzerland; Keuffel & Esser, Ott and others in Germany; the Los Angeles Scientific Instrument Company ('Lasico') and Keuffel & Esser (originally a subsidiary of the German company) in the USA; W.F. Stanley in Britain ('Allbrit'); and a variety of businesses in Japan. However, individual planimeters may occasionally be found with the markings of distributors. These can include a few of the indicator makers, such as Elliott Brothers of London, but usually prove to be drawing- or mathematical-equipment suppliers.

In addition to conventional Amsler-type polar planimeters, there have been several idiosyncratic designs. In the USA, for example, J.L. Robertson & Sons of New York offered Lippincott and Willis planimeters, and the Trill Indicator Company, successor to Robertson, was still offering improved Willis-type planimeters in 1916. These differ greatly from the Amsler type in construction, though the underlying theory of operation was identical.

A typical Willis 1901-type or 'Improved' planimeter, showing the integrating wheel and the rotatable boxwood scale. Author's collection.

An alternative method of assessing the indicator diagram was provided by the 'Averageometer', patented in the USA in 1882 by John Coffin, made by the Thompson & Bushnell Company of New York City and favoured by (amongst others) Ashcroft, maker of Tabor-type indicators. This was a form of linear planimeter combined with a board-like base with a metal channel set into the left edge and a clamp sliding in another channel placed horizontally across the centre. To use an Averageometer, the diagram was attached to the board with its vertical edges against the fixed clamp on the left and the adjustable clamp to the right. The point of the trace arm was then taken around the diagram in the usual way, allowing the integrating wheel to record movement. Most Averagers had wheels with a circumference of 2.5in and a six-inch trace arm, giving a total of 15 for one turn of the integrating wheel.