Notes:
Elliotts made a broad range of steam engine indicators—including Richards, Darke, Wayne and Simplex patterns. However, the British market seems to have been surrendered to Dobbie McInnes Ltd and few Elliott-made examples can be reliably dated later than 1910.
This Simplex design consists of a small diameter brass piston; a vented brass cylinder; an internal, single wound spring, which can be changed; a small drum with a coil spring; a roll of paper inside the drum, which is hand fed; and a heavy brass slide and stylus.
An engine indicator is an instrument for graphically recording the pressure versus piston displacement through an engine stroke cycle. Engineers use the resulting diagram to check the design and performance of the engine.
A mechanical indicator consists of a piston, spring, stylus, and recording system. The gas pressure of the cylinder deflects the piston and pushes against the spring, creating a linear relationship between the gas pressure and the deflection of the piston against the spring. The deflection is recorded by the stylus on a rotating drum that is connected to the piston. Most indicators incorporate a mechanical linkage to amplify the movement of the piston to increase the scale of the record.
Patented in Britain by Moses Wayne in September 1894, the Simplex is easily recognised by the 'sugar tong' or 'V' spring compressed between the top cap and the tip of the piston-rod extension. The spring could be removed simply by sliding it sideways. The piston unit could then be detached by unscrewing the locking ring and then lifting the piston and pointer assembly upward and out of the slotted tubular housing. Vigorously promoted for the ease with which the spring could be changed and regulated, and the manufacturing advantages possessed by the distinctive spring ('lends itself to accuracy of calibration in any range', 'a simplicity of attachment [that] will recommend it to all engineers'), the Simplex was not particularly successful; production, apparently confined to1897–1901, was comparatively small. Two sizes were made: 'Pattern A' for normal speed ranges, and this unit, a 'Pattern B', with a small drum, for high-speed use.
The Indicator is stored in a solid wooden box with a number of compartments for springs rulers etc.
The springs were distinguished to ensure they were used with the correct indicator and the box contains a number of springs each uniquely marked. A typical example reads 'B’' over '1/170' and '+255', showing a spring requiring a pressure of 170lb/sq.in to compress it by an inch and that 255lb/sq.in above atmospheric pressure was the maximum permissible. The box contains a number of ‘rulers’ and a typical example reads ’10 to the inch’ over the company name. One ‘ruler’ is marked ’60 to the inch’ over the manufacturer ‘Dobbie McInnes Ltd Glasgow’
The problems with the Simplex are said to have concerned the excessive inertia of the heavyweight pantograph assembly, though the unique tong-springs may not have been popular. They may not have been as durable as the standard coil type, and regulating them may have been much more difficult than Elliott Brothers claimed. In addition, though the lighter springs were easily detached, changing the stiffer patterns required considerable manual strength or the assistance of a compressor.
https://www.archivingindustry.com/Indicator/externalspring.htm
The dimensions given are of the storage box.
