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The solution to this issue is quite simple put chisel tips at both sides of the cursor!
This way, you can use either pair of tips in the middle of the rule, and use the appropriate pair when close to either of the rules ends.
To do that, youd need a cursor, a sliding member that traces a perfect perpendicular line across the rule. Who invented the Cursor (or, as it was sometimes called, Index, indicator, or Runner) is debatable.
There is mention of some form of cursor devised by Isaac Newton in 1675, and another is described by Frenchman Philippe Mouzin in 1837.
they used fixed scales on a wooden rule that allowed distances to be measured and added using a pair of dividers.
This "Gunter's Rule" was the original device introduced by Edmund Gunter in 1620, which remained in use for some two centuries.
He took this design to the Tavernier-Gravet firm the successor to Gravet-Lenoir and had them produce it as a standard item in their product line. The Mannheim slide rule was extremely effective, and became a standard configuration produced by numerous makers well into the following century.
The cursor was of the Chisel type, with two fingers jutting to the left of the sliding rectangle; the thin tips of these fingers would point at the scales.
Engineers, meanwhile, needed to do the generic calculations of multiplication, division, and square roots; and for that purpose James Watt, the inventor of the steam engine that powered the industrial revolution, devised about 1790 a simple and convenient slide rule called the Soho (after the location of Watts workshop in Birmingham).
With Robertsons cursor you can move between the various scales at will (and keep track which is which, given the legend on the cursor itself).
That such a useful idea was left out of the Soho design that came years later is probably due to the fact that before the industrial revolution each inventor would devise his own instrument, and the dissemination of new ideas was limited. The cursor came of age in 1851, thanks to a French artillery officer, engineer, and later professor of mathematics called Amde Mannheim.
Closed frame, bamboo & celluloid construction, missing cursor.
Between 18 they made approximately 15 million slide rules.