Before looking at which type of Scale Rule to select we should explain what a Scale Rule, or Ruler, is used for. When we were at school our first use of a ruler was to draw straight lines on a piece of paper, the old 300mm (12 inch wooden ruler became well used and suitably worn over a period of time, usually inscribed with various notations and doodles. The next use of the ruler was to measure lines and geometric shapes and to help us to draw things to ‘scale’. Scale at this stage was full size, so if the line was to be 50mm (2 inches) long we drew it at 50mm long using the printed scale on our rules.

So why would we need to use a Scale Rule? If we wanted to draw a line that represented the height of the room we are in, which is say 2400mm (8 feet), the line would not fit onto our A4 piece of paper if we tried to draw it at full size, which is a scale of 1:1. We could draw the line at one tenth of full size, 1:10, which would then become a line measuring 240mm (9.45 inches) which would fit onto our piece of paper. To make this easier we could use a Scale Rule which has calibrations on one edge with the dimensions already at a 1:10 scale, we simply look for the 2400mm measurement on the rule and draw our line to scale.

But what if we wanted to draw the whole building to scale so that it would fit on our small piece of paper? We might want to use a scale of 1:50 or 1:100, our Scale Rule would therefore have a number of different scales on in for many types of uses such as drawing buildings, bridges, machines, ships and other structures. One of the main uses of Scale Rules is to measure items which have been drawn to scale, the drawing may have been generated by computer aided design (CAD) without the use of a Scale Rule, however the Scale Rule will still be needed to measure various parts of the drawing during its use in manufacturing or construction.

Back now to ‘Which Scale Rule?’ and the types available. The most popular and economical is the flat, or Oval Scale Rule, this usually has two scales on each of the four measuring faces, there are various selections of scales available for different uses and applications. The Oval Scale Ruler has a slight curve, or aerofoil shape, in its cross section which makes it easier to pick up and helps to stop smudging when drawing lines. Two standard lengths are available, 150mm (6 inches) and 300mm (12 inches).

The next type we will discuss is the Triangular Scale Ruler which has a three pointed star shaped cross section with scales on each of the six measuring faces. The sides are often colour coded for easy identification of the scale being used, the three dimensional shape makes it very easy to find on a desk or drawing board and user friendly to pick up and use.

Finally the extruded aluminium Rapid Rule is the top end of the prestige Scale Rules and has a very attractive appearance with a novel scale rod which is turned to select the scale being used.