How to read the scales with graduation length of 10 inches, e.g. C, D, CF, DF, CIF
The following should be noted:
The slide rule does not show the actual place of decimals to which a number belongs. For example, the 6 shown on the slide rule may equally well denote 6, 0.6, 60, 600, 6000 or 0.006, and so forth.( An exception is provided by the exponential scales ) The position of the decimal point is ascertained afterwards, by a rough calculation with round figures. In most practical calculations it is known in advance, so that no further rules for determining the decimal point are required. It is the basic scales C and D that give the clearest idea of the way in which the scales are sub.divided. Once familiar with the graduation of these two scales, we shall be able to understand the others likewise.
All scales marked in red run in the opposite direction (reciprocally) from right to left, the exceptions being the extended supplementary graduations which are provided to enable a calculation to be continued in the case of border line values just below 1 (beginning of graduation) or just above 10 (end of graduation).
Let us now have a look at the basic scales C and D on the front of the slide rule, the reading . and setting . exercises being carried out by means of the long cursor line or the index.1 (beginning of scale) or index.l0 (end of scale), as the case may be.
A section of the graduation range from I to 2 (Scales C and D
From guide.number 1 to guide.number 1·1 10 sub.divisions of 10 intervals each ( = 1/100 or 0·01 per graduation mark)
Here an accurate reading can be immediately taken to 3 places (e.g. 1.0.1). By halving the space between two graduation marks, 4 figures can be accurately set (e.g. 18.104.22.168). In all cases the final number must then be a 5.
A section of the graduation range from 2 to 4 (Scales C and D)
From guide.number 3 to guide.number 4 10 sub.divisions of 5 intervals each ( = 1/50 or 0.02 per graduation mark)
Here an accurate reading can be immediately taken to 3 places (e.g. 3.8.2). The last number is then always even (2, 4, 6, 8). If the intermediate spaces are halved, this provides the uneven numbers (1, 3, 5, 7, 9) as well (e.g. 3.8.3).
A section of the graduation.range from 4 to 10 (Scales C and D)
From guide number 8 to 9 and guide.number 10 10 sub.divisions of 2 intervals each ( = 1/20 or 0·05 per graduation mark)
Here an accurate reading can be taken to 3 places, when the last number is a 5 (9.0.5). By halving the intermediate spaces it is even possible to take an accurate reading to 4 places. Here again the last number is always a 5 (22.214.171.124).