1956 --- The ST scale on rules that had Decimal Trig scales were converted to an SRT scale.

1956 0 88,500 88,500

1955b - The scale set was changed. ST scale was changed to SRT on the 4081s only.

1955b (4081-3, 4081-5, 4181-1, 4181-1C, E4181J, 4181-3, 68 1200, 68 1205, 68 1210, 68 1215, 68 1220, 68 1251, 68 1256, 68 1261, 68 1282, & 68 1287) Scale sets: Front: LL02 LL03 DF = CF CIF CI C = D LL3 LL2 Rear: LL01 L K A = B T SRT S = D DI LL1

4181-3 10" Log Log Duplex Decitrig Plastic 4081-3 family

Either index may be used.

To find the product of two numbers, disregard the decimalpoints, opposite either of the numbers on the D scale set the index ofthe C scale, push the hairline of the indicator to the second numberon the C scale, and read the answer under the hairline of the D scale.The decimal point is placed in accordance with the result of a roughcalculation.

Definitions. The central sliding part of the rule is called theslide, the other part the body. The glass runner is called the indi-cator and the line on the indicator is referred to as the hairline.

Accuracy of the slide rule. From thediscussion of § 2 it appears that we read fourfigures of a result on one part of the scaleand three figures on the remaining part.Assuming that the error of a reading is onetenth of the smallest interval following theleft-hand index of D, we conclude that theerror is roughly 1 in 1000 or one tenth of oneper cent. The effect of the assumed errorin judging a distance is inversely propor-tional to the length of the rule. Hencewe associate with a 10-inch slide rule anerror of one tenth of one per cent, with a20-inch slide rule an error of one twentiethof one per cent or 1 part in 2000, and withthe Thacher Cylindrical slide rule an errorof a hundredth of one per cent or one part.in 10,000. The accuracy obtainable withthe 10-inch slide rule is sufficient for manypractical purposes; in any ease the sliderule result serves as a check.