The Use of the Slide Rule Classic Reprint

Excerpt from The Use of the Slide Rule

The need for a small book of this type arose in the work of teaching the use of the slide rule to engineering and industrial students; and this Manual is a direct result of sets of notes issued to classes consisting of engineering students and men of more or less practical experience. The book is not a treatise in any sense, its aim being to develop the ideas of the operator rather than to give empirical rules. Those rules that have been given are for the purpose of training the student in the formulation of processes, and it is not intended that they shall ever be committed to memory.

The examples have been taken largely from Hydraulics and Mechanics, and while the actual field covered by specific problems is narrow, the idea has been to make them fundamental. It is hoped that these examples will serve the purpose of development better than the more specialized problems arising different branches of engineering.

We f' recording or computing data should force himself to express. By the number of significant figures in the result, the accuracy of that result. The frequent habit of carrying results to a greater number of significant figures than the data warrant comes perilously near to lying with figures; it certainly creates a wrong impression as to the accuracy of the result. Certain mathematical constants can, however, be computed to any number of significant figures; for instance a may be expressed as or On the other hand, certain physical constants are very uncertain, even in the third place. Take for example the weight of a cubic foot of water generally given as lbs.; conditions of temperature and solution may easily alter the last figure of the three. With recorded data, however, the number of significant figures, as well as their character, gives very definite information. If we say that light travels 186000 miles per second, we do not mean that it covers 186000 miles to within the smallest fraction of an inch in one second of time, but that the distance covered is nearer 186000 miles than it is 185000 or 187000, and the accuracy is expressed by three significant figures. In this particular instance, it will he noticed that the three zeros to the right of the six, may or may not be significant figures. To prevent this ambiguity it has been suggested that results like the above be written 186 x which obviates the difficulty.

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