Abstract
MESSRS. JOHN DAVIS AND SON, of Derby, the A well known instrument makers, are bringing out a variation of the slide rule which is likely to. increase its value for certain classes of calculation without interfering with the simplicity and convenience of the form with which we are all familiar. The lower groove on the outside of the rule, which ordinarily is only wide enough to hold the inturned edge of the cursor, is made wider, so as to take one of the tongues of a spare slide, and this slide is held in place when required by two light aluminium clips which grasp the ends of the rule and of the spare slide while leaving the usual slide free to move. An extra cursor is also provided which is long enough to grasp both the rule and the extra slide. By this means any rare or special scales' upon the extra slide are for the time being equivalent to scales upon the rule, and these may be read against scales upon the other slide by means of the long cursor. If desired, the extra slide can take the place of the ordinary slide, or may be removed altogether when the rule, if provided with an ordinary cursor adapted to the altered lower groove, becomes an ordinary slide rule. In the example submitted,- the extra slide carries what are called E and —E scales. The E scale is a log log scale, and is always being re-invented; it was called a P line or power line by Lieut. Thomson, who showed it at the Inventions Exhibition, and it was long before invented by Dr. Roget. This P or E line is very handy, for it at once enables the logarithm of any number on any scale, i.e. to any base, to be read according to its position against an ordinary A line, while fractional or high powers of numbers are read with equal facility. Compound interest, pressures and volumes of gases under isothermal or adiabatic conditions are readily evaluated with the aid of the E line read against an A line. If, however, a pair of E slides are used, one in the usual position and one attached below the rule by means of the clips, then against any value, say of v, on one, the cursor will show the value of vγ on the other, γ having any desired value according to the relative position of the two slides.
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B., C. A New Slide Rule . Nature 72, 45–46 (1905). https://doi.org/10.1038/072045b0
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DOI: https://doi.org/10.1038/072045b0