All modern slide rules are now fitted with a _cursor_ or _runner_, which
usually consists of a light metal frame moving under spring control in
grooves in the edges of the stock of the rule. This frame carries a
piece of glass, mica or transparent celluloid, about 1 in. square,
across the centre of which a fine reference line is drawn exactly at
right angles to the line of scales. To “set the cursor” to any value on
the scales of the rule, the frame is taken between the thumb and
forefinger and adjusted in position until the line falls exactly upon
the graduation, or upon an estimated value, between a pair of
graduations, as the case may be. Having fixed one number in this way,
another value on either of the scales on the slide may be similarly
adjusted in reference to the cursor line. The cursor will be found very
convenient in making such settings, especially when either or both of
the numbers are located by eye estimation. It also finds a very
important use in referring the readings of the upper scale to those of
the lower, or _vice versa_, while as an aid in continued multiplication
and division and complex calculations generally, its value is
inestimable.
_Multiple Line Cursors._—Cursors can be obtained with _two_ lines, the
distance between them being that between 7·854 and 10 on the A scale.
The use of this cursor is explained on page 57. Another multiple line
cursor has short lines engraved on it, corresponding to the main
graduations from 95 to 105 on the respective scales. This is useful for
adding or deducting small percentages.
_The Broken Line Cursor._—To facilitate setting, broken line cursors are
made, in which the hair-line is not continued across the scales, but has
two gaps, as shown in Fig. 6.
_The Pointed Cursor_ has an index or pointer, extending over the
bevelled edge of the rule, on which is a scale of inches. It is useful
for summing the lengths of the ordinates of indicator diagrams, and also
for plotting lengths representing the logarithms of numbers, sometimes
required in graphic calculations.
_The Goulding Cursor._—It has been pointed out that in order to obtain
the third or fourth figure of a reading on the 10 in. slide rule, it is
frequently necessary to depend upon the operator’s ability to mentally
subdivide the space within which the reading falls. This subdivision can
be mechanically effected by the aid of the Goulding Cursor (Fig. 7),
which consists of a frame fitting into the usual grooves in the rule,
and carrying a metal plate faced with celluloid, upon which is engraved
a triangular scale A B C. The portion carrying the chisel edges E is not
fixed to the cursor proper, but slides on the latter, so that the index
marks on the projecting prongs can be moved slightly along the scales of
the rule, this movement being effected by the short end of the bent
lever F working in the slot as shown. D is a pointer which can be moved
along F under spring control. As illustrating the method of use, we will
assume that 1 on C is placed to 155 on D, and that we require to read
the value on D under 27 on C. This is seen to lie between 4150 and 4200,
so setting the pointer D to the line B C—always the first operation—we
move the whole along the rule until the index line on the lower prong
agrees with 4200. We then move F across the scale until the index line
agrees with 4100, set the pointer D to the line A C, and move the lever
back until the index line agrees with 27 on the slide. It will then be
found that the pointer D gives 85 on A B as the value of the
supplementary figures, and hence the complete reading is 4185.
[Illustration: FIG. 6.]
[Illustration: FIG. 8.]
[Illustration: FIG. 7.]
[Illustration: FIG. 9.]
_Magnifying Cursors_ are of assistance in reading the scales, and in a
good and direct light are very helpful. In one form an ordinary lens is
carried by two light arms hinged to the upper and lower edges of the
cursor, so that it can be folded down to the face of the rule when not
in use. A more compact form, shown in Fig. 8, consists of a strip of
plano-convex glass, on the under-side of which is the hair-line. In a
cursor made by Nestler of Lahr, the plano-convex strip is fixed on the
ordinary cursor. The magnifying power is about 2, so that a 5 in. rule,
having the same number of graduations as a 10 in. rule, can be read with
equal facility, by the aid of this cursor.
The Digit-registering Cursor, supplied by Mr. A. W. Faber, London, and
shown in Fig. 9, has a semicircular scale running from 0 at the centre
upward to −6 and downward to +6. A small finger enables the operator to
register the number of digits to be added or subtracted at the end of a
lengthy operation, as explained at page 28.