Many prefer to use the upper scales A and B, in preference to C and D.
The disadvantage is that as the scales are only one-half the length of C
or D, the graduation does not permit of the same degree of accuracy
being obtained as when working with the lower scales. But the result can
always be read directly from the rule without ever having to change the
position of the slide after it has been initially set. Hence, it
obviates the uncertainty as to the direction in which the slide is to be
moved in making a setting.
When the A and B scales are employed, it is understood that the
left-hand pair of scales are to be used in the same manner as C and D,
and so far the rules relating to the latter are entirely applicable. But
in this case the slide is always moved to the right, so that in
multiplication the product is found either upon the left or right scales
of A. If it is found on the left A scale, the rule for the number of
digits in the product is found as for the C and D scales, and is equal
to the _sum of the digits in the two factors, minus 1_; but if found on
the right-hand A scale, the number of digits in the product is equal to
the sum of the digits in the two factors.
In division, similar modifications are necessary. If when moving the
slide to the right the division can be completely effected by using the
L.H. scale of A, the quotient (read on A above the L.H. of index B) has
a number of digits equal to the number in the dividend, less the number
in the divisor, _plus 1_. But if the division necessitates the use of
both the A scales, the number of digits in the quotient equals the
number in the dividend, less the number in the divisor.