A circuit similar to that of the Beckman DB
expansion unit could be used merely to expand any
section of the extinction scale by multiplying by a
constant factor the corresponding range of the
logarithmic voltage delivered by the signal amplifier.
But this would give a scale not linearly related
to the extinction; moreover, the properties of the
logarithmic scale are such that the degree of nonlinearity
would be different for the different sections
of the extinction scale. The problem can, however,
be solved in a different way; and this paper describes
an accurate but cheap and easily constructed unit
for the Beckman DK2 instrument. This unit, by
the turning of a single knob, allows any 0.1 section
of the extinction scale to be brought to fill the
whole chart of an auxiliary ('slave') recorder, while
preserving complete linearity in extinction throughout
the whole range from 0 to 2. Thus, if the slave
recorder has the same chart size as the Beckman
DK2 instrument, the unit gives a fixed tenfold
scale expansion. The device has given a very
satisfactory performance during two years of almost
constant use.