These interesting risetime calculators were in use at Tektronix in the early days (note the 51X-style knobs and the Portland address). They consist of a small battery-powered analog computer specifically for calculating cascaded risetime calculations. Risetime is defined as the time taken by a signal to transition from a defined low state to a defined high state, often 10% to 90% of its final value.
This calculator was used to determine the overall rise time of three cascaded blocks, each with its own risetime. The resulting risetime is the square root of the sum of the individual risetimes squared.
The museum has a risetime calculator. Note the unique battery holder in the upper left corner of the rear photo. The trimmers match the resistors to the potentiometers. The individual risetimes are set using the A, B and C knobs and the meter is then nulled using the F knob which displays the cascaded risetime.
This analysis of the circuit demonstrates how analog computers function.