Paper

Allied's Parallel-Reistance and Series-Capacitance Calculator

This two-sided cardboard rule implements the equation 1/A + 1/B = 1/C. A and B are either the sizes of resistors arranged in parallel, or capacitors arranged in series. C, the effective total resistance/capacitance, is calculated by the rule.

It fits in a paper sleeve which bears the instructions for use. The slider is dated 1945. It has two sides, and 6 scales. Each scale covers a different order of magnitude (e.g. 1 to 10, 10 to 100 etc.). The black numbers are for resistance and the red ones for capacitance.

Radio Shack Metric Converter

Double sided and really pretty useful.