Photograph of Square D calculator, Southern California, 1931. "The Fast Wire and Motor Calculator, Copyright 1931 by Frank A Short, Los Angeles, Patents Pending, Motor Data Based on 1930-31 National Electric Codes; Feet One Way; Wire Size; Percent Drop; Amperes; 440 Volt 3Ø, 1Ø-2Ø-D.C., System, 220-3Ø, 208-3Ø, 220 Volt, Index, 110-3Ø, 120 Volt, 110 Volt; Wire Calculator Based on Resistance Drop Only, To Find Wire Size or Distance: Set amperes to proper system index, turning disk in slot at top of calculator, then turn knob until allowable percent drop appears in round window-distances. To Find Percent Drop: Set amperes to proper system index - turn knob until selected wire size appears opposite the distance to center of load-read percent drop through round window. To Find Amperes: Set selected wire size opposite to distance then turn lower disk until allowable percent drop appears in round window-read amperes at proper system index.; Amperes, R.C. Wire, Cond. 3 WS, [T]o Find Amps. Non-Inductive Load, Multiply KW. by Constant., System, Voltage, Constant, 1 Ø or D.C., 2 Ø, 3 Ø, Current in Common Wire of 3 Wire 2 Phase System = L41 x Current in Phase Wires" -- signage on calculator.