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Leeds & Northrup “dial-a-resistor” Wheatstone Bridge Antique Telephone - $75 (Framingham)
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Leeds & Northrup “dial-a-resistor” Wheatstone Bridge Antique Telephone
Leeds & Northrup precision decade resistance box—essentially a “dial-a-resistor” that lets you create a precise two-terminal standard resistance.
The three linked dials marked 100, 10, and 1 represent decades in ohms. You can set any value from 0 to 999 Ω, calculated as
100 × (hundreds dial) + 10×(tens dial) + 1 × (ones dial)
Each dial has 20 detents (0–9 repeated twice). This “make-before-break, carry” design ensures the contact never opens while turning, and rolling past 9 automatically advances the next higher dial—useful for smooth adjustments without interrupting a circuit.
The two screw posts are the input/output terminals, with the selected resistance appearing between them. Inside, the precision windings (typically manganin or constantan) provide stable, low-drift resistance steps.
How to use/check it
Connect a multimeter across the two posts.
Set the dials (e.g., 3-4-7 → ~347 Ω, within tolerance).
Use only modest current/voltage—these boxes were designed for low-power precision applications such as telephony, bridges, calibration, or substitution, not as high-wattage rheostats.
These resistance boxes were often paired with Wheatstone bridges and other laboratory instruments for precise measurement work
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