McMurdo Silver Model 904 Capacitance Resistance Bridge

Circa 1946

A precision laboratory-grade instrument developed in the immediate post-WWII era for high-accuracy electronic testing.

Use Cases:

The Model 904 is a wide-range bridge designed to measure the electrical characteristics of components with high precision.

• Measurement Capabilities: It directly measures the capacitance (10 mmfd to 1000 mfd) and power factor of all capacitor types (air, ceramic, mica, paper, oil, electrolytic), as well as resistance values from 10 ohms to 1000 megohms.

• Leakage Testing: It features a built-in, continuously variable DC polarizing voltage(0–500V) used for leakage and insulation resistance testing, particularly useful for electrolytic capacitors.

• Fields of Use: It was primarily utilized in radio design laboratories, professional repair shops, and educational laboratories for school-level electrical experiments.

Historical Significance:

• Democratizing Precision: The Model 904 is significant for bringing laboratory-level accuracy to the mass market. McMurdo Silver applied production techniques learned during WWII to offer precision tools previously too costly for independent technicians.

• Transitional Technology: It represents the transition to "modern" post-war testing equipment, utilizing a logarithmic scale to maintain consistent percentage accuracy across its entire range, rather than just at the center of the dial.

• The "Magic Eye": It uses a 6E5 "magic eye" tube as a null indicator, a hallmark of mid-century high-end test gear that provided a visual way to find the most accurate measurement point.

(H-19cm x W-31cm x D-16cm)