Ericsson Thermistor Bridge Type 4 Milliammeter Wheatstone Lab Test Instrument

Ericsson Thermistor Bridge Type 4 Milliammeter Wheatstone Lab Test Instrument

The Ericsson Thermistor Bridge Type 4 is a vintage benchtop bridge measurement instrument manufactured by Telefon A.B. L.M. Ericsson of Sweden. It uses a Wheatstone-style bridge circuit with thermistor elements to measure small electrical quantities, and features two analog panel meters — a milliammeter and a microammeter — along with separate Coarse Balance and Fine Balance controls for precision null-point adjustment. This is a mid-20th century instrument, estimated to date from the 1940s to 1960s based on its construction style, panel engraving, and multi-voltage selector design.

Specifications & What's Included

• Instrument type: Thermistor Bridge
• Designation: Type 4 (engraved on front panel)
• Manufacturer: Telefon A.B. L.M. Ericsson, Sweden
• Inventory number: YE600, Department: K (per ownership plate)
• Front panel meter 1: Milliamperes (Sensitive Research brand analog meter)
• Front panel meter 2: Microamperes (analog panel meter)
• Front panel controls: Coarse Balance knob, Fine Balance knob, Modulator Control knob, Balance Amplifier knob
• Front panel switches/inputs: Main Switch toggle, External Detector toggle, Thermistor input connector, Socket connector
• Rear panel: 3-pin round power inlet, fuse holder, slide toggle switch, ventilation panel
• Voltage selector options (rear): AVI 30V / A052 240V / A052 200V
• Enclosure: Gray painted steel, benchtop form factor with side carry handles
• Includes: Main instrument unit only

Applications

Thermistor bridges like this Ericsson Type 4 were used in mid-century laboratory and telecommunications environments to measure resistance changes via thermistor elements, which change resistance predictably with temperature. In the telecommunications sector — LM Ericsson's core field — thermistor bridges were used in RF power measurement, where a thermistor element absorbs RF energy and its resulting resistance change is detected and balanced by the bridge circuit. The dual-meter design with Coarse and Fine Balance controls is characteristic of precision null-detection instruments used for sensitive low-level measurements. Equipment of this type was found in Swedish Televerket facilities, military communications labs, and research institutions during the postwar period. Today, instruments like this are collected by vintage electronics enthusiasts and restored by hobbyists interested in mid-century Scandinavian test equipment.