Serfass RCM 15B1 Conductivity Bridge  
Written by AnubisTTP on 2006-12-11  


The Serfass RCM 15B1 conductivity bridge functions as a comparator. The component to be tested is attached to the device, and the user manipulates the controls to match the internal configuration of the device to that of the tested component. The eye tube provides visual feedback to this process: as the device is brought closer to matching the value of the tested component, the illuminated wedge on the tube becomes narrower. When the wedge of light in the eye tube is at its minimum width, the comparator has been successfully tuned to the component and the component's value can be read from the state of the controls. Comparators of are little use to the modern electronics engineer, as modern equipment can provide much faster and more accurate measurements.


The tuning controls on the Serfass conductivity bridge consist of three mechanical knobs. A multiplier knob sets the range of the instrument, and the dial control knob controls the tuning by means of a large mechanical disc below the eye tube. A sensitivity control is also provided to make measuring an unknown component easier, by altering how close the device must be tuned to the actual value of the test component before the eye tube begins to react. This unit can measure both resistance and conductance: values on the left side of the multiplier knob are for resistance, and the right side values indicate conductance.

The RCM 15B1 has only four electron tubes: one each of 6AU6, 6AQ5, 6X4, and the 6E5 eye tube.

Serfass RCM 15B1 Conductivity Bridge
Serfass RCM 15B1 conductivity bridge. A date at the bottom of the instruction card indicates that this device was most likely manufactured in 1958.

Serfass Conductivity Bridge Tuning Eye
The most noticeable feature of the RCM 15B1 is the end-view eye tube poking out of the center of the front panel. A full range of eye tube behavior can be seen by attaching a test component to the comparator and turning the knobs.

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