ESI Potentiometric voltmeter bridge
model 300PVB

Ser no.550005

ESI Potentiometric voltmeter bridge. This is a versatile instrument which can be used as a Voltmeter, Ammeter, Kelvin Bridge, Comparison Bridge, Ratiometer, & Null Detector. It can be used with internal or external generator voltages and/or the built in or external detector.

Brief specification:

Voltage: measured or generated: 1 mV to 5 volts with 5 digit resolution, accuracy: �0.01%, Voltage input: up to 500 V with 1 M ohm input resistance �0.02%. Reference: unsaturated standard cell and zener diode.
Resistance:0.1 ohm to 100 M ohms with 5 digit resolution, accuracy �0.01%. Four terminal Measuring system.
Current: 1 mA to 10 amps with internal shunts same resolution and accuracy as potential measurement. Range extendable with external shunts.
Ratio: Voltage or resistance with external voltage source, Range of 5 digit dials multiplied by 1, 0.1 or 0.01. Accuracy: � 0.01% of reading.
Current source and calibration shunts: range DC supply current programmable from 10 mA to 10 A DC shunts: ranges 5 uA to 15 amps. DC or AC up to 20 kHz.

Requires Five 1.5V "D" cells and two 8.4V Mercury Batteries (Two PP3 9Volt batteries in series can be substituted).

Inside view with amplifier removed

Brief working instructions in lid

  Corroded batteries and battery compartment  and DC amplifier with casing removed

I was given this instrument which formed part of a collection of many instruments collected by Bob Evans by his daughter Alice Kirby. Unfortunately it had spent many neglected years with a set of leaking batteries which had leaked and corroded the casing, the battery clips and contacts. I removed the battery compartment and washed and scrubbed it and made a new contact and battery clip, The casing and battery cover were scraped, sanded and painted. A fresh set of batteries including two PP3s in series were inserted (see second picture) and operation as a bridge proved satisfactory only if the internal amplifier was removed and an external detector was used (a digital multimeter). The standard cell proved to be OK and operation as a voltmeter also proved to be accurate. The amplifier responds to bridge unbalance and the sensitivity and zero controls seem effective but it somehow indicates  erroneously. I have opened up the amplifier as can be seen above. As yet I have not found a solution to this problem, the manual has a wiring diagram but has no information about the amplifier.