Telecom Laboratory Services used this precision bridge in association with other measuring instruments to monitor the quality of its telecommunications lines and ancillary equipment and to calibrate the field equipment used to measure telephone transmission equipment. This was a highly precise and accurate device at its time of manufacture, used for measuring electrical resistances. The unit was used from 1966 to c. 1990 and it was likely replaced by faster and easier to use digital test equipment.

As part of the suite of items from the Telecom testing laboratory it is an example of the analogue electrical testing equipment used in the quality control of the phone system in Australia in the 1960-1990 period. While the use of this particular precision bridge was related to telecommunications, similar units would also have been used in physical science research, development and quality control situations whenever electrical resistances needed to be measured with high precision and accuracy.

The precision bridge is used to calibrate known resistances and to determine unknown resistances over the range of approximately 1 ohm to 100 megohms. To use the instrument a galvanometer and standard cell batteries are connected to the terminals. Resistances to be measured are connected to the metal terminals on the left centre of the panel.

This precision bridge was used with the guarded nanovolt detector (also from Telecom Lab Services). The guard terminals of the bridge allow connection between other measuring apparatus to put them at the same electrical potential. This reduces or eliminates leakage and static currents particularly present in humid conditions and increases the accuracy of the precision bridge especially when measuring high resistances.

The earliest date of manufacture is inferred as 1964 from the date of an article about the new precision resistance bridge made by Cambridge Instrument Co. The Telecom laboratory manager said this particular instrument was purchased new in 1966.

This instrument was used by Telecom Laboratory Services, The Boulevard, Strathfield, Sydney. Alan Ogier, Telecom lab manager, said the instrument was originally purchased 4 March 1966 for $1003.00

Precision bridge, type 43328, with thermometers (2), metal / wood / plastic / glass / mercury / cardboard / fibrous padding, made by Cambridge Instrument Co Ltd / Apex, England / Japan, 1966, used by Telecom Laboratory Services, Strathfield, New South Wales, Australia, c. 1966-1990

The operating controls of the precision bridge are fitted into a grey metal panel which is screwed into a wooden box. The instrument has 12 black [plastic] rotary switches, two small rotary switches, 11 plastic and two metal terminals and two small buttons (press keys).

Two battery, one guard and two galvanometer terminals are at the top right side of the operating panel. Two terminals each for 'EXT. KEY B' (external key for the battery) and 'EXT. KEY G' (for the galvanometer) and one for guard are on the top left of the panel. One terminal for guard and two metal terminals for the test resistance are on the left centre of the panel.

The 'GUARD RATIO' rotary switch on the upper right selects 'OFF', 10/1, 100/1 or 1000/1. The guard ratio is the ratio of the measurement-circuit current to guard-path current. Selecting a higher guard ratio (when using higher input voltages) decreases the accuracy of measurements. The two rotary dials below the guard ratio dial are marked 'RATIO X' and 'RATIO +'. They select the bridge ratios for A or B coils at settings of 10, 100, 1000 or 10,000. Three smaller rotary dials in the upper left part of the panel are eleven position switches in steps of 2, marked 0-20. The dials are marked 'X0.01 OHM ZERO', 'X0.2 OHM ZERO' and 'X4 OHM ZERO'. Six rotary dials along the lower face of the instrument are ten position switches to select resistances in decades of 0.1, 1.0, 10, 100, 1000, and 10000 ohms to give a range of variable resistances from 0.1-90 000 ohms. Two buttons (press keys) on the left bottom row of the panel are for engaging the battery and galvanometer when making measurements. There are two small rotary switches at the centre bottom row of the panel. One is the reversing switch for the battery and the other is to select shunts (resistances) of 10, 100, 1k, 10k and 100k ohms to be placed in series with the battery.

Two identical mercury thermometers, each bent into a right angle, have scales reading from -5°C to +50°C in 0.1°C graduations. To measure the temperature of coils inside unit the mercury bulb end of one thermometer is placed into the hole in front of the box and the outer angled part of the thermometer placed into the two supporting clips on the front side of the box. The thermometers are packed in a cardboard box within two layers of soft fibrous padding material.

Made: 1966


Used: 1966 - 1990

Markings engraved on metal top panel of the instrument include:
'PRECISION BRIDGE'
'TYPE No 43328'
'No L-393157'
'CAMBRIDGE'
''INSTRUMENT Co. Ltd.'
'ENGLAND'
(company logo)

Printed and typed information on the paper sticker on the front of the unit from the Telecom MQA Northern Region State Calibration Centre includes 'DATE 01.05.91' and 'THIS UNIT IS NOT CALIBRATED. CALIBRATE ON DEMAND' and 'CALIBRATION DUE: MAY 1993'

The backs of the thermometers are engraved 'APEX (within a triangle outline) JAPAN 3" 1MM' and 'H004' and 'H005' are marked in ink.

2008/187/8

1966

195 mm

612 mm

430 mm