Telecom Laboratory Services used these vacuum thermocouple units with other equipment to measure very small alternating currents and voltages over a range of frequencies as part of its procedures to monitor the quality of its telecommunications lines and ancillary equipment. Vacuum thermocouples were replaced by faster, more accurate and easier to use electronic and digital test equipment.

As part of the suite of items from the Telecom testing laboratory these vacuum thermocouple units are an example of the analogue electrical testing equipment used as part of the quality control of the phone system in Australia in the [1910]-1990 period. Vacuum thermocouples would also have been used during the early to mid 20th century in physical science research, development and quality control situations whenever small alternating currents and voltages needed to be measured over a range of frequencies.

Vacuum thermocouples are used to measure small AC currents or voltages with the same accuracy over a wide range of frequencies.

The reading of a galvanometer, ammeter or voltmeter connected to a vacuum thermocouple is proportional to the root mean square value of the alternating voltage being measured. The thermocouple converts the ac current or voltage into a direct current to drive the meter. The system will stay calibrated from input voltage frequencies of 10-20 Hz to about 50 MHz. The input AC current or voltage heats the heater filament which is adjacent or touching the thermocouple junction within the thermocouple unit. The heater circuit and thermocouple are enclosed in a vacuum tube to eliminate temperature changes caused by conduction, convection and radiation losses through air. The output current of the thermocouple is proportional to the mean squared heater current (or voltage).

For the meter to be calibrated in RMS volts the meter being used would have a square root scale.

The date range for manufacture is an estimate based on description of similar devices in a 1941 handbook for electrical engineers (see reference).

This instrument was used by Telecom Laboratory Services, The Boulevard, Strathfield, Sydney.

Vacuum thermocouple (2), wood / metal / Bakelite / paper, made by Standard, England, probably 1910-1950, used by Telecom Laboratory Services, Strathfield, New South Wales, Australia, [1910]-1990

The vacuum thermocouple is in a cube shaped wooden box with four black terminals on top and a paper panel of specifications on the front side. Two terminals are marked 'G+' and 'G-' for connecting a galvanometer. The other two terminals are marked "H' presumably for connecting the alternating current (AC) source to be measured to the heater side of the thermocouple circuit.

Made: 1910 - 1950


Used: 1910 - 1990

Each thermocouple has '199' or '200' handwritten in white ink on the top surface.
The paper panel on each box is a yellow printed form with the serial number and specifications handwritten:
For the thermocouple marked '199':
'215 SL'
'STANDARD'
'MADE IN ENGLAND'
'VACUUM THERMOCOUPLE TYPE 4002J'
'Serial No. J.G24.'
'Approximate Constants at 18°C.'
'HEATER: COUPLE:'
'637. ohms. 12 ohms.'
' 1.5 milliamps. for 5 millivolts*'
' 9.5 milliamps.+ for 15 millivolts*'
'+Maximum Safe Heater Current.'
'*Open Circuit E.M.F.'
For thermocouple marked '200' the specific markings are:
'VACUUM THERMOCOUPLE TYPE 4002J'
'Serial No. N.Q.21'
'Approximate Constants at 18°C.'
'HEATER: COUPLE:'
'607. ohms. 12 ohms.'
'1.7 milliamps. for 5 millivolts*'
'3.7 milliamps.+ for 15 millivolts*'

2008/187/18

1910 - 1950

96 mm

76 mm

76 mm