Dual-coil, superconducting, on-off switch measures small, thermoelectrically generated voltages produced by thermocouples in a liquid helium bath. Placed in a shunt configuration between the thermocouple and the measuring device, the measuring device sees the sum of the voltage to be measured and the spurious thermoelectric voltages

Govednik, R. E.

Huebener, R. P.

English

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AEC-NASA TECH BRIEF 
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April 1968	 Brief 68-10087 
AEC-NASA Tech Briefs describe innovations resulting from the research and development program 
of the U.S. AEC or from AEC-NASA interagency efforts. They are issued to encourage commercial 
application. Tech Briefs are published by NASA and may be purchased, at 15 cents each, from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Superconducting Switch Permits Measurement of Small Voltages at

Cryogenic Temperatures 
The problem: 
To measure small, thermoelectrically generated 
voltages produced by thermocouples in a liquid 
helium bath. Such measurements are made difficult 
by relatively large, spurious, thermoelectric voltages 
generated in the leads which connect the thermocouple 
(at low temperatures) with the measuring device (at 
room temperatures).
The solution: 
A small, dual-coil, superconducting, on—off switch 
placed in a shunt configuration between the thermo-
couple (voltage source) and the measuring device. 
When the switch is nonsuperconducting, the measur-
ing device sees the sum of the voltage to be measured 
and the spurious thermoelectric voltages. When the 
switch is superconducting, only the spurious voltages 
Am (continued overleaf) 
This document was prepared under the sponsorship of the Atomic 
Energy Commission and/or the National Aeronautics and Space 
Administration. Neither the United States Government nor any per-
son acting on behalf of the United States Government assumes any
liability resulting from the use of the information contained in this 
document, or warrants that the use of any information, apparatus, 
method, or process disclosed in this document may not infringe pri-
vately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19680000087 2020-03-12T05:48:32+00:00Z
are detected. The switch, in combination with an 
accurate potentiometer, will yield a sensitivity of 
I x 10- 9 volt. 
How it's done: 
The switch consists of two, identical, ferrite potenti-
ometer cores. A bobbin, placed between the potenti-
ometer cores, carries 800 windings of 0.005-inch-
diameter niobium wire. After the potentiometer cores 
are screwed together with the bobbin within, a coil of 
fine, tantalum wire is wound noninductively into the 
air gap between the surfaces of the potentiometer 
cores: 7.4rnmof 0.002-inch diameter tantalum wire is 
used and is separated from the niobium coil by copper 
foil. Both the tantalum and niobium wire are teflon-
coated. The switch dimensions are 3.5 cm in diameter 
by 1.5 cm thick. 
In operation, the switch is immersed in liquid 
helium outside the vacuum can of the cryostat. The 
tantalum coil of the switch is connected to the thermo-
couple within that vacuum can via short pieces of 
niobium wire spot-welded to the tantalum wire. With 
no current passing through the niobium coil, the 
tantalum coil is superconducting (with a resistance 
of I x 10- 5 ohm), and the measuring device can 
detect only the spurious voltages. A current of 6 ma or 
more passed through the niobium coil activates the 
switch by causing the tantalum coil to pass into a 
nonsuperconducting state with a resistance of 45 ohms 
at 4.2° K. (The resistance of the tantalum coil at room
temperature is 1040 ohms.) In this state, the meas-
uring device detects the sum of the thermocouple 
voltage and the spurious voltage since the tantalum 
coil impedance is very large relative to the output 
impedance of the thermocouple. 
Notes: 
1. Additional details may be found in Review of 
Scientific Instruments. vol. 37, no. 12, p. 1675-
1676 (December 1966). 
2. Inquiries concerning this innovation may be di-
rected to:
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B68-10087 
Source: R. P. Huebener and R. E. Govednik 
Solid State Sciences Division 
(ARG-90260) 
Patent status: 
Inquiries about obtaining rights for commercial 
use of this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Argonne, Illinois 60439 
Brief 68-10087	 Category 01


Superconducting switch permits measurement of small voltages at cryogenic temperatures

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