Secohmmeter (inductance measurement)

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Rheometer 1.jpg
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Secohmmeter (inductance measurement)




April 2021: this device has been reidentified as a secohmmeter and will be updated with information soon.  Exciting find!

Physical Description: Rectangular brass box with removable lid on top of black iron base. Brass box is mostly hollow. A shaft with a heavy wheel on it runs through the box; the wheel is inside the box. On either side of the box, centered around the shaft, are braces that hold four thin metal arms which contact a cylinder set around the shaft. Paired letters BR and BA are arranged radially around the long-end brace; letters GA and BA are arranged radially around the short-end brace. The shaft is not centered in the box; it is mounted about a quarter of the way toward one end. It also protrudes about twice as far on one side than the other. The long end is capped by a wider cylinder that has a groove running around it that appears to fit a belt. The base has four holes in the corners for mounting.

Functional description: If the object is in fact a rheometer, it is used by attaching a belt to the cylinder and rotating it while the box is filled with a liquid or slurry. The thin metal arms pass over differently-colored squares on the shaft when it is spun, which could serve to measure the reaction of the contents to the force exerted by the wheel. The primary issue with this theory is that the box was not sealed; holes in the sides for screws have too large of tolerances to prevent fluid from leaking. However, this may be caused by missing parts or a niche feature unique to this device.


Lynette Webber and Shaina Royer with cataloging by Timothy Maze, Liam Anderson, Zach Nicholas, Peter Amundsen


in use from 1890–1920


Given its lack of markings, it was possibly constructed at Michigan Tech.




physical object


United States

Physical Dimensions

[base] 25.5 x 11.3 x 2.2 cm
[brass box] 19.1 x 10.3 x 7.0 cm
[shaft] 21 cm x 0.9cm diam


brass, steel, insulator (fibre)





History of the Object

"Secohmmeter" is a meter for measuring self-induction (what is today simpy called inductance), a quantity measured by the "secohm" or "second-ohm" [i.e., resistance unit], also known as the "Henry" (after Joseph Henry), or "H": the inductance induced in a circuit by a rate of change of current of one amp per second and a resulting electromotive force of one volt.


Physics storage


Ayrton, W. and J. Perry, "The Secohmmeter," Nature 36, no. 919 (1887): 129–132. DOI:10.1038/036129a0.
"A New Form of Secohmmeter," Science 13, no. 325 (Apr. 26, 1889): 313-315 []

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Lynette Webber and Shaina Royer with cataloging by Timothy Maze, Liam Anderson, Zach Nicholas, Peter Amundsen, “Secohmmeter (inductance measurement),” Michigan Tech Inventory of Historic Scientific Instruments, accessed May 7, 2021,