A unit of concentration of radioactive material, = 3.64 emans = 3.64 × 10⁻¹³ curies per cubic centimeter, defined in 1930 by the International Radium-Standards Committee as that quantity of radon per liter (i.e., radium emanation without the decay products of the radon) which will maintain an ionization saturation current of 0.001 cgs electrostatic units of current in air. In SI units, it is about 13.5 × 10⁻³ becquerels per cubic centimeter. Abbr., M.E.
The unit is named for the Austrian physicist Heinrich Mache (1876–1954).
A major use of the Mache unit was in evaluations of the radioactivity of mineral springs — supposed at the time to be beneficial. In the time in which discovery of radioactivity created a sensation, “taking the waters” was a common recreational and therapeutic activity. Visitors to springs and spas might soak in one pool for psoriasis, and drink water from another spring for liver ailments. That the water of a particular spring was significantly radioactive became an attraction. Mache himself measured the radioactivity of springs, and such measurements were often made with an instrument called the Mache-Meyer fontactometer.
“The Mache-Meyer fontactometer¹⁷ was the third instrument used. Its ionization chamber had the relatively large volume of 14 liters. The central electrode is 25 cm. long and 0.5 cm. in diameter. The fontactometer has been widely used especially on the continent of Europe for determining the activity of natural waters.”
17. Phys[ikalische]. Zeit[schrift]., 10, 860 (1909).
T. H. Leaming, Herman Schlundt and Julius Underwood.
Comparison of the ionization currents due to equal quantities of radium emanation in different types of electroscopes.
Transactions of the American Electrochemical Society, vol. 30 (1917) Pages 365-378.
For a table of measurements of the radioactivity of various springs (not just in Colorado), all in Mache units, see
The radioactivity of some Colorado springs.
Journal of Physical Chemistry, vol 18, no. 8, pages 662-666 (1914).
For amusing (horrifying?) examples of the use of the Mache unit in quack medicine, visit the Oak Ridge Associated Universities site:
Mache Unit (M. E.) is a concentration unit referred to the Rn content of 1 liter of water or gas, etc. It is that quantity of Rn per liter which without decay products and with complete utilization of the α-particles can maintain by its ionization of air a saturation current of 10⁻³ e.s.u..
1 M.E. corresponds to 3.64·10⁻¹⁰ curie/liter = 3.64 Eman.
M. Curie, A. Debierne, A. S. Eve, H. Geiger, O. Hahn, S. C. Lind, St. Meyer, E. Rutherford and E. Schweidler.
The Radioactive Constants as of 1930. Report of the International Radium-Standards Commission.
Reviews of Modern Physics, volume 3, number 3, pages 427-445 (July 1931). Quote from page 432.
We hope that the custom of expressing activities of water and gas samples in terms of Mache units will soon be superseded entirely by stating directly the radium content or curies per liter.
Leaming et al (1917), cited above. Page 377.
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