String Electrometer of Carl Braestrup (1930s, early 1940s)
electrometer was produced for Carl Braestrup, probably in the 1930s.
picture of it appears in his
paper "X-ray Protection in Diagnostic Radiology" that appeared
in the February 1942 issue of Radiology ( p. 207-216).
It was used to measure the charge on a condenser-type thimble ion
chamber before and after a radiation exposure in an x-ray beam. The
decrease in the charge was related to the exposure via a calibration.
the above photo, the small black thimble chamber is seen at the end of a
16 " long chrome-plated probe. The
black viewing tube and the thimble probe, both pointing in the same
direction (towards the left), can be
The chrome cylinder with the holes in it on the right of the unit is
a protective housing for the light bulb. The metal plate on the end of the
meter says "Braestrup type Roentgen Meter, Made by F. Sellmer,
Brooklyn New York"
The chrome cylinder with the holes in it on the right of the unit is a protective housing for the light bulb. The metal plate on the end of the meter says "Braestrup type Roentgen Meter, Made by F. Sellmer, Brooklyn New York"
In the previously referenced paper, it is described as a “modified Wulf type r-meter” which would imply that the fiber was suspended between two identical plates of opposite charges. However, on inspection it appears to be more of a modified Wulf electroscope.
The photo to the right is a close- up of the electrometer/electroscope with the viewing tube removed. The quartz fiber is barely discernable a few millimeters to the right of the vertical supporting rod. The fiber is connected at the bottom to a quartz bow that keeps the fiber taut.
For additional information about string electrometers, click here.
B. Braestrup (1897 – 1982) was born in
Donated by Richard Mooney
Copyright 1999, Oak Ridge Associated Universities