The MacLagan Penetrometer (ca. 1910-1915)
Penetrometers are used to determine the optimal exposure settings for performing an X-ray of a particular part of the body. They typically consist of a series of metal (usually aluminum) plates of increasing thickness. Sometimes they are arranged in the form of stair steps.
This particular unit, the MacLagan Penetrometer, was manufactured by the McIntosh Electrical Company, or as it was later known, the McIntosh Battery and Optical Company of Chicago. I am not sure when the firm changed its name, other than that it was done by 1912. McIntosh seems to have gotten into the X-ray business by producing static high voltage machines. The Hogan Silent XRay Transformer, referred to on the plate (below left), was a featured product in their 1912 catalog.
The penetrometer contains ten metal disks arranged on the inside like the numbers of a clock, and each disk has a number cut through it so that it shows up on an X-ray image (photo above right). According to the plate, the number indicates that a particular disk corresponds to so many inches of solid tissue. As such, the images of the various disks on a test X-ray would indicate the appearance of body parts that correspond to the indicated thickness.
The above advertisement is from the Chicago Medical Recorder, Volume 37, 1915. Normally $5, but on sale for $2! Wow.
Size: 12" long, 3.5" diameter, 1" thick
X-ray image courtesy of David Allard.
Grigg, E.R.N. The Trail of the Invisible Light. Charles C Thomas 1965. pages 51 and 634.