The Sex Detector made its debut around 1920. It was a gadget, sold by "Sex-Detector Laboratories," that promised to be able to detect the gender of an egg — or any piece of biological matter whose sex one might want to find out (oysters, butterflies, caterpillars, beetles, worms). It supposedly even worked on blood. So police could use it to discover the sex of a criminal.
It was basically an empty rifle shell suspended on a piece of string. When held over an egg (or whatever) it would reveal through the direction of its motion the sex of the chick inside.
It was probably more accurately described as an idiot detector... the idiot being the one holding the string.
For a while it was heavily advertised in poultry journals, but when inspectors at the U.S. Dept of Agriculture investigated the efficacy of the device, they found it to be useless. It worked no better than a piece of cardboard attached to a thread. Advertisements for the product were banned.
The Leghorn World - Feb 1921
Wilmington Evening Journal - May 4, 1928
Williams News - July 8, 1921
San Francisco Chronicle - Oct 17, 1920
St. Louis Post-Dispatch - Feb 5, 1922