Desco 19350 2 State LED Wrist Strap Touch Tester, 3/16 W x 2.1" L x 2.9" H, 6VDC/4VDC Test

The Touch Tester is an "on demand" wrist strap tester that also functions as a common-point ground for two wrist straps and a mat, when the wearer of a wrist strap touches the metallic surface of the Touch Tester, a path for current flow is completed from the wearer's skin, through the wrist strap, to the metallic surface to ground. - This action will cause the green "test good" LED to light (and the optional audible signal to sound) as long as the resistance of the circuit path is between 750k - 10 M ohms.