The wrench can be see on top of an oscilloscope face. The dot of light from the oscilloscope traces out regular lines left to right and top to bottom. When the moving dot goes behind the wrench its light is blocked. A photodiode mounted above the wrench picks up the unblocked light from the moving dot. The signal from this diode is then displayed by a long persistence phosphor on the front screen.
Visitors can move the wrench about or replace it with their own hand.
They can also change the scan rate of the dot on the oscilloscope, making it move fast so that all of the lines are shown at once or making it go so slow that only one broken line is shown at one time. They can also invert the brightness of the displayed lines, that is, they can make the blocked parts light and the unblocked parts dark. This exhibit shows how a television picture can be built up by scan lines. Many spacecraft radio their pictures back to earth in a scan line format. The scan line format is therefore a basic building block of modern technology.