Stern's Tone variator, 1897.
© Science Museum / Science & Society Picture Library
Invented by psychologist Wilhelm Stern, the tone variator is an instrument that was devised to test an individual's ability to judge very small differences in pitch. Stern invented this in order to study a person's sensitivity to 'change'. This marked a significant shift in the progres of psychophical research as, previously, this research had investigated a person's sensitivity to differences in constant stimuli.