Template:Are You Sure/April 9

• ... that physicist and academic Thomas Johann Seebeck (9 April 1770 – 10 December 1831) discovered the thermoelectric effect, where a junction of dissimilar metals produces an electric current when exposed to a temperature gradient?

• ... that mathematician and electrical engineer Charles Proteus Steinmetz (9 April 1865 – 26 October 1923) promoted the development of alternating current, formulating mathematical theories which will advanced the expansion of the electric power industry in the United States?

• ... that Édouard-Léon Scott de Martinville (25 April 1817 – 26 April 1879) made the oldest known recording of an audible human voice, a visual recording of audio data; that de Martinville managed to sell several phonautographs to scientific laboratories for use in the investigation of sound, proving useful in the study of vowel sounds; that the phonautographs initiated further research into tools able to image sound, such as Koenig's manometric flame?