A null-reading photometer head is described which employs a Photox cell integrally mounted with a graphite-graphite commutator in such manner that the direct current in an external instrument is reversed synchronously with the exposure of the cell to standard and unknown source. The candlepower of a Mazda lamp may be read directly on a permanent scale.
EARL D. WILSON, "A Simple Photox Photometer Head," J. Opt. Soc. Am. 29, 35-36 (1939)