An Audible Photometer

Abstract

IN your issue of September 22 (vol. xxiv. p. 491, British Association Reports) Mr. Lant Carpenter mentions an idea of his about an “audible photometer.” The same idea occurred to me some six months ago. As my plan seems to be much more simple than Mr. Carpenter's, dispensing with intermitting beams and rotating disks, perhaps you will allow me a little space to describe it. I require only one photophonic receiver, whereas Mr. Carpenter mentions “two precisely similar receivers,” which is difficult, if not wholly impossible, to obtain. A is a small battery, B an induction-coil with the ordinary vibrating magnetic interrupter, and with a high-resistance secondary coil; c is the Wheatstonebridge combination, S E a selenium cell, with its working surface turned to the scale E; R is a high resistance of about the value of the selenium cell; a is a sliding contact, T a high-resistance telephone. Now I place on the scale E a standard candle at a distance d from selenium cell, and move the sliding contact till no sound is emitted from the telephone, Then the wire connections are left wholly unaltered, and the candle is taken away. Now I place the light I wish to compare with the standard candle on the scale E, and move it along the scale till the telephone is again silent. Be the distance of the light from the selenium cell now D, then its luminous intensity is