The instrument consists of a vacuum photoelectric cell feeding into a vacuum tube voltmeter with logarithmic input characteristic. The voltmeter employs a type 6D6 tube which is provided with a properly chosen biasing resistor in the cathode lead. This gives a plate current proportional to the logarithm of the input voltage. Since this relationship never extends to zero plate current, a portion of the plate current is balanced out, giving in effect, a suppressed zero instrument. Suitable shunts and a choice of input resistors afford several ranges of sensitivity. The output meter can be calibrated directly in concentration units. Other methods of achieving the same result are discussed.
© 1935 Optical Society of AmericaFull Article | PDF Article