240 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



of an a-c magnet driven with 200 cps, as shown in (5-1)7. The mag- 

 netic field deflects the electrons (both photoelectric and thermionic 

 emission) and modulates the electronic current with 400 cycles, while 

 the leakage current remains unmodulated. The a-c component is 

 amplified in a subsequent a-c amplifier. The method permits with 



simple means the measurement of light 

 intensities down to 3 x 10~ 7 lumen at a 

 Cof/iode signal-to-noise ratio of 1. 



Shot effect, i.e., the random fluctuation 

 of the electron current through the tube, 

 causes a variational (a-c) current, super- 

 imposed on the tube current. The frequency 

 of the variational current is uniformly 

 Fig. (5-1)7. Arrangement distributed over the frequency spectrum, 



for the separation of leakage i ,i i i , r ,i • 



, • ■ . tT but the measurable part ot this current is 



and emission current by i 



magnetic modulation of the limited by the band width A/ of the associ- 

 emission current [from H. a t e d equipment. The mean square value 



P. Kalmus and G.O. Striker, r , , , . 



Rev. Sci. Instr., 19, 7!. of the current IS 



(1948); faj permission], p _ 3 2 x 10~ 19 / Af 



where /„ is the average emission current in the tube. If the load 

 resistance connected to the tube is R, the mean square value of the 

 noise output voltage caused by shot effect is 



E 2 S = 3.2 x 10" 19 /i? 2 A/ 



Johnson noise, i.e., a randomly varying potential difference arising 

 between the terminals of a resistor, caused by thermal agitation of 

 the electrons, is 



El = UctB A/ 

 At room temperature 



E'f h = 1.6 X lO- 20 ^ Af 



where k is the Boltzmann constant, / the absolute temperature of the 

 tube, and R is either the value of the load resistance or of the resistive 

 component of the complex load impedance. 



The total noise voltage, comprising shot noise and Johnson noise, 

 is, at room temperature (t = 293°K), 



^v.rms = [3-2 X 10-™R(IR - 0.05) A/]' 



a. Vacuum Photoelectric Cells. The standard circuit for vacuum 

 photoelectric cells is shown in Fig. (5-1)1. In general, these cells 

 have a moderate sensitivity; the luminous sensitivity is between 10 



