588 BELL SYSTEM TECHNICAL JOURNAL 



For a fixed potential the magnitude of this conductance is nearly a 

 linear function of the illumination. With a suitable potential in series 

 with the cell, then, there is obtained a current the amplitude of which 

 is proportional to the quantity of light reaching the cell. 



In order to connect the photoelectric cell to the amplifier, there is 

 introduced in series with the cell and its polarizing battery a pure 

 resistance the voltage drop across which is used to control the grid 

 potential of the first tube. It is desirable, of course, to make this 

 resistance high in order to have available as much voltage as possible. 

 Its value is, however, limited by two considerations. The added 

 series conductance must not be so low that it appreciably disturbs the 

 linear relation between the illumination and the total conductance of 

 the circuit. The voltage drop must also be so small, in comparison 

 with the total potential in the circuit, that the photoelectric cell 

 operates at an approximately constant polarizing potential. 



In view of the extremely small voltage of the electric signal wave 

 as delivered by the photoelectric cell circuit, it is essential that great 

 care be taken to prevent such interference as may enter the initial 

 amplifier stages from approaching a comparable magnitude. The 

 most troublesome sources of interference are electrostatic induction, 

 electromagnetic induction, mechanical vibration, and acoustic vibra- 

 tion. By mechanical vibration is meant disturbances transmitted 

 through the supports as the result of building vibrations and similar 

 phenomena. By acoustic vibrations are meant impulses transmitted 

 through the air which strike the several elements of the amplifier and 

 cause motion which results in variations in their electrical constants. 

 Electrical disturbances are reduced to a minimum by placing the 

 amplifier as close as possible to the photoelectric cells, thereby keeping 

 the leads short, which avoids electrostatic pick-up and also prevents 

 the formation of closed loops of any appreciable size, thus avoiding 

 electromagnetic induction. The amplifier is provided with a very 

 complete electrical shield and both the shielded amplifier and the 

 photoelectric cells are placed in a carefully shielded cabinet. 



The tubes used, namely, the so-called "peanut" tubes, are, under 

 ordinary conditions, remarkably free from any microphonic action. 

 At the very low signal levels used, however, certain extra precautions 

 have to be taken against this effect. In addition to lining the amplifier 

 box with sound-absorbing material, the tubes themselves have been 

 wrapped in felt and placed within a heavy lead case. This prevents 

 such acoustic disturbances as reach the interior of the amplifier con- 

 tainer from having any noticeable effect on the tube. The lead 

 container is supported entirely by an elastic suspension and thus 



