--/ Cl.OCK-COXI N(>l.l.r./> TCNtiWG FORK 155 



kIvcs a iH-gativo |X)ti-nlial to tlu- j;rul sutVicii-nt to cut oft all space 

 current, ami tlio relay Ri remains unoperatwl. When the ix-ndulum 

 cuts olT the li^ht to the photo-electric ceil, the resistance of the cell 

 rises imniediaielv and the jjrid voltage tlrops to a very small value. 

 Enough space current will pass now to operate the relay /?. and a 



OPTICAL SYSTEM 



Fig. 4 — -Circuit for Obtaining Electrical Impulses from Standard Clock I'sing Photo- 

 electric Cell 



signal is transmitted of the same duration as the time the light is cut 

 off the cell by the pendulum. There is no appreciable time lag in 

 the photo-electric cell or vacuum tube. 



The principal requirement in setting up this circuit is to obtain 

 a vacuum tube ha\ing a resistance between filament and grid includ- 

 ing wiring, which is under all conditions considerably greater than 

 the minimum resistance of the photo-electric cell. If this resistance 

 drops much lower, the circuit Ijecomes inoperative even though no 

 additional grid leak is used. 



The only irregularity introduced in this system is in the operation 

 of the relay, and as this is a fast operating relay this error will be 



