W h e n here is no i l l u m i n a t io n , there is no curre nt in p h o to tu b e 1 and
resistor 2. A n e g a tiv e p o te n tia l w ith respect to the cath o d e is a p p lie d
to g rid a of trio d e 3. C o n s e q u e n tly , tu b e 3 does n ot c o n d u c t a curre nt.
W h e n illu m i n a t e d , p h o to tu b e 1 becomes curre nt c o n d u c tin g . A t th is,
g rid a of tu b e 3 is connected th ro u g h p h o to tu b e 1 to th e p lu s p la te v o lt age
a n d the p o te n tia l of the g rid becomes p o s itiv e . This produces a p la te
cu rre n t in tu b e 3, w h ic h energizes the c o il of a solenoid-operated relay
(n o t shown), th ereby t r ip p in g the re la y .
$433$SmE,Re$