30 PHOTO-TELEGRAPHY 



is placed as one arm of a Wheatstone bridge, 

 a second cell S^ 2 being placed on the opposite 

 arm. W is a regulating resistance, and BI and 

 B 2 two batteries of about 100 volts, BI being 

 provided with a compensating variable resistance 

 W 2 . The galvanometer is of the "string" form, 

 i.e., two fine wires XY move laterally in the field 

 of a powerful electromagnet, whose pole pieces MM 

 are tunnelled with a hole. A small piece of 



Tim e 



FIG. n. 



77, 



FIG. 12. . 



aluminium or magnesium foil is stuck to the wires 

 in the centre, and this shutter just cuts off the 

 light which would pass from a Nernst lamp N 

 through the poles. If current passes through 

 the wires they are laterally displaced, and 

 the beam of light can then reach the second 

 selenium cell S^ 2 . As the current transmitted to 

 the receiving station passes there into a precisely 

 similar galvanometer, the circuit is closed. 



Now let us see what happens when a bright part 



