COMPENSATION 31 



in the photograph causes light to be cast on the cell 

 S^i. The equilibrium of the bridge is at once upset, 

 current therefore passes through XY, the shutter 

 is displaced, and light falls, a fraction of a second 

 later, on to S^- We can represent the effect for an 

 illumination I by the two curves below (Fig. 11) 

 on opposite sides of the time axis. Add the two 

 ordinates and you get the " dead beat " curve shown 

 in Fig. 12. The effect is almost instantaneous, and 

 when the illumination ceases the current drops at 

 once to zero. This effect can only be obtained, 



77 me 



FIG. 13. 



needless to say, when the two cells are well 

 matched ; it is easy to have over -compensation, as 

 shown in Fig. 13, where the current is brought 

 below zero, and time is thus lost in regaining a 

 normal condition. 



It is not difficult to show that for good compensa- 

 tion, assuming the equations of the two cells be 



(ii) ^ = 03*!.* -*"' 

 we must have the condition fulfilled 



At 



