COMPASS AND PLUMB-BOB METHODS 



127 



the contact rings and is screwed to the casing. The 

 brackets D, D' inside B carry the vertical rod E with the 

 plumb bobs F. These bobs can swing freely in one plane 

 only. The plumb-bob system is attached to a switch arm 



G, having a strip of platinum 

 on its upper end so that its 

 movement causes the arm to 

 describe an arc about the pivot 

 /. The said strip presses on 

 the commutator H carried on 

 rod E, which rod also carries the 

 resistance coil J connected to H. 

 The alternation of conductor and 

 non-conductor strips in H causes 

 alternations of current and 



/,■ 



n 



Fig. 71. 



ZZvl 



Fig. 72. 



Fig. 73. 



cut-out as the switch arm G moves, as shown in Fig. 72, 

 or it may be arranged, as in Fig. 73, to start from a maxi- 

 mum resistance and gradually cut out the resistance as 

 G moves over H. Or, again, it can be arranged to increase 

 the resistance from the minimum according to the inter- 

 spacing of the platinum and ebonite segments in the com- 



