COMPASS AND PLUMB-BOB METHODS 



145 



min. to come to rest. The needle position is illustrated in 

 Fig. 86 and is seen to be at the intersection of two equipo- 

 tential lines the dotted circle being the actual range of the 

 plummet needle which is thus the reading needle range. 

 Differing connections, of course, vary the mesh of equi- 

 potentials, and a connection suited to a person with uneven 



Fig. 86. — Equipotential lines in receiv- 

 ing-dish. First arrangement. 



Fig. 85. — Clinophone wiring diagram. 



Fig. 87. — Equipotential lines in receiv- 

 ing-dish. Third arrangement. 



hearing is found by short-circuiting the electrodes in adja- 

 cent pairs and coupling an earphone between the pair using 

 the good ear only. Such an arrangement (Fig. 87) will 

 give a locus of minimum noise points as a straight line. 

 Short-circuiting on another cardinal point electrode we get 

 another minimum noise line. The intersection of these two 

 loci can be easily and exactly fixed and is the point sought; 

 now read off its dip and bearing in the dish scale. The 

 average reading error of an observation is about 5 min. of 

 arc with an 18i^-in. plumb bob, and this may be reduced by 

 carrying a longer plumb line or by having a check reader 

 and alternative connections. 



