XXVIII.] 



SHOCKS AND CURRENTS. 



175 



extra-current be the same at make and break, this inequality will 

 disappear. 



(a.) Connect the terminals of a Daniell's cell with the top 

 binding screws of an induction coil, as in fig. 103, and to the 



FIG. 103. Arrangement to approximately equalise M. and B. shocks. 

 S. Secondary coil ; K. Key in deriving circuit, D. D. 



P. Primary, 



same induction coil terminals connect two other wires with a 

 make and break key (K) in their circuit (" deriving circuit," D, D). 

 Thus the primary current is never broken. 



(b.) Arrange the secondary coil with short-circuiting key and 

 electrodes. 



(c.) On closing the key in the deriving circuit the current in 

 the primary coil is diminished, and on opening it the primary 

 current is increased. Induced currents of opposite directions 

 are thereby produced, which, though weaker than the make 

 induction shock, are approximately equal to each other. 



6. To Eliminate either M. or B. Shocks. For this purpose the " Rotating 

 Key " devised by Gregor Brodie is most useful. It consists of a horizontal 

 axis supported on two ebonite uprights fixed to an ebonite base (fig. 104). 



FIG. 104. Brodie's "Rotating Key" to eliminate the Af. or B. shock. 



This axis consists of two metal rods, A B and C D, united together by an 

 insulating piece of ebonite, K. A B passes through a cup, E, cut in the upright 

 and filled with mercury. The other rod, C D, is similarly connected to the 



