166 



THE SIGNS OF LIFE 



the primary circuit, and (2) opening the galvanometer circuit, in 

 quick and regular succession. 



This is very simply effected in the first of these keys by making 



B C A 



Object, ' 



FlG. 68. A B C Key. The primary circuit of the inductorium is completed 

 through the pools / /. Connection between G G short-circuits the galvanometer. 

 Of the three pools in the centre of the key, the middle is connected to A, via the 

 keyboard and galvanometer, the pool under W to C, and the remaining pool to B. 

 The three arms of the rocker are of unequal length, that connecting G G being a 

 little longer than the centre arm, and this again longer than the connection between 

 p p. On rocking the arms to the left, and then letting go the weight, the following 

 events take place in order : 



(a] The circuit// is broken, and an induction current passes through B A. 

 (K) ,, B A is broken. 

 (c) ,, A C is made. 



(r/) ,, ,, G G is broken, and any potential difference between A and C 

 acts upon the galvanometer. 



Thus the in luction shock caused by breaking p p passes through A B, A is then 

 connected to C, and not till then is the galvanometer short-circuit G G broken, and 

 the current from A C allowed to pass. To take the current from B C, the connections 

 A and B are transposed. 



the curved wire in the galvanometer short-circuit a little longer 

 than that in the primary circuit, so that the hreak shock takes 



