82 



Dr. A. D. Waller. An Attempt to Estimate 



Fig. i. 



To a keyboard having four plugs and plug-holes 1, 2, 3, 4 are connected — 



1. A compensator to balance any accidental current in circuit and to measure 



E.M.F. of reaction. 



2. An iuduction coil to supply the stimulus, preferably a single break shock, 



the make being cut out. 



3. The object under examination. 



4. A galvanometer. 



The procedure is as follows : — 



With 3 and 4 unplugged any current that may be present in the object is shown 

 by the galvanometer. Sucb current is balanced by manipulation of the com- 

 pensator unplugged at 1. "When exact compensation is obtained the galvanometer 

 can be plugged and unplugged at 4 without any deflection from zero. 



With the galvanometer plugged at 4 a single induction shock is now sent 

 through the object (with 1, 2, and 3 unplugged). Immediately afterwards the 

 galvanometer is unplugged, and the deflection (caused by the after- current) is 

 noted. 



The E.M.F. causing it is approximately estimated by comparison with the 

 deflection by a known E.M.F. from the compensator. 



tion shock in the positive direction ; and in the negative direction in 

 response to an induction shock in the negative direction. 



2. The same bean after removal of a horizontal slice from its under 



