190 ELECTRICAL CONSIDERATIONS. 



Connect the C pole of the cell to the key by a wire, then wire 

 the other side of the key to the top binding screw A of the P coil, 

 wire B to the zinc pole of the cell. 



Withdraw the S coil to 20 cm of the scale, and let one co-worker 

 hold the electrodes against his wetted lip whilst the other makes 

 the trials. 



Make and break the P circuit with the key. Do this smartly, once 

 or twice only, and after each trial push the S coil half a cm 

 towards the P coil. 



Let the co-worker indicate when he feels the " shock," and whether 

 he does so at closure or at opening. 



Note the position of the coil as soon as the minimal break shock 

 is felt ; it is perceived first. 



Proceed with further trials until the make shock is also felt. 



Read off the position of the S coil. 



It is considerably nearer to the P coil. 



The break shock is the stronger of the two. 



Continue the approximation of the S coil by short steps to the 

 P coil, the shocks will be stronger each time until finally unbearable. 



The strength of a stimulus can therefore be varied by changing the 

 relative position of the S coil. It may approximately be assumed to 

 change inversely with the square of the distance between the two 

 coils. 



Next take the S coil out of the slide and place it end on, and 

 close up to, the P coil. Whilst making arid breaking the P circuit 

 turn the S coil so that its axis shall be ultimately set at right 

 angles to that of the P coil. 



The shocks will rapidly diminish and disappear as the position of 

 the S coil is changed. 



Explanation : When the battery current at closure of the circuit is rising in 

 strength in the primary, an opposing induction current is thereby generated in 

 the P coil itself, which retards the battery current from attaining its full strength 

 as soon as it otherwise would, and consequently the effect upon the S coil is 

 not so sudden a one. 



On breaking the P circuit an induction current is likewise generated which 

 has the same direction as the disappearing battery current, and consequently it 

 retards change of the electrical condition but does not interfere much with the 

 suddenness of the subsequent drop in potential, and therefore the effect upon the 

 S coil is greater than at closure. 



This opening extra current never reaches its full development, as it originates 

 at the moment that the P circuit is being broken ; to circulate at all it must 

 needs fly across the air gap at the contact E, which rapidly increases and so 

 quenches the flow. 



