18 



EXPERIMENTAL PHYSIOLOGY 



with a battery and keys in the way shown in fig. 25. Place the 

 electrodes on the tongue. Make and break the battery circuit by 

 closing and opening the key, k 1 . If this is done when the primary 

 coil is included in the circuit (i.e., with k z open as in the diagram) 



Fio. 25. EXPERIMENT FOR SHOWING EFFECT OF EXTRA CURRENT. 



the stimulus is sharp owing to the " extra " currents, but if the coil is 

 shunted out by closing k 2 the stimulus is hardly perceptible to the 

 tongue. 



6. Instead of placing the simple key in the primary circuit place 

 it in a side circuit (fig. 26). On closing and opening the key. shocks 

 are still produced in the secondary circuit, although the current 



PIG. 26. DIAGRAM OF EXPERIMENT TO SHOW THE EFFECT OF CLOSING AND OPENING A SIDE 

 CIRCUIT TO PRIMARY COIL, k, KEY m SIDE CIRCUIT ; V , KEY IN SECONDARY CIRCUIT. 



through the primary coil is not made and broken, but only strength- 

 ened and weakened. The make and break shocks in the secondary 

 coil are now more uniform, but are both weaker. This is the same 

 effect as is obtained for rapidly interrupted shocks by the use of the 

 Helmholtz wire (see p. 16). 



7. Take the secondary coil out and place it across the direction of 

 the primary coil instead of in its usual position. The making and 

 breaking of the primary circuit now produce no effect on the secondary 

 circuit, but induced currents begin to show themselves if the secondary 

 coil is placed obliquely to the primary, and are strongest when the two 



