10 ELEMENTARY PHYSIOLOGY. I. 



Daniell, or other such cell, the former consideration only 

 has to be borne in mind. The reason for using a short- 

 circuit key in the secondary circuit will be obvious from 

 5 c, Less. II. 



Put the index of the secondary coil at 12 cm. of the 

 scale, open the key of the secondary coil and place the 

 electrodes on the lip or tongue. 



Close the key in the primary circuit. This sets up an 

 induction current in the secondary coil and a brief shock 

 will be felt. After a short interval open the key in the 

 primary circuit; this sets up another induction current 

 in the secondary coil, and causes a rather stronger shock. 

 Thus the break induction current is a stronger stimulus 

 than the make induction current. 



The stimulating effect depends, amongst other factors, upon the 

 rate of electrical change. When the primary current is made, a brief 

 induced current is set up in the primary coils in the opposite direction. 

 This delays the rate of rise of the primary current at make. When 

 the primary circuit is broken the current falls at once to zero. Thus 

 the induced current in the secondary coil is stronger at break than 

 at make, since it depends on the rate of change of current in the 

 primary circuit. 



Move the secondary coil away from the primary about 

 2 cm. at a time, and note that the make induction cur- 

 rent ceases to be felt before the break induction current. 

 Write down the position of the secondary coil at which 

 each induced current can just be felt. 



Since both the make and the break induction cur- 

 rents are currents which rise to a maximum and then 

 fall again to zero each has the possibility of stimulating 

 twice, during the rise and during the fall as in the case 

 of the make and break of a constant current. In con- 

 sequence, however, of the brief duration of the induced 



