14 



EXPERIMENTAL PHYSIOLOGY 



are connected to the same binding screws. If now the pulley be rotated 

 in the direction of the hands of a watch, and the sectors are in the position 

 drawn in the figure, the sequence of events is: — i. the spring a is brought 

 into contact with c, and therefore the secondary coil is short-circuited; 

 ii. the spring b is brought into contact with D, thus closing the primary; a 

 make shock is therefore induced in the secondary, which is, however, short- 

 circuited because the spring a is still depressed ; iii. the sector v 1 glides off 

 the spring a, which flies up, and the secondary coil is no longer short- 

 circuited ; iv. the sector v' leaves the spring b, which flies up and breaks the 

 primary circuit, and the break shock now passes to the electrodes and through 

 a nerve or muscle laid upon them. By fixing the sector v- a little in advance 

 of v 1 , only make shocks would be sent through the electrodes. When a more 

 rapid series of stimuli is required, two notched wheels are provided to replace 

 the sectors ; these close and open the keys six times in each revolution, and 

 one, as with the sectors, may be set a little in advance of the other, and so 

 either make or break shocks sent through the electrodes as desired. 



Fit up the key as directed, and placing the electrodes upon the tongue, 

 rotate the key, and show that the one or other shock can be cut out as 

 required. 



Experiment 1. — Show that the break shock is greater tban the make 

 shock in the following way. Connect the primary coil with a battery and 

 mercury or spring ke}' as in fig. 20. To the secondary coil attach a pair of 

 wires, and remove the coil to some distance from the primary. Hold the two 



PC 



Fig. 20. — Arrangement of Apparatus for Making Use of 

 Single Induced Shocks. 



iree ends of the wires on the tip of the tongue, and make and break the primary 

 circuit by opening and closing the mercury key. At first nothing is felt. Now 

 gradually move up the secondary coil, testing each new position by opening 

 and closing the key in the primary circuit. At last a position will be found at 

 which a shock is perceived at break and none at make. Make a note of the posi- 

 tion of the secondary coil with respect to the fixed scale. Move up the secon- 

 dary still further, noting that the break shock becomes progressively stronger, 

 and at last a position is reached at which a shock is felt on making the current. 

 This position is to be noted and contrasted with that previously observed for 



the break shock. The experiment also 

 clearly shows how convenient the 

 coil is for modifying the strength of 

 stimulus to any required degree. 



Experiment 2. — To demonstrate 

 the break extra-current arrange the 

 apparatus as in fig. 21, applying the 

 Fig. 21. — Arrangement of Apparatus to electrodes e to the tongue. First close 

 Show the Break Extra-current. the key K, ; on now closing the key K, 



the current is short-circuited, and 

 none passes through the tongue ; on opening k, all the current passes through 



