48 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



alteration of temperature, an unexpected noise, or the cessation of a monotonous 

 sound, as exemplified by the common experience that a sleeper is awakened 



^^1* 



FIG. 10. Induction apparatus : a, primary coil; 6, secondary coil; c, the automatic interrupter. 



when reading aloud abruptly ceases, attract the attention, although a continu- 

 ous sensory irritation may be unnoticed. This physiological law of the nervous 

 system would seem to have a psychological bearing as well. 



FIG. 11. Schema of induction apparatus. 



Irritating Effect of Induced Electric Currents. Within certain limits, the 

 more rapid the change in intensity of an electric current the greater its power to 



irritate. This probably accounts in part 

 for the fact that the induced current is a 

 more powerful irritant to nerves than the 

 direct galvanic current. Induced currents 

 are usually obtained by means of an induc- 

 tion apparatus (see Fig. 10). 



The ordinary induction apparatus employed 

 in the laboratory (see Fig. 11) consists of a coil of 

 wire, p, which may be connected with the ter- 

 minals of a battery, fe, arid a second coil, *, wholly 

 independent of the first, which is connected with 

 electrodes, e. At the instant that the key, k, in 

 the primary circuit is closed, and the battery cur- 

 rent enters the primary coil, an induced current 

 is developed in the secondary coil, and the nerve 

 resting on the electrodes is irritated. The in- 

 duced current is of exceedingly short duration, 

 suddenly rising to full intensity and falling to 

 zero. As long as the battery current continues to 

 flow constantly through the primary coil, there is 

 no change in the electrical condition of the sec- 

 ondary coil, but at the instant the primary current is broken another induced current of short 

 duration is set up in the secondary coil, and again the nerve receives a shock. The rise and 



FIG. 12. Schema of the relative intensity 

 of induction currents (after Hermann, Hand- 

 buch der Physiologic, Bd. ii. S. 37) : P, abscissa 

 for the primary current; S, abscissa for the 

 secondary current; 1, curve of the rise of 

 intensity of the primary current when made ; 

 2, curve of the rise and fall of intensity of 

 the corresponding induced current; 3, curve 

 of fall of the intensity of the primary cur- 

 rent when it is broken ; 4, curve of the rise 

 and fall of intensity of the corresponding in- 

 duced current. 



