MUSCLE AND NERVE 



29 



remains the same as long as the electromotive force and the resistance 

 do not change. A current of this kind is known as a constant or gal- 

 vanic current. 



If a coil of insulated wire is arranged around a ring of iron, it will 

 be found that the passage of a constant current through the iron sets 

 up an electric variation in the outside spool of wire. This secondary 

 current, however, develops only on the make and break of the primary 

 current traversing the iron. An electric energy appearing in this form 

 is called an induced current. Moreover, since the primary current may 

 be made and broken at long and short intervals, we recognize single 

 make and break shocks and rapidly repeated shocks. The latter form 

 the so-called quickly interrupted or "tetanic" current. It will be noted 

 that the induced current is independent of the primary current, and 



FlG. 7. NON-POLARIZABLE ELECTRODES. 



M, Muscle or nerve; C, cotton or camel's-hair brush; S, solution of zinc sulphate; Z, 



amalgamated zinc. 



develops only on the make and break of the primary current. In be- 

 tween these two points no induction takes place, although the primary 

 current continues to flow. 



5. The Induction Coil. The apparatus by means of which the 

 induced current is obtained is known as an inductorium. It consists 

 of about 130 coils of insulated copper wire of medium thickness, the 

 terminals of which are connected with a key and the two elements of 

 a battery. These connections constitute the primary circuit. The 

 core of the primary coil is filled with a bundle of iron wire coated with 

 shellac. A second spiral consisting of about 6000 coils of insulated 

 copper wire of a thickness of 6.1 mm. is adjusted around the primary 

 coil in such a way that it may be moved nearer to or farther away from 

 the primary. The two ends of the secondary wire are fastened to 



