62 PHYSIOLOGY OF MUSCLE AND NERVE 



Different Types of Current. If the two poles of a Voltaic battery 

 are connected with one another by wires and a simple key, the current 

 begins to flow as soon as the bridge is closed and ceases to flow as soon 

 as it is opened. Moreover, provided that the electromotive force 

 land the resistance remain the same, the current must retain a definite 

 strength or volume from its make to its break. A current of this kind 

 is characterized as a constant or galvanic current. It must be kept in 

 mind, however, that the flow of an electrical current is not identical 

 with that of water through a pipe, but consists merely of a transfer of 

 energy in the form of electricity. The nature of this force is not 

 known. 



In 1831, Faraday wound two coils of insulated wire around a ring 

 of iron, the ends of which he connected with a galvanometer. On 

 passing a galvanic current through the iron, he found that the needle 

 of the galvanometer was deflected first on the make and again on the 

 break of this current. This deflection was only of momentary dura- 

 tion, but clearly proved that the primary current also produced a 

 current in the second closed circuit of wires. Peculiarly enough, the 

 secondary current appeared only at the very moment when the bat- 

 tery current was made and broken. He obtained very similar results 

 with coils placed next to one another on wooden cylinders and also 

 with the aid of a magnet surrounded by a coil of wire. A current 

 which is produced in a closed secondary circuit whenever the current 

 flowing through a neighboring primary circuit is made or broken, is 

 called an induced current. Inasmuch as this induction may be re- 

 peated either at longer intervals or in very rapid succession, we recog- 

 nize single as well as quickly repeated induction shocks. The former 

 represent widely separated make and break shocks, while the latter 

 are made to follow one another in such rapid succession that they give 

 rise to an almost constant flow of stimuli. The latter constitute the 

 so-called "tetanic" current. 



The Induction Coil. The induction apparatus devised by DuBois- 

 Reymond, consists of a spiral of about 130 coils of insulated copper 

 wire of medium thickness, the ends of which are connected through a 

 key with the two elements of a battery. These connections form the 

 primary circuit. The core inside the primary coil is filled with a 

 bundle of straight pieces of thin iron wire coated with shellac. A sec- 

 ond spiral containing about 6000 coils of insulated copper wire of a 

 thickness of 0.1 mm., is placed around the primary coil in such a way 

 that it may be pushed completely over it or farther away from it. 

 The two ends of this secondary coil are continued onward to the elec- 

 trodes. These connections form the secondary circuit. 



At the very moment when the primary current is made, a current is also set up 

 for a brief period of time in the secondary circuit. It should be emphasized, how- 

 ever, that this secondary current is- merely induced, and is therefore absolutely in- 

 dependent of the primary current. This fact may be made more evident by plac- 

 i ing the secondary coil at some distance from the primary so that there is an empty 



