INDUCED OR FARADIC ELECTRICITY. 549 



circuit, which explains the enfeeblement of the galvanic current. This phenomenon is called 

 " external secondary resistance " (du Bois-Reymond). 



329. INDUCTION EXTRA-CURRENT MAGNETIC-INDUCTION. Induction of the 



Extra-Current. If a galvanic element is closed by means of a short arc of wire, at the moment 

 the circuit is again opened or broken, a slight spark is observed. If, however, the circuit is 

 made or closed by means of a very long wire rolled in a coil, then on breaking the circuit there 

 is a strong spark. If the wires be connected with two electrodes, so that a person can hold one 

 in each hand, the current at the moment it is opened must pass through the person's body, 

 then there is a violent shock communicated to the hand. This phenomenon is due to a current 

 induced in the long spiral of wire which Faraday called the extra -current. It is caused thus : 

 When the circuit is closed by means of the spiral wire, the galvanic current passing along it 

 excites an electric current in the adjoining coils of the same spiral. At the moment of closing 

 or making the circuit in the spiral, the induced current is in the opposite direction to the 

 galvanic current in the circuit ; hence its strength is lessened, and it causes no shock. At 

 the moment of opening, however, the induced current has the same direction as the galvanic 

 stream, and hence its action is strengthened. 



Magnetisation of Iron. If a rod of soft iron be placed in the cavity of a spiral of copper 

 wire, then the soft iron remains magnetic as long as a galvanic current circulates in the 

 spiral. If one end of the iron rod be directed towards the observer, and the other away from 

 him, and if, further, the positive current traverse the spiral in the same direction as the 

 hands of a clock, then the end of the magnet directed towards the person is the negative pole 

 of the magnet. The power of the magnet depends upon the number of spiral windings, and on 

 the thickness of the iron bar. As soon as the current is opened, the magnetism of the iron 

 rod disappears. 



Induced or Faradic Current. If a very long, insulated, wire be coiled into the form of a 

 spiral roll, which we may call the secondary spiral, and if a similar spiral, the primary spiral, 

 be placed near the former, and the ends of the wire of the primary spiral be connected with 

 the poles of a constant battery, every time the current in the primary circuit is made (closed), 

 or broken (opened), a current takes place, or, as it is said, is induced in the secondary spiral. 

 If the primary circuit be kept closed, and if the secondary spiral be brought nearer to, or 

 removed further from, the primary spiral, a current is also induced in the secondary spiral 

 {Faraday, 1832). The current in the secondary circuit is called the induced or Faradic 

 current. When the primary circuit is closed, or when the two spirals are brought nearer to 

 each other, the current in the secondary spiral has a direction opposite to that in the primary 

 spiral, while the current produced by opening the primary circuit, or by removing the spirals 

 further apart, has the same direction as the primary. During the time the primary circuit is 

 closed, or when both spirals remain at the same distance from each other, there is no current in 

 the secondary spiral. 



Difference' between the Opening [break] and Closing [make] Shocks. The opening and closing 

 shocks in the secondary spiral are distinguished from each other in the following respects (fig. 

 390) : The amount of electricity is the same, during the opening, as during the closing shock, 

 but during the opening shock, the electricity rapidly reaches its maximum of intensity and 

 lasts but a short time, while during the closing shock, it gradually increases, but does not 

 reach the high maximum, and this occurs more slowly. [In fig. 390, P x and S are the abscissae 

 of the primary (inducing) and induced currents respectively. The- vertical lines or ordinates 

 represent the intensity of the current, while the length of the abscissa indicates its duration. 

 Curve 1 indicates the course of the primary current, and 2, that in the secondary spiral 

 (induced) when the current is closed, while at I the primary current is suddenly opened, when 

 it gives rise to the induced current, 4, in the secondary spiral.] The cause of the difference is 

 the following : When the primary circuit is closed, there is developed in it the extra-current, 

 which is opposite in direction to the primary current. Hence, it opposes considerable resistance 

 to the complete development of the strength of the primary current, so that the current induced 

 in the secondary spiral must also develop slowly. But when the primary spiral is opened, the 

 extra-current in the latter has the same direction as the primary current there is no extra 

 resistance. The rapid and intense action of the opening induction shock is of great physio- 

 logical importance. 



Break or Opening Shock. [On applying a single induction shock to a nerve or a muscle, the 

 effect is greater with the break or opening shock. If the secondary spiral be separated from the 

 primary, so that the induced currents are not sufficient to cause contraction of a muscle when 

 applied to its motor nerve, then, on gradually approximating the secondary to the primary 

 spiral, the break or opening shock will cause a contraction before the closing one does so.] 



Helmholtz's Modification. Under certain circumstances, it is desirable to equalise the make 

 and break shocks. This may be done by greatly weakening the extra- current, which may be 

 accomplished by making the primary spiral of only a few coils of wire. V. Helmholtz 

 accomplishes the same result by introducing a secondary circuit into the primary current. By 

 this arrangement the current in the primary spiral never completely disappears, but by 



