SELF-INDUCTION. VOLTAIC INDUCTION. 645 



INDUCTION. THE EXTRA CURRENT. MAGNETIZATION OF IRON BY 



THE GALVANIC CURRENT. VOLTAIC INDUCTION. UNIPOLAR 



INDUCTION-EFFECTS. MAGNETO-INDUCTION. 



If a galvanic element be closed by means of a short curved wire a feeble spark 

 will be observed at the moment when the circuit is again opened. If, however, 

 the closure is effected by means of a long wire wound into a coil a strong spark 

 is observed on opening the circuit. If two handles are attached to the closing 

 wire and held in the hands so that the current (through interruption of the wire- 

 conduction between the two handles) at the moment of opening is conducted 

 only by the body, a severe shock is felt at the moment of opening. This phe- 

 nomenon is due to a current induced in the long, coiled spiral, which Faraday 

 designated the extra current. The cause for its development is as follows: If 

 the circuit is closed through the spiral wire, the galvanic current passing through 

 the latter induces an electrical current in the adjacent turns of the same spiral. 

 This induction-current is, at the moment of closure in the spiral, opposite in 

 direction to the galvanic current in the circuit. Therefore, its effect is limited 

 and it likewise causes no shock. At the moment of opening, this induction-current 

 has, however, the same direction as the current in the circuit and, therefore, its 

 effect is intensified. 



Electrical apparatus, which, therefore, is so constructed that the irritation to 

 which it gives rise results from interruption of the circuit in a spiral conductor 

 is designated extra-current apparatus. 



If a soft-iron rod be introduced into the cavity of a coiled wire spiral, it becomes 

 magnetic so long as an electrical (galvanic) current passes through the spiral. If 

 one extremity of the iron rod is turned toward the observer, and the other in 

 the opposite direction, and if further the positive current passes through the 

 spiral in the direction of the hands of a clock, the extremity of the rod turned 

 toward the observer is the negative pole of the magnet. The strength of a magnet 

 thus produced depends upon the strength of the galvanic current, the number 

 of spiral turns and the thickness of the iron rod. As soon as the current is opened 

 the magnetism in the iron bar disappears. 



If a spiral roll be made of a long insulated wire, which may be designated 

 the secondary spiral; if, further, a similar wire spiral designated the primary 

 spiral be placed in the vicinity of the first, and the ends of the primary spiral are 

 connected with the poles of a galvanic element, an electrical current is generated 

 in the secondary spiral when the primary current is closed, or when opened after 

 having been closed. A current, likewise, appears in the secondary spiral if this 

 is brought closer to or removed further from a closed primary spiral (through 

 which a current is constantly passing). The current appearing in the secondary 

 spiral is designated the induced or faradic current. The process of this induc- 

 tion has been designated voltaic induction or electrodynamic distribution. The 

 current developed in the secondary spiral on closure of the primary current 

 or on approximation of the two coils to each other passes in the direction oppo- 

 site to that of the primary current. On the other hand the current induced 

 on opening the primary current or on separation of the two spirals from each 

 other has the same direction as the primary current. While the primary current 

 is closed or when the distance between the two spirals remains unchanged no 

 current is demonstrable in the secondary spiral. 



The currents developed in the secondary spiral on opening and closing the 

 circuit differ from each other in the following particulars. Although the amount 

 of electricity neutralized on opening and closing the current is the same, so that 

 the same effect from both can be demonstrated by means of electrolysis, as also 

 by means of a galvanometer, the electricity at once attains its maximum intensity 

 and continues for a short time with the opening current, while the electricity in- 

 creases but gradually, does not reach an equally high maximum and flows for 

 a much longer time with the closing current. The reason for this important 

 difference is as follows: With the closure of the primary circuit, there develops in 

 the primary spiral the extra current, which passes in a direction opposite to that of 

 the primary current. It, therefore, offers resistance to the more rapid development 

 of the primary current to its full strength. The current induced in the secondary 

 spiral therefore develops slowly. As, however, on opening the primary spiral the 

 extra current in the latter passes in the same direction as the primary current, 

 the disturbing influence mentioned disappears. The more rapid and profound 



