55Q 



UNIPOLAR INDUCTION. 



alternately making and breaking this secondary circuit where the resistance is much less, it is 

 alternately weakened and strengthened. 



[In tig. 391 a wire is introduced between a and /, while the binding screw, /, is separated 

 from the platinum contact, c, of Neef s hammer, but, at the same time the screw, d, is raised so 

 that it touches Neef s hammer. The current passes from the battery, K, through the pillar, a, 



Fig. 390. 



Fig. 390. Scheme of the induced currents. P,, abscissa of the primary, and S , of the secondary 

 current ; A, beginning, and E. end of the inducing current ; 1, curve of the primary 

 current weakened by the extra-current ; 3, where the primary current is opened; 2 and 4. 

 corresponding currents induced in the secondary spiral; P 2 , height, i.e., the strength of 

 the constant inducing current; 5 and 7, the curve of the inducing current when it is 

 opened and closed during Helmholtz's modification ; 6 and 8, the corresponding currents 

 induced in the secondary circuit. Fig. 391. Helmholtz's modification of Neefs hammer. 

 As long as c is not in contact with d, g h remains magnetic ; thus c is attracted to d and 

 a secondary circuit, a, b, c, d, e is formed; c then springs back again, and thus the process 

 goes on. A new wire is introduced to connect a with/. K, battery. 



to/ in the direction of the arrow, through the primary spiral, P, to the coil of soft wire, g, and 

 back to the battery, through h and e. But g is magnetised thereby, and when it is so, it attracts 

 c and makes it touch the screw d. Thus a secondary circuit, or short circuit, is formed through 

 a, b, c, d, c, which weakens the current passing through the electro-magnet, g, so that the 

 elastic metallic spring flies up again and the current through the primary spiral is long-circuited, 

 and thus the process is repeated. In fig. 390 the lines 1 and 7 indicate the course of the current 

 in the primary circuit at closing (), and opening (e). It must be remembered that in this 

 arrangement there is always a current passing through the primary spiral, P (fig. 391). The 

 dotted lines, 6 and 8, above and below S , represent the course of the opening (a) and closing 

 shocks (c) in the secondary spiral. Even with this arrangement the opening is still slightly 

 stronger than the closing shock.] The two shocks, however, may be completely equalised by 

 placing a resistance coil or rheostat in the short circuit, which increases the resistance, and thus 

 increases the current through the primary spiral when the short circuit is closed. 



Unipolar Induction. When there is a very rapid current in the primary spiral, not only is 

 there a current induced in the secondary spiral, when its free ends are closed, e.g., by being 

 connected with an animal tissue, but there is also a current when one wire is attached to a 

 binding screw connected with one end of the wire of the secondary spiral (p. 537). A muscle 

 of a frog's leg, when connected with this wire, contracts, and this is called a unipolar induced 

 contraction. It usually occurs when the primary circuit is opened. The occurrence of these 

 contractions is favoured, when the other end of the spiral is placed in connection with the 

 ground, and when the frog's, muscle preparation is not completely insulated. 



Magneto-Induction. If a magnet be brought near to, or thrust into the interior of, a coil of 

 wire, it excites a current, and also when a piece of soft iron is suddenly rendered magnetic or 

 suddenly demagnetised. The direction of the current so induced in the spiral is exactly the 

 same as that with Faradic electricity, i.e., the occurrence of the magnetism, on approximating 

 the spiral to a magnet, excites an induced current in a direction opposite to that supposed to 

 circulate in the magnet. Conversely, the demagnetisation, or the removal of the spiral from 

 the magnet, causes a current in the same direction. 



Acoustic Tetanus. If a magnet be rapidly moved to and fro near a spiral, which can easily 

 be done by fixing a vibrating magnetic rod at one end and allowing the other end to swing 

 freely near the spiral, then the pitch of the note of the vibrating rod gives us the rapidity of the 

 induction shocks. If a frog's nerve-muscle preparation be stimulated, we get what Grossmann 

 called "acoustic tetanus." 



