646 MAGNETO-INDUCTION. INDUCTION APPARATUS. 



action of the opening current is of great significance with relation to the physi- 

 ological employment of induction-currents. 



It may naturally be desirable under some circumstances to remove this ine- 

 quality in the closing and opening shocks. This end can be attained by greatly 

 weakening the extra current. This is accomplished simply by giving the primary 

 spiral only a few turns, v. Helmholtz has attained the same object by introducing 

 a secondary circuit in the primary circuit. By this means the current never dis- 

 appears entirely in the primary spiral, but it is alternately weakened and strength- 

 ened by the alternate closing and opening of this secondary circuit of much less 

 resistance. 



If a current is made to appear or disappear in the primary coil with great 

 rapidity, the induction-current develops in the secondary spiral not alone when 

 the free extremities of the spiral wire, which may be connected with some part 

 of an animal, are closed, but also when only one extremity of the wire is made 

 to divert the current by the contact. There occur, therefore, on contact with only 

 one extremity of the secondary spiral contractions in the frog-preparation that 

 are designated unipolar induced contractions. Generally they appear only on 

 opening the primary circuit. The occurrence of these contractions is favored by 

 connecting the other extremity of the spiral in diverting contact with the earth, 

 and also if the frog-preparation is not completely insulated. 



Brief consideration may now be given to so-called magneto-induction. Ac- 

 cording to Ampere one may conceive of a magnetic bar as surrounded perma- 

 nently by electrical currents in such a manner that if the south pole be directed 

 toward the observer the currents pass around each transverse section of the bar 

 like the hands of a clock. On this assumption it will be readily understood that 

 a magnet will develop a current in a wire coil near by as soon as the two are ap- 

 proximated, and also if a piece of soft iron is suddenly rendered magnetic or 

 suddenly loses its magnetism. The direction of the currents thus induced in the 

 coil is the same as that of those induced on voltaic induction : that is the develop- 

 ment of magnetism or the approximation of a coil of wire to a magnet gives rise 

 to an induced current in a direction opposite to that of the current assumed to 

 be present in the magnet; conversely, the disappearance of the magnetism or the 

 separation of the coil from the magnet gives rise to a current in the same direction. 



Approximation and separation of a magnet and a coiled wire may be effected 

 in rapid succession if a magnetic bar that is fastened at one extremity is permitted 

 to vibrate freely in the vicinity of the coil. The pitch of the note of such a rod 

 will then naturally indicate the rapidity of the movement and thereby at the same 

 time the number of induced shocks Grossmann's acoustic current-shocks and the 

 resulting acoustic tetanus in the frog-preparation. 



DU BOIS-REYMOND'S SLIDING INDUCTION-APPARATUS. 

 PIXII-SAXTON'S MAGNETO-INDUCTION MACHINE. 



The sliding apparatus is an improved modification of the magneto-electro- 

 motor of Neef for physiological purposes. The apparatus is readily comprehensible 

 from the accompanying sketch (Fig. 226). A wire passes from one pole (a) of the 

 galvanic battery (D) to the metallic column (S), from the upper extremity of 

 which an easily vibrating metallic spring (F) projects in a horizontal direction 

 and is provided at its free extremity with a rectangular strip of iron (e). An 

 adjustable screw (b) is approximated to the middle of the spring from above 

 so that contact between the two takes place. From the screw (b) passes an 

 insulated copper wire (c) to a hollow spiral (x x) , within which are placed a number 

 of rods of soft iron (i i) insulated by a coating of varnish. From the spiral the 

 wire (d) passes on to a horseshoe of soft iron (H), which it surrounds in spiral 

 turns, passing finally from this back again (at f) to the battery (g) . 



While the current is closed in this manner, it must effect the following results : 

 t renders the horseshoe (H) magnetic and it in consequence at once attracts 

 the movable strip of iron (e, Neef's hammer). By this means the contact of 

 the spring (F) with the screw (b) is broken. The current is thus interrupted, 

 the horseshoe accordingly loses its magnetism, and it releases e, which is drawn 

 upward by the spring, so that contact takes place again at b. This new contact 

 causes renewed magnetization of H and attraction and release are thus 

 repeated^in rapid succession, in consequence of which the primary current between 

 b and b is alternately opened and closed with equal frequency. 



