552 



MAGNETO-INDUCTION. 



improved form of this apparatus (fig. 394) the secondary spiral is equiposed over a pulley 

 with a back weight, so that it can move easily in a vertical direction to and from the primary 

 spiral A de Watteville has used a form similar to this for a long time.] 



According to the law of induction ( 329), when the primary circuit is closed, a current is in- 

 duced in the secondary circuit in a direction the reverse of that in the primary, while, when it 

 is opened the induced current has the same direction. Further, according to the laws of 

 macneto-induction, the magnetisation of the iron rods (i, i) within the primary spiral L(, , ar), 

 causes a reverse current in the secondary spiral (K, K), while the demagnetisation of the iron 

 rods, on opening the primary circuit, causes an induced current 

 in the same direction. Thus, we explain the much more powerful 

 action of the opening or break shock as compared with the closing 

 or make shock (p. 481). [The direction of the inducing current 

 remains the same, while the induced currents are constantly 

 reversed 1 



The magneto-induction (R) apparatus of Pixii, as improved 

 by Stbhrer, consists of a very powerful horse-shoe steel magnet 

 (fig. 395). Opposite its two poles (N and S) is a horse-shoe- 

 shaped piece of iron (H), which rotates on a horizontal axis {a, b). 

 On the ends of the horse-shoe are fixed wooden bobbins (c, d), 

 with an insulated wire coiled round them. When the horse- 

 shoe is at rest, as in the figure, it becomes magnetised by the 



Fig. 393. Fig. 394. 



Fig. 393. Induction apparatus of du Bois-Reymond. R', primary, R", secondary spiral ; B, 

 board on which R" moves ; 1, scale ; + - , wires from battery ; P', F', pillars ; H, Neefs 

 hammer ; B', electro-magnet ; S', binding screw touching the steel spring (H) ; S" and 

 S"', binding screws to which to attach wires where Neefs hammer is not required. Fig. 

 394. New form of du Bois-Reymond's inductorium. 



steel magnet, while in the wires of both bobbins (c and d) an electric current is developed every 

 time the horse-shoe is demagnetised, and again magnetised. When the bobbins rotate in 

 front of the magnet, as each coil approaches one pole, a current is induced, and similarly when 

 it is carried past the pole of the magnet, so that four currents are induced in each coil by a single 

 rotation. By means of Stohrer's commutator {m, n) attached to the spindle {a, b), and the 

 divided metal plates (y, z) which pass to the electrodes, the two currents induced in the bobbins 

 are obtained in the same direction. 



Keys, or arrangements for opening or closing a circuit, are of great use. Fig. 392, II, shows 

 a scheme of the friction key of du Bois-Reymond, introduced into the secondary circuit. It 

 consists of two brass bars {z and y) fixed to a plate of ebonite, and as long as the key is down 

 on the metal bridge (y, r, z) it is "short-circuited," i.e., the conduction is so good through the 

 thick brass bars that none of the current goes through the wires leading from the left of the 

 key. When the bridge (/) is lifted the current is opened. [Fig. 396 shows the form of the 

 key, 9 being a screw wherewith to clamp it to the table.] Similarly the key electrodes (III) 

 may be used, the current being made as soon as the spring connecting-plate (e) is raised by 

 pressing upon k. This instrument is opened by the hand ; a, b are the wires from the battery 

 or induction machine;; r, r, those going to the tissue ; G, the handle of the instrument. 



[Plug Key. Other forms of keys are in use, e.g., fig. 397, the plug key, the two brass plates 

 to which the wires are attached being fixed on a plate of ebonite. The brass plug is used to 

 connect the two brass plates. All these are dry contacts, but sometimes a fluid contact is used 



