XXVII.] INDUCTION MACHINE ELECTRODES. 167 



galvanic current. The secondary coil (R") consists of 6000 turns 

 of thin insulated copper wire arranged on a wooden bobbin ; the 

 whole spiral can be moved along the board (B) to which a milli- 

 metre scale (1) is attached, so that the distance of the secondary 

 from the primary spiral may be ascertained. At one end of the 

 apparatus is a Wagner's hammer as adapted by Neef, which 

 is an automatic arrangement for making and breaking the primary 

 circuit. When JNeef s hammer is used to obtain what is called an 

 interrupted current, or "repeated shocks," the wires from the 

 battery are connected as in the figure, but when single shocks are 

 required, the wires from the battery are connected with a key, and 

 this again with the two terminals of the primary spiral, S" 

 and S'". 



Suppose we place the secondary coil hard up over the primary, 

 and consider this as zero, then an index on the side of the slot will 

 give the distance in millimetres of the secondary from the primary 

 coil, the current being strongest when the secondary coil is com- 

 pletely over the primary, and diminishing as the secondary is 

 removed from the primary. 



3. New Form of Inductorium. Fig. 96 shows an inductorium where the 

 secondary spiral moves vertically in a slot, and is compensated by means of a 

 counterpoise, so that it moves easily. It is used in the same way as the 

 other form. 



4. Graduated Induction Apparatus. In the ordinary apparatus the dis- 

 tance between the secondary and primary spirals is indicated by a millimetre 

 scale attached to the instrument. When the secondary spiral is moved along 

 equal distances, there is not a corresponding increase or decrease in the in- 

 duced current ; on the contrary, the strength of the induced currents under- 

 goes a very unequal change. Fick arid Kronecker use a graduated induction 

 apparatus ; one side of the slot is provided with a millimetre scale, and the 

 other is divided into units. 



5. Bowditch's Rotating Secondary Spiral. The secondary spiral is with- 

 drawn from the primary to the unit mark 30 on the scale. The secondary 

 spiral rotates on a vertical axis, so that it can be placed at varying angles with 

 the primary. In proportion as it is rotated from its conaxial position the 

 current is diminished. The student may test this by removing the secondary 

 spiral from the slot and placing it at variable angles to the primary spiral. 



6. Ewald's Sledge Coil. This coil is, with the exception of the interrupting 

 arrangement, in every respect similar to the ordinary Du Bois-Reymond coil ; 

 the iron core (fig. 95, K) is arranged movable, and the secondary coil slides 

 over the primary and can be adjusted in any position by means of a rack and 

 pinion arrangement. The interrupter consists of an upright electro-magnet, 

 over the poles of which swings a small steel bar-magnet ; this magnet forms 

 the bottom end of a pendulum which swings with very little friction, anu is 

 counterbalanced on its upper end by a small weight. 



The electro-magnet, when traversed by the current, becomes magnetised in 

 such a way that its poles are the same as those of the little bar-magnet above 

 it, thus repelling the latter, the swing of which is limited by the stop 

 spring B. 



