chap, iv.] Du BOIS-REYMOND'S COIL. 43 



automatically making and breaking the primary 

 circuit. Let the current be brought to the binding 

 screw K in the far-off pillar, and let the negative 

 electrode be bound to z. The current passes up the 

 pillar and along the spring s extended horizontally, 

 till it reaches the binding screw s', which makes 

 contact with its platinum point on a small piece 

 of platinum placed on the upper surface of the spring 

 at that spot. The current passes up from s' to s x/ , 

 then round R 1 , and out from it by a wire to the 

 electro-magnetic pillars b'. These consist of cores 

 formed of tubes of soft iron wound round with copper 

 wire. The electricity flows along the wire round both 

 columns, and then gains the screw z, by which it 

 reaches the negative electrode. When the current 

 passes round the pillars b' they become magnetic, and 

 attract the wedge-shaped piece of soft iron s, which 

 is just above them, and is fixed to the end of the 

 spring. The magnetised pillars attract the soft iron ; 

 this withdraws the spring from contact with the screw 

 point s', and the current is interrupted. The pillars 

 lose their magnetism, and fail to retain the spring, 

 which flies back by its own elasticity, and re-establishes 

 the current. Thus, by alternate magnetisation and 

 demagnetisation of the pillars the current is established 

 and interrupted rapidly, and so a rapid series of induc- 

 tion currents is obtained. The hammer interrupter 

 may be put out of use by connecting one wire of the 

 battery directly to s", and carrying the other wire to 

 s'". From s" the current passes straight round the 

 primary coil, then to s'", and so back to the battery. 

 To interrupt the current, then, the circuit must be 

 broken by loosening a wire, or by the use of a key. 

 Because of the sliding arrangement of the secondary 

 coil this induction apparatus has also been called the 

 " sledge " inductorium. 



Extra current. From what has been said about 



