754 EFFECTS OF THE INDUCTION COIL. 



brass, 10 or 12 cm long and 7 or 8 cm wide ; they are hammered 

 round with the mallet upon a circular bar of wood, so as to form 

 cylinders of the stated length and about 2 or 2 cm> 5 in diameter ; the 

 sides -which meet need not be joined by solder. The covering is 

 removed at both ends from two pieces of copper wire, 0'5 or l m long 

 and O mm> 6 thick, and each wire is soldered with one end into the 

 inside of one of the handles ; the other ends are soldered to stouter 

 pieces of copper- wire, about 2 cm .long and l mm thick, by which they 

 are clamped to the binding screws K l and K 2 . 



If the primary current is that of two strong Grove 

 or Bunsen cells the induced currents are so powerful as 

 to pass through the iion- conducting air. Two wires of 

 copper, brass, or iron, l mm thick, are bent as shown in 

 fig. 378, and fixed in the clamps ^ and & 2 ; when the 

 distance between the opposite ends is not more than 1 

 or 2 mm sparks are seen to pass between the ends. 



An apparatus double the size of that assumed in fig. 378 gives 

 sparks as long as from 6 to 10 mm . The spark of the induction coil 

 produces the same effects as the spark accompanying the discharge 

 of frictional electricity. Even the spark of the apparatus fig. 378 is 

 sufficient to ignite gas ; for igniting ether, or the mixture of potassic 

 chlorate and sulphide of antimony, and perforating paper, a coil 

 of about double the size is required. With larger coils, which give 

 sparks of from 6 to 60 cm in length, glass plates several centimetres 

 thick may be perforated, paper or wood ignited, and various other 

 remarkable effects obtained. 



Induction coils even of the small size above de- 

 scribed are especially convenient for exhibiting the 

 beautiful appearances of the electric light in the rare- 

 fied space of Geissler's tubes (compare page 638). The 

 two ends of platinum wire which project from a 

 Geissler's tube are connected with the clamps ^ and L 2 ; 

 when the coil is joined to the battery a beautiful 

 luminous trail is seen inside the tube, which appears t< 

 be constant, but consists in reality of successive flashes 

 of electric light which only last as long as each induce< 



