752 THE INDUCTION COIL. 



inducing wire is made of moderately thick insulated 

 copper wire ; it is coiled in several layers round the 

 bundle of iron wire. On this primary wire is coiled 

 the secondary wire, which is also of copper, but is 

 much iiner and longer so as to form as many turns 

 as possible. This coil of fine wire is usually protected 

 by a covering of some stout fabric. The ends of the 

 secondary wire lead to the binding- screws k and k. 2 . 

 The terminals of the battery are clamped in the binding- 

 screws K^ and K^ of which the former only is seen in the 

 figure. The clamp K-^ is immediately connected with 

 one end of the primary coil ; the other end of this coil 

 is in metallic connection with the small spring / which 

 carries on the top, just opposite to the projecting bundle 

 of iron wire, a small piece of iron e. Upon /, exactly 

 opposite to the point of the screw s, is soldered a small 

 piece of platinum foil ; this is in contact with the 

 end of the screw s, which is also tipped with platinum. 

 The small upright of brass which carries the screw 5 is 

 in conducting connection with the binding-screw 7T 2 . 

 The connecting wires are all hidden in the wooden base 

 of the apparatus, which further contains a special con- 

 trivance, interposed in the primary circuit and cal- 

 culated to increase considerably the intensity of the 

 induced currents, called the c condenser.' For the 

 theory of its a.ction the student must consult larger 

 works, as also for the explanation of the more intense 

 inductive action of a bundle of iron wires compared 

 with that of a massive bar of soft iron. 



When KI and K 2 are connected with the poles of a 

 battery of sufficient strength, the current passes through 

 the primary coil, and the bundle of iron wire becomes 



S 



