364 ELEMENTARY LESSONS ON [CHAP. x. 



to make and break the primary circuit in rapid suc- 

 cession. The result of this is at every " make " to induce 

 in the outer " secondary " circuit a momentary inverse 

 current, and at every " break " a powerful momentary 

 direct current. The currents at " make " are sup- 

 pressed, as explained below : the currents at " break " 

 manifest themselves as a brilliant torrent of sparks 

 between the ends of the secondary wires when brought 

 near enough together. The primary coil is made of 

 stout wire, that it may carry strong currents, and produce 

 a powerful magnetic field at the centre, and is made of 

 few turns to keep the resistance low, and to avoid self- 

 induction of the primary current on itself. The central 

 iron core is for the purpose of increasing, by its great 

 coefficient of magnetic induction, the number of lines- 

 of- force that pass through the coils : it is usually made 

 of a bundle of fine wires to avoid the induction currents, 

 which if it were a solid bar would be set circulating in 

 it, and which would retard its rapidity of magnetisation 

 or demagnetisation. The secondary coil is made with 

 many turns, in order that the coefficient of mutual 

 induction may be large ; and as the electromotive-force 

 of the induced currents will be thousands of volts, its 

 resistance will be immaterial, and it may be made of the 

 thinnest wire that can conveniently be wound. In Mr. 

 Spottiswoode's giant Induction Coil (which yields a 

 spark of 42^ inches' length in air, when worked with 30 

 Grove's cells), the secondary coil contains 280 miles of 

 wire, wound in 340,000 turns, and has a resistance of 

 over 100,000 ohms. 



The interrupters of induction coils are usually self- 

 acting. That of Foucault, shown with the coil in Fig. 

 148, consists of an arm of brass L, which dips a platinum 

 wire into a cup of mercury M, from which it draws the 

 point out, so breaking circuit, in consequence of its 

 other end being attracted toward the core of the coil 

 whenever it is magnetised ; the arm being drawn back 



