INDUCTION COILS. 745 



dips in a layer of mercury covered with water or alcohol. The 

 spark is shorter, for the alcohol rapidly condenses the metallic 

 vapours which facilitate the passage of the extra current. 



A more important improvement was effected by M. Fizeau.* 

 The ends of the principal circuit, on either side the point where 

 the break is made, communicate separately with the armatures of 

 a condenser. The difference of potential of the two points is thus 

 reduced, for the electricity due to the extra current rushes into the 

 condenser. This condenser then discharges itself when the contact 

 is made, so that, by increasing its capacity, we always diminish the 

 spark on breaking ; but we finish by increasing those on making. 

 Experiment shows what capacity gives the best results. 



Finally, the windings of the secondary coil are usually arranged 

 in successive layers, which occupy the entire length coil, and which 

 are separated from each other by insulating layers. For coils of 

 great dimensions, this arrangement has the disadvantage that the 

 adjacent windings of two consecutive layers are separated by a great 

 length of induced wire ; there is thus a great difference of potential 

 between them, which can perforate the insulating layer or, at any 

 rate, produce losses of electricity. To Poggendorfff is due the idea 

 of partitioning the coils that is to say, arranging the secondary wire 

 in a series of successive compartments separated from each other 

 by insulating plates perpendicular to the axis of the coil. The 

 potential of the secondary wire thus increases from one end to the 

 other of the coil, without producing too great a difference between 

 two adjacent layers of the same compartment. 



When the ends of a secondary wire are connected with the 

 armatures of a condenser, the maximum difference of potential 

 diminishes, for the capacity of the induced wire increases ; but this 

 connection is equivalent to the introduction of a conductor ; the 

 discharge is accordingly increased. If the poles are separated by 

 so small a distance that the spark can pass across it, the quantity 

 of electricity corresponding to the discharge increases then with the 

 capacity of the condenser. As the striking distance is, in each case, 

 raised to a maximum, the corresponding energy also increases as the 

 distance diminishes. 



If the striking distance is not to be too much diminished by the 

 introduction of a condenser, the capacity of this condenser must be 

 so small that it can bear a great difference of potential. In that case 



* FIZEAU. Comptes rendtts, Vol. xxxvi., p. 418. 1853. 

 f POGGENDORFF. Pogg. Ann., Vol. xciv., p. 289. 1850. 



