HERTZIAN ^YAVE WIRELESS TELEGRAPHY. 203 



secondary coil. It therefore does not assist us in the direction re- 

 quired, viz., in prolonging the duration of the secondary electromotive 

 force to enable larger capacities to be charged. 



The important point in connection with the working of a coil used 

 for charging a condenser is not the length of spark which the coil 

 can give alone, but the length of spark which can be obtained between 

 small balls attached to the secondary terminals, when these terminals 

 are also connected to the two surfaces of the condenser. Thus, a 

 coil may give a ten-inch spark if worked alone, but on a capacity of 

 one thirtieth of a microfarad it may not be able to give more than 

 a five-millimeter spark. Hence in describing the value of a coil for 

 wireless telegraph purposes, it is not the least use to state the length 

 of spark which the coil will give between the pointed conductors in 

 air, but we must know the spark length which it will give between 

 brass balls, say 1 cm. in diameter, connected to the secondary ter- 

 minals, when these terminals are also short-circuited by a stated 

 capacity, the spark not exceeding that length at which it becomes 

 non-oscillatory. 



A good way of describing the value of an induction coil for wire- 

 less telegraph purposes is to state the length of oscillatory spark 

 which can be produced between balls one centimeter in diameter con- 

 nected to the secondary terminals, when these balls are short-circuited 

 by a condenser having a capacity say of one hundredth of a micro- 

 farad, and also one tenth of a microfarad. 



If a hammer or motor interrupter is employed with the coil, then 

 a primary condenser must be connected across the points between which 

 the primary circuit is broken. This condenser generally consists of 

 sheets of tin-foil alternated with sheets of paraffin paper, and for a ten- 

 inch coil, may have a capacity of about 0.4 or 0.5 of a microfarad.* 



Lord Eayleigh discovered that if the interruption of the primary 

 circuit is sufficiently sudden and complete, as when the primary circuit 

 is severed by a bullet from a gun, the primary condenser can be re- 

 moved and yet the sparks obtained from the secondary circuit are 

 actually longer than those obtained with the condenser and the ordi- 

 nary break, f 



In the use, however, of the coil for Hertzian wave telegraj^hy, 

 with all interrupters except the Wehnelt break, a condenser of suitable 

 capacity must be joined across the break points. 



Turning in the next place to the primary key, or signaling inter- 

 rupter, it is necessary to be able to control the torrent of sparks between 



* For a discussion of the function of the condenser in an ordinary induc- 

 tion coil, see ' The Alternate Current Transformer,' by J. A. Fleming, Vol. 

 II., p. 51. 



t See Lord Eayleigh, Phil. Mag., December, 1901. 



