HERTZIAN WAVE WIRELESS TELEGRAPHY. 205 



Hertzian wave telegraphy, but the fact remains that whatever may be 

 the sensibility of the receiving appliance, the rate at which telegraphy 

 of any kind can be conducted is essentially dependent upon the rate at 

 which the signals can be sent, and this in turn is largely dependent 

 upon the mechanical movement which the key has to make to interrupt 

 the primary circuit, and so interrupt the secondary discharge. 



In order to make the separation of the contact points of the switch 

 as small as possible, and yet prevent an arc being established, various 

 blow-out devices have been employed. The simplest arrangement for 

 this purpose is a powerful permanent magnet so placed that its inter- 

 polar field embraces the contact points and is at right angles to them. 



As already explained, the applicability of the induction coil in 

 wireless telegraphy is limited by the fact of the high resistance of the 

 secondary circuit, and the small current that can be supplied from it. 

 Data are yet wanting to show what is the precise efficiency of the 

 induction coil, as used in Hertzian wave telegraphy, but there are 

 reasons for believing that it does not exceed 50 or 60 per cent. 



Where large condensers have to be charged, in other words, where 

 we have to deal with larger powers, we are obliged to discard the induc- 

 tion coil and to employ the alternating current transformer. But this 

 introduces us to a new class of difficulties. If an alternating current 

 transformer wound for a secondary voltage, say of 20,000 or 30,000 

 volts, has its primary circuit connected to an alternator, then if the 

 secondary terminals, to which are connected two spark balls, are grad- 

 ually brought within striking distance of one another, the moment 

 we do this an alternating current arc starts between these balls. If 

 the transformer is a small one, there is no difficulty in extinguishing 

 this arc by withdrawing the secondary terminals, but if the trans- 

 former is a large one, say of ten or twenty kilowatts, dangerous effects 

 are apt to ensue when such an experiment is tried. The short circuit- 

 ing of the secondary circuit almost entirely annuls the inductance of 

 the primary circuit. There is, therefore, a rush of current into the 

 transformer, and if it is connected to an alternator of low armature 

 resistance, the fuses are generally blown, and other damage done. 



Let us suppose then that the secondary terminals of the trans- 

 former are also connected to a condenser. On bringing together the 

 spark balls connected with the secondary terminals, we may have one 

 or more oscillatory discharges, but the process will not be continuous, 

 because the moment that the alternating current arc starts between 

 the spark balls, it reduces their difference of potential to a compara- 

 tively low value, and hence the charge taken by the condenser is very 

 small, and, moreover, the circuit is not interrupted periodically so as 

 to re-start a train of oscillations. 



When, therefore, we desire to employ an alternating current trans- 



