1912] on High Frpqucncy Currents. 503 



peiidulnin. The long pendulum will be set swinging easier with a 

 blow which conies on and off comparatively slowly, whereas the short 

 pendulum will require a short, quick tap to start it swinging. 



Tliis method of shock excitation, due to Max Wien, is employed 

 by the Telefunken Company in their wireless transmitter, and also 

 in their therapeutic apparatus ; a special type of spark gap being 

 used consisting of large flat metal plates, a very short distance apart, 

 tlie object being to suddenly cool the spark to cause it to go out, and 

 the gap to become insulating again at the earliest possible moment. 



A spark gap of this type connected to a condenser and self-induc- 

 tion can be supplied in the ordinary way, when the frequency of the 

 spark will naturally depend upon tlie frequency of the alternator. 



If a Leyden jar or condenser be charged from a battery through a 

 high resistance, it will take a certain time to charge up ; and if there 

 be also connected to the condenser a discharge circuit containing a 

 spark gap, when the condenser is sufficiently charged a discharge 

 Avill take place through the spark gap, the condenser will then start 

 to charge again and the operation will be repeated. I employed this 

 method soine years ago to work an induction coil. In this case, as I 

 was only using low voltages under 1000 volts, the spark gap was re- 

 placed by a vacuum tube, and the self-induction in the oscillatory 

 circuit was the primary of an induction coil. The connections and 

 data are shown in Fig. 10. By suitably adjusting the pressure in 

 the vacuum tube and the resistance in series with the condenser, it is 

 easy to obtain a rapid series of sparks. If voltages of the order 

 of 1000 volts direct current were generally available, this might form 

 a very convenient method of working a coil. 



It will be noted that the spark frequency is not, in this case, 

 determined by an alternator, but depends upon the time the condenser 

 takes to charge to a sufficiently high voltage to break down the spark 

 gap. By making the resistance small, and the supply voltage high, 

 the condenser will charge more quickly. It is quite easy to cause 

 the discharges to follow one another very rapidly by this means. 



If an attempt be made to employ the successive discharges of a 

 condenser when charged from continuous current, a difficulty is 

 encountered if large powers are dealt with. This difficulty seems to 

 reside in the properties of the spark gap, which tends to become 

 completely conducting, or, what is technically called " arcs," when 

 large powers, say 10 k.w. or over, are employed. 



Mr. Galletti set himself to get over this difficulty by employing 

 a number of such circuits, each with their own spark gap, and con- 

 necting these together so that they would all help to produce the 

 oscillations. 



Of the methods proposed by him I will only refer to one which 

 I have seen in operation. The principle of it is illustrated in Fig. 11. 

 It consists of splitting the condenser into two parts, one of which — 

 Ca — is common to all the discharge circuits. The action of this 



