502 Mr. W. Diiddell [May 17, 



are ol)tainable. Taking the trans- Atlantic case, where the frequency 

 of the oscillations is about 5<),000 per second, and, assuming that 

 there are 50 or 60 oscillations in the aerial for each discharge, tlie 

 total time the oscillations in the aerial last is just over r^^oot-li of a 

 second. As the spark frequency is about 600 per second the oscilla- 

 tions are therefore practically joined up, and there are no completely 

 idle intervals between them. 



I now come to another class of method which still depends upon 

 the discharge of a condenser. If I have a body like a pendulum or 

 a spring capable of oscillations, I can make it oscillate by pulling it 

 away from its position of rest and then letting it go. I can also 

 make it oscillate l)y giving it a blow or shock, and I can make it 

 oscillate by applying to it a number of impulses at properly timed 

 intervals. All three of these methods are employed in the production 

 of high frequency currents. The first is the simple discharge of the 

 Leyden jar without any auxiliary apparatus ; the third is the method 

 usually employed in wireless telegraphy for obtaining and maintain- 

 ing a long series of oscillations in the aerial by tuning it and loosely 

 coupling it to the Leyden jar circuit in which the discharge takes 

 place. The second method also involves a discharge of a condenser. 

 In this case the spark gap is so arranged that it suppresses the oscil- 

 lations of the condenser — that is to say, it lireaks down, allows a large 

 rush of current, and then becomes insulating and stops anything 

 further from happening. To illustrate this I have constructed a 

 little valve which works with water A vertical tube of water repre- 

 sents the condenser, and this is connected to the ^'alve. The height 

 of the water in the tube is gently increased, which corresponds to 

 increasing the quantity of electricity in the condenser, and the pres- 

 sure of the water, represented l)y its height, corresponds to the 

 voltage on the condenser. "When the pressure of water is sufficient 

 the valve suddenly opens, corresponding to the spark gap breaking 

 down, and allows the water to flow out from the vertical tube, so 

 discharging it. As the valve only works in one direction oscillations 

 cannot take place. "VYe have thus a model of Avhat is called a 

 quenched spark, which goes out directly the condenser is discharged. 

 A discharge of this type can be caused to act either by magnetic 

 induction or otherwise on a circuit consisting of a condenser and a 

 self-induction. The effect of the rush of current when the jar dis- 

 charges is to give the second circuit a shock and set up oscillations 

 in it. 



It might be thought at first that in this method of producing 

 oscillations there was no relationship between the nature of the swing 

 in the first, or discharge, circuit and the frequency of the oscillations 

 in the second circuit. Without going into the theory of couphng, 

 which would take me too far, I would point out that for the best 

 result there is a relationship, and this can be easily seen if one con- 

 siders the question of starting oscillations in, say, a short and a long 



