500 Mr. W. Duddell [May 17, 



did not always flow in the same direction, and we owe to Lord Kelvin 

 the mathematical investigation of this subject, and to Feddersen its 

 experimental demonstration. If I charge a Leyden jar, or any other 

 condenser, and allow it to discharge through a very high resistance, 

 a current will flow, which gradually dies away as the jar discharges. 

 If, however, I discharge the condenser through a sufficiently low 

 resistance, it will be found that the current continues to flow in the 

 same direction after the jar is completely discharged, and then 

 charges it up in the opposite direction, the current gradually getting 

 smaller. When the jar is charged to a certain extent in the opposite 

 direction to its original direction it starts to discharge again, this time 

 in the opposite direction. The current again continues to flow after 

 the jar is discharged, and charges it up in the original direction. 

 This cycle of operation is repeated, the energy of the charge in the 

 jar gradually getting less and less, until it dies away. As a simple 

 analogy, a pendulum, or a weight hanging on a spring, when dis- 

 placed oscillates, the oscillations growing smaller and smaller until 

 they die away. This is so well known that I need not go any further 

 into the matter. The frequency of these oscillations is determined 

 by the capacity of the condenser — that is to say, the quantity of 

 electricity it can hold when charged to 1 volt and the self-induction 

 of the circuit — the capacity of the condenser corresponding to the 

 weakness of the spring, and the self-induction to the mass of the 

 weight in our spring and weight analogy. 



It is quite easy to obtain by this method almost any frequency — 

 for instance, anything between 1 and 100,000,000 are comparatively 

 easily obtainable, and I have even demonstrated frequencies as low 

 as i in this lecture theatre. 



The fundamental difference between these high frequency currents 

 and those generated by alternators is that they die away — that is to 

 say, that the amplitude gradually decreases and comes to zero. The 

 condenser then has to be re-charged, and the process repeats itself. 

 Now, a single discharge of this sort is not of much practical use. It 

 is necessary to charge the condenser and repeat the discharges in 

 rapid succession if any useful work is to be done. Many methods 

 have been proposed for this purpose. Some of the early successful 

 experiments were those of Tesla, shown in this theatre on February 8, 

 1892. 



The condenser is allowed to discharge through a circuit which 

 consists of the necessary self-induction and a spark gap in series. 

 Now, a spark gap possesses the property of acting like a safety valve 

 whicli will stand a certain steam-pressure, and will then suddenly 

 open and let the steam out freely. The spark gap will stand a 

 certain electrical pressure, and then will open and let the electricity 

 pass. When once the electrical pressure or voltage has been raised to 

 a sufficiently high value to break down the spark gap, the spark gap 

 changes from being an insulator which prevented the flow of electricity 



