^^^™ 



328 Prof. J. J. Thomson on the Discharge of Electricity 



portion rushes through the dielectric to discharge itself into 

 the wire connecting the coatings of the jars, an intermediate 

 position being shown in fig. 5. These tubes while rushing 

 through the dielectric produce, as already stated, magnetic 

 forces ; some of them on their way to the discharging wire 

 will pass through the discharge-tube; if they congregate there 

 in sufficient density, discharge will take place through the 

 rarefied gas. 



The discharge of the jar is oscillatory, and we have only 

 followed the motion of the tubes during a part of the oscilla- 

 tion ; when, however, this tube enters the wire between the 

 jars a tube of opposite kind emerges from it ; the same thing 

 happens when the other portion enters the spark-gap. These 

 go through the same processes as the tubes we have followed, 

 but in the reverse order, until we get again two short tubes 

 in the jars, but opposite in sign to the original ones ; the 

 process is then repeated and so on as long as the vibrations 

 last. 



In order to see what are the most advantageous di- 

 mensions to give to our apparatus, let us consider on what 

 the maximum electromotive force in the secondary depends. 

 Let us take the case of a condenser of capacity C discharging 

 through a circuit whose coefficient of self-induction is L ; 

 then, if the potential difference between the plates of the con- 

 denser is initially V , the current 7 at the time t is (sup- 

 posing as a very rough approximation that there is no decay 

 in the vibrations) given by the equation 



cv„ . t 



7= — 7f7^ sin 



VLC VLC* 



The rate of variation of this, 7, is therefore 



L cos v'LC 



So that if M is the coefficient of self-induction between the 

 primary and a secondary circuit, the maximum electromotive 

 force round the secondary is MV /L, which for a given spark- 

 length is independent of the capacity of the condenser. In 

 practice it is advisable, however, to have as much energy in the 

 jars to start with as possible, and better results are got with 

 large jars than with small ones. Using a six-plate Wimshurst 

 machine I got very good results with two " gallon jars;" with 

 a large induction-coil the best results were got with two 

 " pint and a half jars." 



The best number of turns to use in the primary coil C 



