DlsrHAIUil-: OF ELKCTRICITV. 235 



motion of the tubes (luviiij^ u |);iii of the osciUatioii; when, however, 

 this tube euterib the wire between the Jars a tube of oi)i)osite kind 

 emerges from it; the same thing happens when theother portion enters 

 the spark gap. These go through the same processes as the tubes we 

 have foHowed, but in tlu' 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 ou as long as the vibrations last. 



In order to see what are the most adsautageous dimensions to 

 give to our apparatus, let us consider on what the maximum electro- 

 motive force in the secondary depends. Let us take the case of a con- 

 denser of capacity C discharging thiough a- circuit whose coefiticieut of 

 self-induction is L; then, if the potential difference between the plates 

 of the condenser is initially Vn, the current ;/ at the time / is (suppos- 

 ing as a very rough approximation that there is no decay in the vibra- 

 tions) given by the equation 



CVo . t 



The rate of variation of this, y, is therefore 



Vo t 



So that if iNI is the coefficient of self-induction between the primary 

 and a secondary circuit, the maximum electro-motive force around the 

 secondary is MV,, /L, Avhich for a given spark-length is independent of 

 the capacity of the condenser. In practice it is advisable, iiowever, to 

 liav^e as nuu'h energy in tlie jars to start with as possible, and better 

 results are got with large jars than with small ones. Using a six-plate 

 Wimshurst maciiine 1 got very good results with two " gallon jars; " 

 with a large induction coil the best results Avere got with two " pint- 

 and-a-halt jars,'' 



The best number ol' turns to use in the primary coil (J depends ui)on 

 the size of the leads; if all the circuit were available for this coil one 

 luin would give the laigest electro-motive tbrce, because, though lor a 

 given rate of change of the current in the primary the <'ffect on the sec- 

 ondary increases with the number of turns, the rate of change of the 

 current varies inversely as the self-indu(;tion of the i>rinuiry, so that if 

 all the circuit is in the coil (J, since an increase in the number of turns 

 will increase the vself-induetion of the circuit faster than the mutual in- 

 duction, it will diminisli the eh'ctn)-motiv<' force round the secondary, 

 in practice however it is not possible to have the whole of the wire 

 connecting the coatings of the jar in the coil C; and in this case an in- 

 crease in the number of tuiiis may increase tlie nmtual induction more 

 than the self-induction, and so be advantageous. Tlie best result will 

 be obtained when the self-induction in the coil C is e((nal to that of 

 the remainder of the circuit. It is very easy to find by actual trial 



