of Induced Electric Sparks. 197 



10. Throughout the above experiments the most curious 

 effects of induction were exhibited by all metallic bodies in 

 the neighbourhood of the apparatus. From any of these 

 within a range of several yards sparks of a similar reciproca- 

 ting nature could be drawn by bringing near an earth-wire or 

 any conductor of large surface, whether insulated or not. 



11. It was found, as in a preceding case stated at the close 

 of § 3, that no sparks could be drawn from Q (fig. 3) when 

 w was brought up to actual contact with T. That is, an in- 

 termittent current (as is that in the secondary wire of the 

 coil) passing through a closed circuit does not evoke that kind 

 of disturbance in a neighbouring conductor which is evoked 

 when the intermittent current has to overleap an interval of 

 air, however short. This result proved that the appearance of 

 a momentary charge upon Q when w and T were separated 

 was the result of induction from a momentary charge upon P, 

 and not merely from a current passing through P. The well- 

 known phenomenon of the statical charge produced upon the 

 separated terminals of the secondary circuit of an induction- 

 coil will account for the presence of single momentary charges 

 upon P and upon T. These charges must accumulate through 

 a short but definite period of time until they have acquired 

 sufficient density to overleap with a spark the resistance 

 offered by the layer of air iv T — the density of the accumula- 

 ting charge depending on the dryness of the atmosphere, the 

 perfectness of the insulation, and the distance between w and 

 T. Though this period be very short, such charges will 

 necessarily, by the inductive influence of their presence, dis- 

 turb the electrical equilibrium of all near conductors such as Q. 

 The equally well-known phenomenon of the return-stroke, in- 

 vestigated by Lord Mahon in 1779, will account for the ap- 

 pearance of a return-spark after an interval perhaps not 

 exceeding the thousandth part of a second, thus establishing 

 the reciprocating nature of the discharges passing between 

 Q and C. 



12. In the case of the earlier experiments (§ 2), where an 

 interrupted primary current passes round an electromagnet, 

 similar conditions virtually exist. The extra-current conse- 

 quent on breaking the primary is a current of high electro- 

 motive force, like the secondary current of the coil. It, too, 

 has a sensible interval to overleap, though the distance in this 

 case is not constant. It, too, must accumulate upon its con- 

 ductor in order to acquire sufficient density to overleap the 

 gap in the circuit. It, too, can therefore, during a distinct 

 minute period of time, analyze the neutral electricity of the 

 electromagnet-core across the dielectric of silk or cotton that 



