and its Effects. 127 



electric egg. The more the latter is exhausted the less the effect 

 of the condenser, which effect disappears altogether (as iu me- 

 tallic contact) when the exhaustion is carried to a line, or less, 

 of mercurj^ pressure ; provided only that the induction current 

 itself is sufficiently intense. For a similar reason an induction 

 coil which possesses a short wire produces much weaker sparks 

 in free air than one with a long wire ; whereas with the former 

 the luminous phsenomena in a well-exhausted electric egg are 

 very little less developed than with the latter. I observed this 

 with the induction wire only 1200 feet long, 



Fizeau does not appear to have thoroughly understood the 

 action of the condenser. Amongst other things, in order to 

 explain the action of the instrument, he only considers a single 

 interruption of the inducing current, consequently only a single 

 charge of the condenser, and never mentions what further occurs. 

 But it is evident, in order that the condenser may continue to 

 serve its purpose, it must be discharged before every interruption 

 of the current ; and this actually takes place every time the cii"- 

 cuit is closed after it has been broken. At both these times 

 sparks are visible ; in fact, when a condenser is employed, Neef s 

 hammer occasions a double series of sparks, which, although they 

 cannot be separated by the eye, owing to their quick succession, 

 are nevertheless distinguishable in character. The spark, on 

 breaking contact, is always weakened by the condenser, and the 

 degree in which it is so weakened may be considered as a mea- 

 sure of the condenser's action* j but the spark caused by its 

 discharge is proportionally strengthened. 



Thus it appears, that, by the addition of a condenser, the 

 luminous phenomena at the hammer are, according to circum- 

 stances, apparently unchanged, increased, or diminished. For 

 example, when a double wire around the ])rimary coil was em- 

 ployed, together with a soft iron core, and a battery of two of 

 Grove's elements, the introduction of the 8-feet condenser caused 

 the luminous pha?nomenon at the hammer to deci'ease ; with the 

 2-feet condenser it was, on the contrary, increased, without any 

 particular change in the sparks of the induction coil being per- 

 ceptible iu the two cases. By employing one of Grove's elements, 

 with which the sparks at the hammer arc always weak, and are 

 still further weakened by the condenser, the discharge sparks 

 are scarcely perceptible, although they can be heard, and by their 

 irregular and somewhat sonorous stroke they can be easily distiu- 



* When the metallic continuity of the induction circviit is unbroken, 

 the sparks at the liammer are always considerably diuiiuislied. Even when 

 the induction circuit is interrupted by a stratum of air so thin that the in- 

 duction sparks form a continuous line, the spiirks at the hammer are always 

 diminished. 



