20 i\Ii'. W. R. Grove on the Slrice seen hi 



which I am surprised did not more readily present itself: it is 

 simply to make an interruption in the secondary circuit, besides 

 that formed by the vacuum ; such second interruption to be in 

 air, and of the extreme length of the spark, so that not more 

 than one spark should pass at a time across the intervening air, 

 and consequently not more than one discharge through the 

 vacuous space. 



Using the- air-pump with phosphorus and a stick of potash in 

 the receiver, I connected by a wire one of the secondary ter- 

 minals of a RuhmkorfF coil with the rod which entered the 

 receiver at the top. The wire from the second terminal was 

 attached to a glass stand or pillar, so that its end projected; to 

 another glass stand was attached a wire with one end also pro- 

 jecting, and the other end was fixed to the brass work of the air- 

 pump. The two projecting ends could be made to touch, or to 

 remain at any desired distance from each other. 



The ends of the wires being in contact, the coil apparatus was 

 set at work, and the strife very beautifully exhibited in the re- 

 ceiver; the projecting wires were now gradually separated, and 

 the stri?e for some time continued visible and until the points of 

 the wires were so far apart that an occasional spark only passed 

 from point to point when the stri?e disappeared, and a uniform 

 luminous cloud was prodviced in the receiver. Even in this 

 extreme case, however, there is apt to be a double discharge, 

 which any one who has not watched closely the phseuomenon 

 with eye and ear, would not detect, but would pronounce the dis- 

 charge to be single. 



From vmavoidable minute differences in the action of the con- 

 tact breaker, discharges will, all conditions being apparently the 

 same, vary in strength and length; those accustomed to the 

 apparatus will know that, by separating the terminals beyond 

 the normal distance of the discharge, sparks will still pass at 

 occasional, distant and irregular intervals, proving the slightly 

 varying intensity of the apparatus. 



Now, when the points are sej)arated to the maximum for the 

 experiment which I have detailed above, two classes of sparks 

 will be perceived in the air, — the one a thin blue spark giving to 

 the ear a single sharp sound, and the other a burred yellow 

 spark with a sound not so clear and metallic, but attended with 

 a slight whizz : this contrast of sound maybe roughly compared 

 to that presented by a whole or cracked piece of porcelain when 

 struck ; it requires attention and habit to distinguish these two 

 classes ; but when the power is attained, it will be seen that with 

 the first or single discbarge there ai'e no strife in the receiver, 

 with the second, which I believe to be a double discharge, striae 

 are visible. 



