STRIDE IN THE ELECTRIC DISCHARGE. 381 



there are not two discharges when an apparently single dis- 

 ruption of the primary circuit takes place. 



I had repeatedly and in vain attempted to get rid of this 

 difficulty, and to produce what I could feel assured was a single 

 induced discharge, when a very simple plan occurred to me, 

 which I am surprised did not more readily present itself ; it 

 is 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 

 terminals 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 

 projecting, 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 striae very beautifully exhibited in 

 the receiver ; the projecting wires were now gradually sepa- 

 rated, and the striae 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 ; the striae now disap- 

 peared, and a uniform luminous cloud was produced in the 

 receiver. Even in this extreme case, however, there is apt to 

 be a double discharge, which anyone who has not watched 

 closely the phenomenon with eye and ear would not detect, 

 but would pronounce the discharge to be single. 



From unavoidable minute differences in the action of the 

 contact-breaker, discharges will, all conditions being appa- 

 rently the same, vary in strength and length ; those accus- 

 tomed to the apparatus will know that, by separating the ter- 

 minals 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. 



