I 



Objects , and on the Oscillating Electric Spark. 255 



magnification. That this pilot-spark is due to the discharge 

 of the knobs is certain, because in the first place it is a 

 quarter period behind the centre of the first spark of the 

 oscillation proper, and in the second place because in a photo- 

 raph taken under similar conditions, but after a small 

 eyden-jar had been directly connected to the terminals, the 

 pilot-spark was far brighter than before and was even brighter 

 than any of the succeeding alternating elements, while it occu- 

 pied the same place, namely one quarter period before the 

 centre of the first spark of the oscillation proper. The very 

 exact analogy between the motion of the electricity in an 

 oscillating spark and of the bob of a common pendulum may 

 be worth tracing : — 



The electric displacement gradu- 

 ally increases as the machine is 

 worked until the electromotive force 

 is sufficient to break the air-gap 

 which held the electricity on one 

 side. 



The steady oscillation of the elec- 

 tricity then begins, and in a quarter 

 period the electric Telocity is at its 

 greatest value, the current is strong- 

 est, or the spark is brightest ; the 

 current then gradually gets less 

 until for a moment it ceases and 

 the spark goes out. 



The electric displacement is now 

 nearly the same in amount as at 

 first, but in the opposite direction, 

 it therefore starts back again, and 

 so swings backwards and forwards 

 as the knobs become alternately + 

 and — (as is indicated in some of 

 the photographs by the brighter 

 glow alternately at each end of the 

 spark), but getting less at every 

 swing, owing partly to electrical 

 resistance wastefully produciug 

 heat and partly to electromagnetic 

 radiation which will give rise to 

 oscillating currents in neighbouring- 

 conductors, especially if they have 

 the same period, until at last the 

 whole store of energy has been 

 wasted and the electricity is at rest. 



In order to increase the total number of oscillations, I have 

 introduced what I call a trap, which consists of a second air- 

 gap in the circuit kept open by a prop while a strong spring- 

 tends to close it. The spark-gap is then made very short, but 

 the potential can be raised up to any degree that the con- 



T2 



The displacement of the bob 

 gradually increases as the thread 

 which draws it to one side is pulled 

 until the force is sufficient to break 

 the thread which held the bob on 

 one side. 



The steady oscillation of the pen- 

 dulum then begins, and in a quarter 

 period the velocity is at its greatest 

 value ; the velocity then gradually 

 gets less until for a moment the 

 bob is at rest. 



The displacement of the bob is 

 now nearly the same as at first, but 

 in the opposite direction ; it there- 

 fore starts back again and so swings 

 backwards and forwards, resting 

 alternately on one side and on the 

 other, but getting less at every 

 swing, owing partly to resistance 

 wastefully producing heat and 

 partly to wave-motion transmitted 

 through the supporting wall which 

 will cause other pendulums to 

 start swinging, especially if they 

 have the same period, until at last 

 the whole store of energy has been 

 wasted and the bob is at rest. 



