INFLUENCE OF PRESSURE ON STRIKING DISTANCE. 199 



enormous, that the potential of an electrified cloud is out of pro- 

 portion with those obtained by ordinary frictional machines. 



823. INFLUENCE OF PRESSURE. When the pressure of a gas is 

 diminished, the electromotive force corresponding to a given striking 

 distance decreases rapidly. Experiments agree in showing that the 

 law of variation is very closely represented by the branch of a 

 hyperbola, the real axis corresponding to the pressures, and the 

 imaginary axis to differences of potential. 



With highly rarefied gases the electromotive force only diminishes 

 to a certain limit of pressure, beyond which it again increases with 

 extreme rapidity. There is then a pressure for which the resistance 

 to the production of the discharge passes through a minimum. This 

 limit varies in different gases, and for one and the same gas it is 



Fig. 158. 



lower the narrower is the tube. De la Rue and Miiller found that 

 for air it varied from three millimetres in wide tubes to 0-38 mm. in 

 narrow ones. As the pressure diminishes further, the spark does not 

 pass, whatever be the electromotive force. All" experiments tend 

 thus to show that matter is necessary for the transport of electricity, 

 and that the molecules of the dielectric serve as the vehicle. The 

 effect of a gradual diminution of the pressure is to give more freedom 

 to the motion of the gaseous molecules, but beyond a certain limit 

 the diminution of the number is no longer compensated by the 

 greater freedom of their movements. 



824. MEASUREMENT OF CHARGES BY THE SPARK. The spark 

 is especially used in measuring charges. The simplest arrangement 

 is that of Lane's jar (Fig. 158). It consists of a Leyden jar, the 

 outer coating of which is connected with a knob B, which can be 



