[fiekheller] to determine MINIMUM SPARK POTENTIALS IDS 



A measure of the current jjassing through the discharge chamber 

 could be obtained from the readings on either of the Weston instruments, 

 but owing to a difference in the sensibilit}' of the two instruments the 

 milliammeter was used to obtain approximate readings and the voltmeter 

 when accuracy was required. 



In making the observations a voltage sufficiently high to cause a 

 discharge to take place was hrst applied and then the pressure in the 

 discharge chamber was gradually lowered and at various stages the 

 y-ressure in the discharge chamber ascertained from the corresponding 

 leading on the McLeod gauge. 



Sets of such readings were taken in hydrogen for 400, 500, 600, 

 700, and 900 volts, and the various current and pressure readings cor- 



responding to each of these potentials are recorded in Table I. Curves 

 illustrating these readings are also shewn in Fig. V. 



From these curves it will be seen that with each voltage the current 

 over a limited range of pressures was practically constant, but that 

 below this range the current fell away as the pressure was lowered and 

 ultimately dropped to zero at a certain definite pressure for each voltage. 

 This drop in the current indicated of course that the resistance of the 

 spark gap increased as the pressure was lowered. As the conductivity 

 of the circuit apart from the spark gap remained constant for all 

 pressures, this rise in the resistance of the gap was therefore accompanied 

 hj a corresponding rise in th.e potential difference between the two 

 electrodes in the discharge chamber. It follows then that when the 

 current ceased to pass the potential differepco between the electrodes was 

 the same exactly as that which was supplied by the battery and was 

 measured by the electrostatic voltmeter reading. We have then in these 

 final current readings a means of ascertaining the pressures at which 

 the different applied voltages ceased to cause current to pass, or what is 

 practically the same thing, the spark potentials corresponding to different 

 pressures in the discharge chamber. 



Sec. in., 1909. 7 



