On the Phosphorescent Glow in Gases. 343 



connected with the terminals o£ a powerful Wimshurst 

 machine ; the outer coatings are connected by a coil of a few 

 turns of thick insulated wire. The coil surrounds the vacuum- 

 bulb in which the electrodeless discharge is produced. 

 Whenever a spark passes between the terminals of the Wims- 

 hurst, rapid alternating currents are set up in the coil by the 

 discharge of the jars, and this gives rise to an induced 

 alternating electromotive force in the rarefied gas at pressures 

 that depend somewhat upon the size of the bulb and other 

 circumstances ; but in a bulb of about 12 cms. in diameter 

 the ring-discharge can be made to pass at pressures of about 

 1 cm., although it is then extremely feeble. At pressures of 

 about 0*5 mm. the ring appears most distinct, but as the 

 pressure is reduced it becomes less and less clearly defined, 

 until the electrodeless discharge appears to take place through- 

 out the whole bulb. Within certain limits of pressure (that 

 for air lie between 0*7 mm. and 0*02 mm.), the gas exhibits 

 a most beautiful after-glow or phosphorescence, which is 

 brightest at about 0*1 mm., and it is this after-glow in a 

 gas that has been the subject of investigation. 



The ley den-jars used were both two-gallon jars, and the 

 spark-length between the knobs at the terminals of the 

 electrical machine varied from 2 cms. to about 5 cms. 



The period of the oscillatory currents which are set up by 

 the passage of the spark depends upon the self-induction of 

 the circuit and the capacity of the jars. Theoretically the 

 capacity of the jars does not contribute to the maximum 

 electromotive force round the circuit, since, as it may be 

 shown, the expression for the electromotive force does not 

 involve the capacity of the jars; but it is found that better 

 effects are obtained with large jars than with small ones, 

 the energy to begin with being greater, so that the oscillations 

 last for a longer time. Prof. J. J. Thomson ('Recent 

 Researches/ p. 95) has pointed out that, if M is the mutual 

 induction between the primary and the secondary circuit, 

 consisting of the coil of wire wound round the bulb and 

 the gas in the bulb, the maximum electromotive force 



round the circuit is -j—^ ; V being the initial difference 



of potential between the terminals of the Wimshurst, and L 

 the coefficient of self-induction of the discharging circuit, and 

 the maximum electromotive force being independent of the 



M 



capacity of the jars. It is easy to see that *- is a maximum 



when the self-induction of the coil is equal to that of the 



