344 



Mr. J. B. B. Burke on the 



rest of the circuit ; for if n is the number of turns of wire 

 we put 



M. = /3n and L = L + cm 2 , 



L being the self-induction of circuit not included in the coil. 

 Then 



M_ J3n 

 L ~~ L + aw 2 ' 



which is a maximum when L = cm 2 . But in practice this 

 is not exactly the best system, since it is found advisable to- 

 increase the period of oscillation of the currents. 



Section II. 



(2) A clear idea of the apparatus used may be formed 

 from fig. 2. The air-pump and McLeod gauge are repre- 

 sented on the right-hand side of the figure, and the bulb 

 12 cms. diam v with the long tube 2 m. in length and 5 cms. 

 diameter, is represented on the left. 



Fig. 2. 



!\ 



f 







) 4 



A 



When the electrodeless discharge is sent through the gas 

 in the bulb, as already explained, within the limits of 

 pressure, the phosphorescent after-glow is produced, and it is 

 seen to travel down the tube with a velocity of about 

 2 metres per sec. This velocity, however, may be con- 

 siderably diminished by using narrower tubes. 



It is sometimes difficult to get the discharge to pass, or to 

 obtain the glow, but when once started it increases rapidly 



